nanoscale views

If I could do this with electrons....

2012-02-01T08:20:00.000-06:00

If they aren't already, these people are going to be swimming (or drowning) in DARPA money.

This is damned peculiar....

2012-01-30T20:37:00.000-06:00

There was pretty big hoopla last week about two papers concerning graphene (and it's related material graphene oxide). In Science, Andre Geim's group reported a remarkable result concerning a membrane made from a "paper" comprising layers of graphene oxide flakes. This membrane is apparently extremely leak-tight for gases including the notoriously slippery helium, but essentially transparent (!) to the transport of water vapor. This is very very odd. The argument made by the authors is that the graphene oxide layers are wet by physisorbed water, which can move across the graphene surface nearly frictionlessly (since graphene itself is hydrophobic - that is, it's nonpolar and doesn't interact particularly strongly with the polar water molecules). When the water is removed, the layers compress against one another tightly enough that there are no continuous pathways large enough to allow helium diffusion (or they're clogged up with residual adsorbed water). Assuming this is right, it's pretty cool, and brings to mind the ideal "semipermeable membrane" that's sometimes used as a teaching concept in thermodynamics classes. (Old joke: how do you catch a lion in the desert? A thermodynamicist would take a semipermeable membrane that passes everything except lions, and drag it across the desert to the entrance of a cage. A mathematician would simply map the exterior of the cage to the interior of the cage. Etc.)

Now, the other paper that got a decent amount of attention was this one. The interactions of water with a solid surface are often characterized by a "contact angle", the angle (inside the droplet) with which the water-air interface meets the solid-air interface. When a droplet on a surface "beads up", that angle exceeds 90 degrees (the surface is hydrophobic), while when a droplet wets the surface well, that angle is much less than 90 degrees (the surface is hydrophilic). The authors of this paper claim that a monolayer of graphene on a surface leaves the contact angle completely undisturbed (for surfaces where there is not chemical bonding at work between water and the surface). That's extremely weird, especially in light of the previous paragraph. You'd have to have a situation where the surface interactions of water with graphene are completely determined by the material under the graphene, not by the graphene itself. That is, somehow having graphene there doesn't affect the van der Waals interaction much at all. This is surprising, given past experiments that look at, e.g., the interactions of nanotubes with graphite surfaces, where clearly the van der Waals interaction is nontrivially tied to the graphene geometry, for example. I have a tough time understanding how the interpretations of both of these papers can be correct, though just because it's unintuitive to me doesn't mean it's not true.

(Bonus question: can any of the commenters identify the quote that I used for the title of this post?)

Cold atoms, optical lattices, and condensed matter physics

2012-01-24T13:23:00.000-06:00

Over the last decade, since this experiment in particular, there has been rapidly growing interest in using optically trapped ultracold atoms, traditionally the tool of what people in the game call "atomic/molecular/optical" or "AMO" physics, to study condensed matter problems. Using interfering laser beams, it is possible to make a spatially periodic pattern of optical intensity that acts like a spatially periodic potential energy. Ultracold atoms (they have to be cold so that their kinetic energy is too low for them to fly out of the little potential wells) can be placed in this lattice in a controlled way. The interactions between the atoms can be tuned using clever approaches, so that the interaction is so large that only one atom will like to sit in each little potential minimum. It's also possible to tune the overlap of the potential wells to allow tunneling processes so that the atoms can move (virtually and in real space). With other exceedingly clever modifications, it is even possible to use internal degrees of freedom of the atoms (e.g., nuclear spins), and to introduce effects equivalent to magnetic fields or spin-orbit coupling.

Condensed matter theorists love this stuff - you can actually implement the model problems they've been playing with for ages (e.g., the 2d Hubbard model on a square lattice), and all while maintaining exquisite tunability and control over the microscopic parameters. Moreover, with spectroscopic techniques, you can probe these systems in real space (no need for diffraction experiments to see the periodic arrangement of atoms - just image them!), and pull out microscopic information (population and energy distributions) that is incredibly hard or impossible to get in solid materials. These optical lattice systems are particularly great for examining nonequilibrium dynamics in microscopic detail.

This prompts a couple of questions. First, is this condensed matter physics? Yes, since the systems being modeled are condensed matter systems - that's how we denote theory, right? (Empirically, some optical lattice results are now published in the condensed matter section of Phys Rev Letters, so there you go.) Second, are there condensed matter systems that can't be modeled with these optical lattice methods? Yes. For one example, consider a material like VO₂. First, it's lattice structure is not something readily achievable in an optical lattice. Second, this material undergoes a spontaneous structural change as a function of temperature, due to coupling between the electrons and the lattice. In a cold atom system, you would somehow need the optical lattice itself to change depending on the positions of the atoms stored within it - I don't think anyone has figured out how to do such a thing. I'm sure there are other examples, even in pure single-crystal systems. Bottom line: cold atom techniques for studying certain condensed matter problems are amazing and revolutionary, but there are going to be many CM systems that can't be accessed or modeled that way.

"Low energy nuclear reactions" - again.

2012-01-16T08:30:00.000-06:00

A person at NASA's Langley Research Center appears in a video touting the great benefits that are going to come with the realization of "low energy nuclear reactions", which is a phrase that is meant to be a bit more general (and a bit less tainted) than "cold fusion". Let me take care of the preliminaries right away:

The experimental evidence for any of this stuff remains dodgy at best. I've explained what most scientists would consider a threshold for reproducibility of a real phenomenon, and this just isn't there. There's always a "secret sauce" or very particular and idiosyncratic surface treatment; there are equivocal claims about the presence or absence of fusion products and radiation; etc. (This is the point where a true believer will show up and point out the many documents indexed here, and castigate me for not being sufficiently open-minded. Let's just take that as read.)
For this to be correct, much of our knowledge of nuclear processes would have to be in severe need of correction, despite the fact that it works pretty darn well for things like nuclear reactors and the description of how the sun works.
Just because someone at NASA likes this, or because Brian Josephson likes it, doesn't mean it's automatically real.
Despite claims to the contrary, physicists would love it if something like this turned out to be true - look at the reaction of most physicists to the superluminal neutrino business. It'd be the story of the century. There is not some giant conspiracy of The Establishment trying to suppress this. Again, look at the neutrino situation: everyone agrees that such an extraordinary claim requires extraordinary evidence, presented for public scrutiny in detail.

That being out of the way, I want to comment briefly on the supposed explanation implied by the NASA video, "Method for Enhancement of Surface Plasmon Polaritons to Initiate and Sustain LENR". The proposed explanation, related to "Widom-Larsen theory", is related a bit to muon-catalyzed fusion. The muon is a cousin of the electron, but 200 times heavier. The muon can replace an electron in, e.g., a deuterium molecule, causing the two nuclei to be considerably closer to each other, and enhancing the rate of fusion. Widom and Larsen propose that some collective coupling between nuclei and collective electronic excitations (plasmons) results in electrons with large effective masses, and that this effective mass enhancement allows "heavy" electrons to catalyze fusion reactions. This is exceedingly unlikely to be correct, because (to paraphrase Morbo from Futurama), "Effective mass does not work that way!". At the end of the day, while there are collective excitations of many electrons that act, at condensed matter energy scales, like they are heavy (meaning that their energy increases more slowly as a function of their (crystal) momentum than for a free electron), (1) individual electrons are what participate in things like inverse beta decay, and (2) only a small fraction of the total number of electrons in a metal participate in these "heavy" excitations.

Again, I'd love it if this were real. Show me reproducibility that does not require prior belief to buy, and then we can talk.

From around the web

2012-01-11T21:27:00.000-06:00

While working on several writing projects simultaneously, I've run across some interesting articles and links.

Here is an interesting discussion about whether our ordinary metrics are doing a good job at measuring scientific impact (and therefore encouraging the kinds of collaborative behaviors that tend to advance science). One tricky bit not really addressed here is the challenge of distinguishing when a 12 author paper really involves excellent collaborative work, with everyone contributing to a scientific advance; and when a 12 author paper really represents the work of about 3 people, with others included for contributions (intellectual, financial, or political) of varying small degrees.
This is a (slightly ad-laden) compilation of many online lectures related to condensed matter physics.
Likewise, here are a series of continuing education lectures by Lenny Susskind (who taught me graduate stat mech) on statistical mechanics, and another series on quantum mechanics. I find it very interesting that these are so clearly organized - he must've put a lot of time into planning them.
Here is Phillip Gibbs with a great article about why c stands for the speed of light. I'll admit, I was one of those people he mentions that had read (and naively believed) Asimov's assertion that c stood for the Latin celeritas, meaning "speed". Guess I need to reconsider!
More evidence that Elsevier is just evil. Through lobbyists, they're trying to kill public access to data from publicly funded research if that research has been published in a journal of a for-profit publisher.
And for fun, here is a place (not the only one, I'm sure) that sells serious computer keyboards - the kind with real clicky metal leaf springs and solid metal backplanes. I got one of these a couple of years ago and love it. It reminds me of the best keyboard I've ever used, from an old HP 9000 workstation back in my beginning grad school days.

TOEFL scores

2012-01-04T21:36:00.005-06:00

This is my first attempt at using a blogging app for the iPad. Let's see how it goes....

Over the last few weeks, I've received several emails from foreign students who are would-be applicants to Rice graduate programs, asking me whether I'll be looking for students next year. In these same emails, the students point out that their TOEFL scores fall below Rice's official cutoff of 90, and ask if they can get in anyway. For some I know that cutoffs like this seem unfair - that only physics ability should matter in terms of getting into a grad program. However, we don't set these things just to be arbitrary. Historically, students who cannot meet that language test criterion have a very hard time - they can't generally be put in front of undergrads to teach, they have difficulty in communicating with their instructors, and often the language barrier is sufficiently severe that there is a tendency to hang out with other students who speak their native language rather than to speak English (a situation that can prolong rather than address the issue). I have enormous respect for someone motivated and bright enough to go abroad to a foreign country for grad school in a non-native language - I couldn't have done it - but the language rules are there for rational reasons.

Underappreciated papers (not yours)

2012-01-02T20:57:00.000-06:00

While doing research, scientists and engineers read (at various levels of depth) many papers. Every now and then, you come across one that is really great, yet somehow doesn't seem to have received the attention or appreciation it deserves. I'll pick one here, and hopefully some readers will put their examples in the comments.

One that I like a lot is this paper from Wilson Ho's group at UC Irvine. Here the authors use a scanning tunneling microscope, and demonstrate that when the tunneling current-voltage characteristic, they get rectification of microwaves. That is, when microwaves are applied to the tip-sample junction, the result is a dc current proportional to the square of the microwave amplitude and to the nonlinearity (second derivative of I with respect to V) of the tunnel junction. It's a clean, elegant experiment, with quantitatively accurate comparison of experiment and a simple classical theory - very very nice, and really underappreciated in my view.

Any suggestions of others?

Tidbits.

2011-12-30T08:18:00.001-06:00

First, I have a guest post on the Houston Chronicle's science blog today. Thanks for the opportunity, Eric.

Second, here is a great example of science popularization from the BBC. We should do things like this on US television, instead of having Discovery Channel and TLC show garbage about "alien astronauts" and "ghost hunting".

Third, if you see the latest Sherlock Holmes flick, keep an eye out for subtle details about Prof. Moriarty - there's some fun math/physics stuff hidden in there (pdf) for real devotees of the Holmes canon.

Shifting gears

2011-12-28T18:42:00.000-06:00

One of the most appealing aspects of a career in academic science and engineering is the freedom to choose your area of research. This freedom is extremely rare in an industrial setting, and becoming more so all the time. Taking myself as an example, I was hired as an experimental condensed matter physicist, presumably because my department felt that this was a fruitful area in which they would like to expand and in which they had teaching needs. During the application and interview process, I had to submit a "research plan" document, meant to give the department a sense of what I planned to do. However, as long as I was able to produce good science and bring in sufficient funding to finance that research, the department really had no say-so at all about what I did - no one read my proposals before they went out the door (unless I wanted proposal-writing advice), no one told me what to do scientifically. You would be very hard-pressed to find an industrial setting with that much freedom.

So, how does a scientist or engineer with this much freedom determine what to do and how to allocate intellectual resources? I can only speak for myself, but it would be interesting to hear from others in the comments. I look for problems where (a) I think there are scientific questions that need to be answered, ideally tied to deeper issues that interest me; (b) my background, skill set, or point of view give me what I perceive to be either a competitive advantage or a unique angle on the problem; and (c) there is some credible path for funding. I suspect this is typical, with people weighting these factors variously. Certainly those who run giant "supergroups" in chemistry and materials science by necessity have more of a "That's where the money is" attitude; however, I don't personally know anyone who works in an area in which they have zero intellectual interest just because it's well funded. Getting resources is hard work, and you can't do it effectively if your heart's not in it.

A related question is, when and how do you shift topics? These days, it's increasingly rare to find a person in academic science who picks a narrow specialty and sits there for decades. Research problems actually get solved. Fields evolve. There are competing factors, though, particularly for experimentalists. Once you become invested in a given area (say scanned probe microscopy), this results in a lot of inertia - new tools are expensive and hard to get. It can also be difficult to get into the mainstream of a new topic from the outside, in terms of grants and papers. Jumping on the latest bandwagon is not necessarily the best path to success. On the other hand, remaining in a small niche isn't healthy. All of these are "first-world problems", of course - for someone in research, it's far better to be wrestling with these challenges than the alternative.

students and their mental health

2011-12-17T17:13:00.000-06:00

There was an interesting article earlier this week in the Wall Street Journal, on mental health concerns in college students. It's no secret that mental illness often has an onset in the late teens and early twenties. It's also not a surprise that there are significant stressors associated with college (or graduate school), including being in a new environment w/ a different (possibly much smaller) social support structure, the pressure to succeed academically, the need to budget time much more self-sufficiently than at previous stages of life, and simple things like lack of sleep. As a result, sometimes as a faculty member you come across students who have real problems.

In undergrads, often these issues manifest as persistent erratic or academically self-destructive behavior (failure to hand in assignments, failure to show up for exams). Different faculty members have various ways to deal with this. One approach is to be hands-off - from the privacy and social boundaries perspective, it's challenging to inquire about these behaviors (is a student just having a tough time in college or in a particular class, or is a student afflicted with a debilitating mental health issue, or are is the student somewhere on the continuum in between). The sink-or-swim attitude doesn't really sit well with me, but it's always a challenge to figure out the best way to handle this stuff.

In grad students, these issues can become even more critical - students are older, expectations of self-sufficiency are much higher, and the interactions between faculty and students are somewhere between teacher/student, boss/employee, and collaborator/collaborator. The most important thing, of course, is to ensure that at the end of the day the student is healthy, regardless of degree progress. If the right answer is that a student should take time off or drop out of a program for treatment or convalescence, then that's what has to happen. Of course, it's never that simple, for the student, for the advisor, for the university.

Anyway, I suggest reading the WSJ article if you have access. It's quite thought-provoking.

Universality and "glassy" physics

2011-12-16T20:32:00.000-06:00

One remarkable aspect of Nature is the recurrence of certain mathematically interesting motifs in different contexts. When we see a certain property or relationship that shows up again and again, we tend to call that "universality", and we look for underlying physical reasons to explain its reappearance in many apparently disparate contexts. A great review of one such type of physics was posted on the arxiv the other day.

Physicists commonly talk about highly ordered, idealized systems (like infinite, perfectly periodic crystals), because often such regularity is comparatively simple to describe mathematically. The energy of such a crystal is nicely minimized by the regular arrangement of atoms. At the other extreme are very strongly disordered systems. These disordered systems are often called "glassy" because structural glasses (like the stuff in your display) are an example. In these systems, disorder dominates completely; the "landscape" of energy as a function of configuration is a big mess, with many local minima - a whole statistical distribution of possible configurations, with a whole distribution of energy "barriers" between them. Systems like that crop up all the time in different contexts, and yet share some amazingly universal properties. One of the most dramatic is that when disturbed, these systems take an exceedingly long time to respond completely. Some parts of the system respond fast, others more slowly, and when you add them all together, you get total responses that look logarithmic in time (not exponential, which would indicate a single timescale for relaxation). For example, the deformation response of crumpled paper (!) shows a relaxation that is described by constant*log(t) for more than 6 decades in time! Likewise, the speed of sound or dielectric response in a glass at very low temperatures also shows logarithmic decays. This review gives a great discussion of this - I highly recommend it (even though the papers they cite from my PhD advisor's lab came after I left :-) ).

Higgs or no

2011-12-12T22:04:00.001-06:00

The answer is going to be, to quote the Magic 8-Ball, "Ask again later." Sounds like the folks at CERN are on track to make a more definitive statement about the Higgs boson in about one more Friedman Unit. That won't stop an enormous surge of media attention tomorrow, as CERN tries very hard to have their cake and eat it, too ("We've found [evidence consistent with] the God Particle! At least, it's [evidence not inconsistent with] the God Particle!"). What this exercise will really demonstrate is that many news media figures are statistically illiterate.

I should point out that, with the rumors of a statistically not yet huge bump in the data near 125 GeV, there has suddenly been an uptick in predictions of Higgs bosons with just that mass. How convenient.

Update - Interesting. For the best write-up I've seen about this, check out Prof. Matt Strassler. Seems like the central question is, are the two detectors both seeing something in the same place, or not? That is, is 123-ish GeV the same as 126-ish GeV? Tune in next year, same Stat-time, same Stat-channel! (lame joke for fans of 1960s US TV....)

Nano book recommendation

2011-12-10T13:12:00.000-06:00

My colleague in Rice's history department, Cyrus Mody, has a new book out called Instrumental Community, about the invention and spread of scanned probe microscopy (and microscopists) that's a very interesting read. If you've ever wondered how and why the scanning tunneling microscope and atomic force microscope took off, and why related ideas like the topografiner (pdf) did not, this is the book for you. It also does a great job of giving a sense of the personalities and work environments at places like IBM Zurich, IBM TJ Watson, IBM Almaden, and Bell Labs.

There are a couple of surprising quotes in there. Stan Williams, these days at HP Labs, says that the environment at Bell Labs was so cut-throat that people would sabotage each others' experiments and steal each others' data. Having been a postdoc there, that surprised me greatly, and doesn't gibe with my impressions or stories I'd heard. Any Bell Labs alumni readers out there care to comment?

The book really drives home what has been lost with the drastic decline of long-term industrial R&D in the US. You can see it all happening in slow motion - the constant struggle to explain why these research efforts are not a waste of shareholder resources, as companies become ever more focused on short term profits and stock prices.

Priorities

2011-12-02T08:17:00.001-06:00

My colleagues at Texas A&M University must be so happy to hear that in these troubled economic times, their university is rumored to be offering the current University of Houston football coach a $4M/yr salary to come to College Station. I like college sports as much as the next person, but what does it say about higher education in the US that a public university, dealing with tight budgets, thinks that this is smart?

Antennas for light + ionics at the nanoscale

2011-11-30T20:47:00.000-06:00

A (revised) particularly excellent review article was posted on the arxiv the other day, about metal nanostructures as antennas for light. This seems to be an extremely complete and at the same time reasonably pedagogical treatment of the subject. While in some sense there are no shocking surprises (the basic physics underlying all of this is, after all, Maxwell's equations with complicated boundary conditions and dielectric functions for the metal), there are some great ideas and motifs: the importance of the optical "near field"; the emergence of plasmons, the collective modes of the electrons, which are relevant at the nanoscale but not in macroscopic antennas for, e.g., radio frequencies; the use of such antennas in real quantum optics applications. Great stuff.

I also feel the need for a little bit of shameless self-promotion. My colleague http://physics.ucsd.edu/~diventra/ and I have an article appearing in this month's MRS Bulletin, talking about the importance of ion motion and electrochemistry in nanoscale structures. (Sorry about not having a version on the arxiv at this time. Email me if you'd like a copy.) This article was prompted in part by a growing realization among a number of researchers that the consequences of the motion of ions (often neglected at first glance!) are apparent in a number of nanoscale systems. Working at the nanoscale, it's possible to establish very large electric fields and concentration/chemical potential gradients that can drive diffusion. At the same time, there are large accessible surface areas, and inherently small system dimensions mean that diffusion over physically relevant distances is easier than in macroscale materials. While ionic motion can be an annoyance or an unintended complication, there are likely situations where it can be embraced and engineered for useful applications.

Nano"machines" and dissipation

2011-11-26T12:56:00.000-06:00

There's an article (subscription only, unfortunately) out that has gotten some attention, discussing whether artificial molecular machines will "deliver on their promise". The groups that wrote the article have an extensive track record in synthesizing and characterizing molecules that can undergo directed "mechanical" motion (e.g., translation of a rod-like portion through a ring) under chemical stimuli (e.g., changes in temperature, pH, redox reactions, optical excitation). There is no question that this is some pretty cool stuff, and the chemistry here (both synthetic organic, and physical) is quite sophisticated.

Two points strike me, though. First, the "promise" mentioned in the title is connected, particularly in the press writeup, with Drexlerian nanoassembler visions. Synthetic molecules that can move are impressive, but they are far, far away from the idea of actually constructing arbitrary designer materials one atom at a time (a goal that is likely impossible, in my opinion, for reasons stated convincingly here, among others). They are, however, a possible step on the road to designer, synthetic enzymes, a neat idea.

Second, the writeup particularly mentions how "efficient" the mechanical motions of these molecules are. That is, there is comparatively little dissipation relative to macroscopic machines. This is actually not very surprising, if you think about the microscopic picture of what we think of as macroscopic irreversibility. "Loss" of mechanical energy takes place because energy is transferred from macroscopic degrees of freedom (the motion of a piston) to microscopic degrees of freedom (the near-continuum of vibrational and electronic modes in the metal in the piston and cylinder walls). When the whole system of interest is microscopic, there just aren't many places for the energy to go. This is an example of the finite-phase-space aspect that shows up all the time in truly nanoscale systems.

Superluminal neutrinos - follow-up

2011-11-17T20:45:00.001-06:00

The OPERA collaboration, or at least a large subset of it, has a revised preprint out (and apparently submitted somewhere), with more data on their time-of-flight studies of neutrinos produced at CERN. Tomasso has a nice write-up here. Their previous preprint created quite a stir, since it purported to show evidence of neutrino motion faster than c, the speed of light in vacuum. The general reaction among physicists was, that's really weird, and it's exceedingly likely that something is wrong somewhere in the analysis. One complaint that came up repeatedly was that the pulses used by the group were about 10000 nanoseconds long, and the group was arguing about timing at the 60 ns level. You could readily imagine some issues with their statistics or the functioning of the detector that could be a problem here, since the pulses were so long compared to the effect being reported. To deal with this, the group has now been running for a while with much shorter pulses (a few ns in duration). While they don't have nearly as much data so far (in only a few weeks of running), they do have enough to do some analysis, and so far the results are completely consistent with their earlier report. Funky. Clearly pulse duration systematics or statistics aren't the source of the apparent superluminality, then. So, either neutrinos really are superluminal (still bloody unlikely for a host of reasons), or there is still some weird systematic error in the detector somewhere. (For what it's worth, I'm sure they've looked a million ways at the clock synchronization, etc. now, so that's not likely to be the problem either.)

Update: Matt Strassler has an excellent summary of the situation.

So you want to compete w/ fossil fuels (or silicon)

2011-11-17T14:07:00.000-06:00

Yesterday I went to an interesting talk here by Eric Toone, deputy director of ARPA-E, what is supposed to be the blue-sky high-risk/high-reward development portion of the US Department of Energy. He summarized some basic messages about energy globally and in the US, gave quite a number of examples of projects funded by ARPA-E, and had a series of take-home messages. He also gave the most concise (single-graph) explanation for the failure of Solyndra: they bet on a technology based on CIGS solar cells, and then the price of silicon (an essential component of the main competing technology) fell by 80% over a few months. It was made very clear that ARPA-E aims at a particular stage in the tech transfer process, when the basic science is known, and a technology is right at the edge of development.

The general energy picture was its usual fairly depressing self. There are plenty of fossil fuels (particularly natural gas and coal), but if you think that CO₂ is a concern, then using those blindly is risky. Capital costs make nuclear comparatively uncompetitive (to say nothing of political difficulties following Fukushima). Solar is too expensive to compete w/ fossil fuels. Other renewables are also too expensive and/or not scalable. Biomass is too expensive. Batteries don't come remotely close to competing with, e.g., gasoline in terms of energy density and effective refueling times.

The one thing that really struck me was the similarity of the replacing-fossil-fuels challenge and the replacing-silicon-electronics challenge. Fossil fuels have problems, but they're sooooooo cheap. Likewise, there is a great desire to prolong Moore's law by eventually replacing Si, but Si devices are sooooooo cheap that there's an incredible economic barrier to surmount. When you're competing against a transistor that costs less than a millionth of a cent and has a one-per-billion failure rate over ten years, your non-Si gizmo better be really darn special if you want anyone to take it seriously....

Bad Astronomy day at Rice

2011-11-14T21:10:00.000-06:00

Today we hosted Phil Plait for our annual Rorschach Lecture (see here), a series in honor of Bud Rorschach dedicated to public outreach and science policy. He kept us fully entertained with his Death from the Skies! talk, with a particularly amusing litany of (a small subset of) the scientific flaws in "Armageddon". There was a full house in our big lecture hall - there's no question that astro has very broad popular appeal (though it did bring out the "Obama should be impeached immediately because he's not protecting us from possible asteroid impacts!" crowd).

Teaching - Coleman vs. Feynman

2011-11-06T14:46:00.000-06:00

As pointed out by Peter Woit, Steve Hsu recently posted a link to an interview with (the late) Sidney Coleman, generally viewed as one of the premier theoretical physicists of his generation. Ironically, for someone known as an excellent lecturer, Coleman apparently hated teaching, likening it to "washing dishes" or "waxing floors" - two activities he could do well, from which he derived a small amount of "job well done" satisfaction, but which he would never choose to do voluntarily.

It's fun to contrast this with the view of Richard Feynman, as he put it in Surely You Must Be Joking, Mr. Feynman:

I don't believe I can really do without teaching. The reason is, I have to have something so that when I don't have any ideas and I'm not getting anywhere I can say to myself, "At least I'm living; at least I'm doing something; I am making some contribution" -- it's just psychological.... The questions of the students are often the source of new research. They often ask profound questions that I've thought about at times and then given up on, so to speak, for a while. It wouldn't do me any harm to think about them again and see if I can go any further now. The students may not be able to see the thing I want to answer, or the subtleties I want to think about, but they remind me of a problem by asking questions in the neighborhood of that problem. It's not so easy to remind yourself of these things. So I find that teaching and the students keep life going, and I would never accept any position in which somebody has invented a happy situation for me where I don't have to teach. Never.

I definitely lean toward the Feynman attitude. Teaching - explaining science to others - is fun, important, and helpful to my own work. Perhaps Coleman was simply so powerful in terms of creativity in research that teaching always seemed like an annoying distraction. In these days when there are so many expectations on faculty members beyond teaching, I hope we're not culturally rewarding a drift toward the Coleman position.

Science - what is it up to?

2011-11-01T19:36:00.003-05:00

Hat tip to Phil Plait, the Bad Astronomer, for linking to this video from The Daily Show. My apologies to non-US readers who won't be able to watch this. It's a special report from Asif Mandvi, complete with remarks from a Republican "strategist" / Fox News talking head, who explains how science is inherently corrupt, because only scientists are really qualified to review the work of scientists. Seriously, she really makes that argument, and more.

Update: I've decided to ditch the embedded video. Here's a link to the video on the Daily Show's site, and here's a link that works internationally.

Faculty search process, 2011 version.

2011-10-26T06:38:00.000-05:00

As I have done in past years, I'm revising a past post of mine about the faculty search process. My thoughts on this really haven't changed much, but it's useful to throw this out there rather than hope people see it via google.

Here are the steps in the typical faculty search process:

The search gets authorized. This is a big step - it determines what the position is, exactly: junior vs. junior or senior; a new faculty line vs. a replacement vs. a bridging position (i.e. we'll hire now, and when X retires in three years, we won't look for a replacement then). The main challenges are two-fold: (1) Ideally the department has some strategic plan in place to determine the area that they'd like to fill. Note that not all departments do this - occasionally you'll see a very general ad out there that basically says, "ABC University Dept. of Physics is authorized to search for a tenure-track position in, umm, physics. We want to hire the smartest person that we can, regardless of subject area." The danger with this is that there may actually be divisions within the department about where the position should go, and these divisions can play out in a process where different factions within the department veto each other. This is pretty rare, but not unheard of. (2) The university needs to have the resources in place to make a hire. In tight financial times, this can become more challenging. I know anecdotally of public universities having to cancel searches in 2008/2009 even after the authorization if the budget cuts get too severe. A well-run university will be able to make these judgments with some leadtime and not have to back-track.

The search committee gets put together. In my dept., the chair asks people to serve. If the search is in condensed matter, for example, there will be several condensed matter people on the committee, as well as representation from the other major groups in the department, and one knowledgeable person from outside the department (in chemistry or ECE, for example). The chairperson or chairpeople of the committee meet with the committee or at least those in the focus area, and come up with draft text for the ad. In cross-departmental searches (sometimes there will be a search in an interdisciplinary area like "energy"), a dean would likely put together the committee.

The ad gets placed, and canvassing begins of lots of people who might know promising candidates. A special effort is made to make sure that all qualified women and underrepresented minority candidates know about the position and are asked to apply (the APS has mailing lists to help with this, and direct recommendations are always appreciated - this is in the search plan). Generally, the ad really does list what the department is interested in. It's a huge waste of everyone's time to have an ad that draws a large number of inappropriate (i.e. don't fit the dept.'s needs) applicants. The exception to this is the generic ad like the type I mentioned above. Historically MIT and Berkeley had run the same ad every year, trolling for talent. They seem to do just fine. The other exception is when a university already knows who they want to get for a senior position, and writes an ad so narrow that only one person is really qualified. I've never seen this personally, but I've heard anecdotes.

In the meantime, a search plan is formulated and approved by the dean. The plan details how the search will work, what the timeline is, etc. This plan is largely a checklist to make sure that we follow all the right procedures and don't screw anything up. It also brings to the fore the importance of "beating the bushes" - see above. A couple of people on the search committee will be particularly in charge of oversight on affirmative action/equal opportunity issues.

The dean usually meets with the committee and we go over the plan, including a refresher for everyone on what is or is not appropriate for discussion in an interview (for an obvious example, you can't ask about someone's religion, or their marital status).

Applications come in and are sorted; rec letters are collated. Each candidate has a folder. Every year when I post this, someone argues that it's ridiculous to make references write letters, and that the committee should do a sort first and ask for letters later. I understand this perspective, but I largely disagree. Letters can contain an enormous amount of information, and sometimes it is possible to identify outstanding candidates due to input from the letters that might otherwise be missed. (For example, suppose someone's got an incredible piece of postdoctoral work about to come out that hasn't been published yet. It carries more weight for letters to highlight this, since the candidate isn't exactly unbiased about their own forthcoming publications.) There is a trend toward electronic application review, and that is likely to continue, though it can be complicated if committee members are not very tech-savvy.

The committee begins to review the applications. Generally the members of the committee who are from the target discipline do a first pass, to at least wean out the inevitable applications from people who are not qualified according to the ad (i.e. no PhD; senior people wanting a senior position even though the ad is explicitly for a junior slot; people with research interests or expertise in the wrong area). Applications are roughly rated by everyone into a top, middle, and bottom category. Each committee member comes up with their own ratings, so there is naturally some variability from person to person. Some people are "harsh graders". Some value high impact publications more than numbers of papers. Others place more of an emphasis on the research plan, the teaching statement, or the rec letters. Yes, people do value the teaching statement - we wouldn't waste everyone's time with it if we didn't care. Interestingly, often (not always) the people who are the strongest researchers also have very good ideas and actually care about teaching. This shouldn't be that surprising. Creative people can want to express their creativity in the classroom as well as the lab.

Once all the folders have been reviewed and rated, a relatively short list (say 20-25 or so out of 120 applications) is formed, and the committee meets to hash that down to, in the end, four or five to invite for interviews. In my experience, this happens by consensus, with the target discipline members having a bit more sway in practice since they know the area and can appreciate subtleties - the feasibility and originality of the proposed research, the calibration of the letter writers (are they first-rate folks? Do they always claim every candidate is the best postdoc they've ever seen?). I'm not kidding about consensus; I can't recall a case where there really was a big, hard argument within the committee. I know I've been lucky in this respect, and that other institutions can be much more fiesty. The best, meaning most useful, letters, by the way, are the ones who say things like "This candidate is very much like CCC and DDD were at this stage in their careers." Real comparisons like that are much more helpful than "The candidate is bright, creative, and a good communicator." Regarding research plans, the best ones (for me, anyway) give a good sense of near-term plans, medium-term ideas, and the long-term big picture, all while being relatively brief and written so that a general committee member can understand much of it (why the work is important, what is new) without being an expert in the target field. It's also good to know that, at least at my university, if we come across an applicant that doesn't really fit our needs, but meshes well with an open search in another department, we send over the file. This, like the consensus stuff above, is a benefit of good, nonpathological communication within the department and between departments.

That's pretty much it up to the interview stage. No big secrets. No automated ranking schemes based exclusively on h numbers or citation counts.

Tips for candidates:

Don't wrap your self-worth up in this any more than is unavoidable. It's a game of small numbers, and who gets interviewed where can easily be dominated by factors extrinsic to the candidates - what a department's pressing needs are, what the demographics of a subdiscipline are like, etc. Every candidate takes job searches personally to some degree because of our culture and human nature, but don't feel like this is some evaluation of you as a human being.
Don't automatically limit your job search because of geography unless you have some overwhelming personal reasons. I almost didn't apply to Rice because neither my wife nor I were particularly thrilled about Texas, despite the fact that neither of us had ever actually visited the place. Limiting my search that way would've been a really poor decision - I've now been here 12 years, and we've enjoyed ourselves (my occasional Texas-centric blog posts aside).
Really read the ads carefully and make sure that you don't leave anything out. If a place asks for a teaching statement, put some real thought into what you say - they want to see that you have actually given this some thought, or they wouldn't have asked for it.
Research statements are challenging because you need to appeal to both the specialists on the committee and the people who are way outside your area. My own research statement back in the day was around three pages. If you want to write a lot more, I recommend having a brief (2-3 page) summary at the beginning followed by more details for the specialists. It's good to identify near-term, mid-range, and long-term goals - you need to think about those timescales anyway. Don't get bogged down in specific technique details unless they're essential. You need committee members to come away from the proposal knowing "These are the Scientific Questions I'm trying to answer", not just "These are the kinds of techniques I know". I know that some people may think that research statements are more of an issue for experimentalists, since the statements indicate a lot about lab and equipment needs. Believe me - research statements are important for all candidates. Committee members need to know where you're coming from and what you want to do - what kinds of problems interest you and why. The committee also wants to see that you actually plan ahead. These days it's extremely hard to be successful in academia by "winging it" in terms of your research program.
Be realistic about what undergrads, grad students, and postdocs are each capable of doing. If you're applying for a job at a four-year college, don't propose to do work that would require an experienced grad student putting in 60 hours a week.
Even if they don't ask for it, you need to think about what resources you'll need to accomplish your research goals. This includes equipment for your lab as well as space and shared facilities. Talk to colleagues and get a sense of what the going rate is for start-up in your area. Remember that four-year colleges do not have the resources of major research universities. Start-up packages at a four-year college are likely to be 1/4 of what they would be at a big research school (though there are occasional exceptions). Don't shave pennies - this is the one prime chance you get to ask for stuff! On the other hand, don't make unreasonable requests. No one is going to give a junior person a start-up package comparable to a mid-career scientist.
Pick letter-writers intelligently. Actually check with them that they're willing to write you a nice letter - it's polite and it's common sense. (I should point out that truly negative letters are very rare.) Beyond the obvious two (thesis advisor, postdoctoral mentor), it can sometimes be tough finding an additional person who can really say something about your research or teaching abilities. Sometimes you can ask those two for advice about this. Make sure your letter-writers know the deadlines and the addresses. The more you can do to make life easier for your letter writers, the better.

As always, more feedback in the comments is appreciated.

Science, communication, and the public

2011-10-19T22:52:00.000-05:00

This week's issue of Nature includes an interesting editorial emphasizing how crucial it is that scientists and engineers learn how to communicate their value to the general populace. This is something I've thought about for quite some time, as have a number of other people - see this article in Physics Today (subscription only, I'm afraid), this related blog post, and a discussion in the Houston Chronicle's science blog.

It's hard not to get down about this whole topic. Industrial R&D funding (for projects with more than a year lead time) is a shadow of what it used to be, and looming fiscal austerity may well cripple federally funded basic research. If companies aren't willing to invest for the long term, and government is unable or unwilling to invest for the long term, then technological innovation may shift away from the US. If more of the general public and politicians appreciated that things like the iPad, XBox, the internet, and flat screen TVs didn't come out of nowhere, maybe the situation would be different.

By the way, I find it interesting that the Nature editorial discusses looming cuts to Texas physics departments, a topic I mentioned here and was discussed in the New York Times, and yet our own Houston Chronicle hasn't bothered to write about them. At all. Even on their online science blog. Yes, they're aware of the topic, too. Clearly they've had more newsworthy things to worry about.

A few fun links.

2011-10-17T21:00:00.001-05:00

I'm buried under a couple of pieces of work right now, but I did want to share a couple of fun science videos.

Here is a great example of magnetic levitation via superconductivity.

Those Mythbusters guys had a great time trying to make a giant Newton's Cradle using wrecking balls. It didn't work well (and I assigned a homework problem looking at why this was the case).

I just heard yesterday that there's a full-length version of the theme song to the Big Bang Theory. Pretty educational, though the lyrics imply that there'll be a Big Crunch, and we now know that's unlikely (see this year's Nobel in physics).

Here is a cool collection of videos, from minutephysics. Good stuff!

What's wrong with modern American economics.

2011-10-11T22:06:00.000-05:00

According to this, Google stock may take a hit because their revenues only grew year-over-year by 30% this past quarter. Specifically, analysts are worried because Larry Page said that he cares more about the long term health of the company than goosing the stock price.

What the hell is wrong with these people? It's not enough that Google is making enormous profits. It's not enough that Google's enormous revenues are 30% larger than they were last year. Rather, apparently the free market will penalize Google because they expected the earnings to be 32% larger than last year, and it's apparently a bad thing that the management has talked about prioritizing long-term health and growth. How is this attitude by the financial sector at all a good thing? This attitude is exactly why corporate long-term R&D has been nearly obliterated in the US.

Quasicrystals

2011-10-10T09:01:00.000-05:00

I was going to do a post about quasicrystals and this year's chemistry Nobel, but Don Monroe has done such a good job in his Phys Rev Focus piece that there's not much more to say. Read it!

The big conceptual change brought about by the discovery of quasicrystals was not so much the observation of five-fold and icosahedral symmetries via diffraction. That was certainly surprising, since you can't tile a plane with pentagons; it was very hard to understand how you could end up with a periodic arrangement of atoms that could fill space and give diffraction patterns with those symmetries. The real conceptual shift was realizing that it is possible to have nice, sharp diffraction patterns from nonperiodic (rather, quasiperiodic) arrangements of atoms. The usual arguments about diffraction that are taught in undergrad classes emphasize that diffraction (of electrons or x-rays or neutrons) is very strong (giving 'spots') in particular directions because along those directions, the waves scattered by subsequent planes of atoms all interfere constructively. Changing the direction leads to crests and troughs of waves adding with some complicated phase relationship, generally averaging to not much intensity. In particular symmetry directions, though, the waves scattered by successive planes of atoms arrive in phase, as the distances traveled by the various scattered contributions all differ by integer numbers of wavelengths. Without a periodic arrangement of atoms, it was hard to see how this could happen nicely.

It turns out that quasicrystals really do have a hidden sort of symmetry. They are projections onto three dimensions of structures that would be periodic in a higher dimensional space. The periodicity isn't there in the 3d projection (rather, the atoms are arranged "quasiperiodically" in space), but the 3d projection does contain information about the higher dimensional symmetry, and this comes out when diffraction is done in certain directions. The discovery of these materials spurred scientists had to reevaluate their ideas about what crystallinity really means - that's why it's important. For what it's worth, the best description of this that I've seen in a textbook is in Taylor and Heinonen.

A modest proposal for Google, Intel, or the like.

2011-10-06T20:45:00.000-05:00

A post on quasicrystals will be coming eventually....

Suppose you're an extremely successful tech company, and you want to make a real, significant impact on university research for the long term, because you realize that you need an educated, technically sophisticated workforce. Rather than endowing individual professorships, or setting up one or two research centers, I have a suggestion. Take $250M, and set up research equipment endowments at, say, the 50 top research universities. Give each one $5M, with the proviso that the endowment returns be used for the purchase or maintenance of research equipment, and/or technical staff salary lines, as the institution sees fit. That could buy one good-sized piece of equipment per year, or pay for several technical staff. This would be a way for universities to replenish their research infrastructure over time without being dependent on federal equipment grants (which are undoubtedly useful, but tend to favor the exotic over the essential, and are likely to become increasingly scarce as fiscal austerity takes over for the foreseeable future). Universities could also charge depreciation on that equipment when assessing user fees, making the whole system self-sustaining even beyond endowment returns. Alternately, critical staff lines could be supported. Anyone at a research university knows that a good technical staff member can completely reshape the way facilities (e.g., a cleanroom; a mass spec center) operate. You put all the decision making on the university, with the proviso that they can't spend down the principal. This strategy would boost research productivity across the country over time, get more and better equipment into the hands of future tech workers, and be a charitable write-off for the company that does it. It could really make a difference.

I'm completely serious about this, and would be happy to talk to any corporations (or foundations) about how this might work.

Nobel speculation time again

2011-10-02T20:39:00.001-05:00

It's that time of year again - time to speculate about the Nobel Prizes. Physics gets announced Tuesday, followed by Chemistry the next day. While I feel almost obligated to mention my standard speculation (Aharonov and Berry for geometrical phases), it seems likely that this year's physics prize will be astro-themed, since there hasn't been one of those in a while. Something related to dark matter perhaps (Vera Rubin for galaxy rotation curves?), though direct detection of dark matter may be a necessary precursor for that. Inflationary cosmology gets mentioned (Guth and Linde?) by some. Extrasolar planets? Fine scale structure in the cosmic microwave background as a constraint on what the universe is made of? There certainly has been a lot of astro excitement in the last few years.... Feel free to speculate in the comments.

Update: The 2011 Nobel in Physics has been awarded to Saul Perlmutter, Brian Schmidt, and Adam Riess, for their discovery (via observations of type IA supernova) that not only is the universe expanding, but that expansion is (apparently) accelerating. Makes sense, in that this work certainly altered our whole view of the universe's fate. Combined with other observations (e.g., detailed measurements of the cosmic microwave background), it would now seem that the universe's total energy density is 4% ordinary matter, 23% dark matter (gravitates but otherwise interacts very weakly with the ordinary matter), and 73% "dark energy" (energy density associated with space itself). For a nice summary of the science, see here (pdf). Congratulations to all!

Superluminal neutrinos - a case study in how good science is done

2011-09-22T20:33:00.000-05:00

As many people have now heard, the OPERA collaboration is reporting a very surprising observation. The OPERA experiment is part of CERN, and is an experiment meant to study neutrino flavor oscillations. The idea is, the proton beam at CERN creates a beam of neutrinos. Since neutrinos hardly interact with normal matter, they move in a straight line right through the earth, and pass through the experimental station in Gran Sasso, Italy, where some small fraction of them are then detected. There are (according to the Standard Model) three flavors of neutrinos, the electron neutrino, muon neutrino, and tau neutrino. It has been determined experimentally that those flavors are not exact "mass eigenstates". That means that if you start off with a tau neutrino of particular energy, for example, and let it propagate for a while, it will change into a muon neutrino with some probability that oscillates in time. Anyway, OPERA wanted to study this phenomenon, and in doing so, they measured the time it takes neutrinos to go from their production point at CERN to the detector in Gran Sasso, using precisely synchronized special clocks. They also used differential GPS to measure the distance between the production point and the detector to within 20 cm. Dividing the distance by the time, they found much to their surprise that the neutrinos appear to traverse the distance about 60 ns faster than would be expected if they traveled at the speed of light in vacuum.

So, what could be going on here? There are a few possibilities. First, they could have the distance measurement wrong. This seems unlikely, given the use of differential GPS and the sensitivity (they could clearly see the change in the distance due to a 2009 earthquake, as shown in Fig. 7 of the paper). Second, they could have a problem in their synchronization of the clocks. That seems more likely to me, given that the procedure is comparatively complicated. Third, there is some other weird systematic at work that they haven't found. Fourth, neutrinos are actually tachyons. That would be all kinds of awesome, but given how challenging it would be to reconcile that with special relativity and causality, I'm not holding my breath.

Why is this an example of good science? The collaboration spent three years looking hard at their data, analyzing it many different ways, checking and cross-checking. They are keenly aware that a claim of FTL neutrinos would be the very definition of "extraordinary" in the scientific sense, and would therefore require extraordinary evidence. Unable to find the (highly likely) flaw in their analysis and data, they are showing everything publicly, and asking for more investigation. I want to point out, this is the diametric opposite of what happens in what I will term bad science (ahem. Italian ecat guys, I'm looking at you.). This is how real experimental science works - they're asking for independent reproduction or complementary investigation. I hope science journalists emphasize this aspect of the story, rather than massively sensationalizing it or portraying the scientists as fools if and when a flaw is found.

State of Texas threatens physics departments at smaller public universities

2011-09-15T20:45:00.003-05:00

This article is both sad and frustrating. The coordinating body of the Texas state government that runs the public universities in this state has recommended that a number of places shut down their physics departments. In particular, this affects two schools near Rice that are historically African American serving, Prairie View A and M and Texas Southern. (Unfortunately, the article doesn't have a link to the actual Texas Higher Education Coordinating Board recommendations, so I don't have any further information, like which other universities here may be affected.)

Depressingly updated: see NY Times story here.

I understand that financial times are tight for the state. (Look at the "Texas Miracle" in action as we slash the state's education budget.) The bit that really galls me is the rationale: enrollment in the upper division courses is small, so we should eliminate the whole department. This idea that somehow the only valuable and cost effective courses are those with large enrollment is ridiculous, and it seems to have infected the public university system in this state, driven by misguided, bean-counting thinktank types. If you follow this reasoning all the way, we should only have large service courses, and never have upper division, specialized courses in anything, and of course all of these should be taught by non-tenure-track, non-research-active instructors. That would surely cut costs. It would also be a disaster in the long run. As is stated in this article, if you used the same criteria in terms of size of upper division courses across the country, you'd end up shutting down 2/3 of the physics departments in the US, to say nothing of other disciplines. I can't imagine the situation is any better in, e.g., math, or chemical engineering, or any technical discipline. I'd also love to see numbers about how much collegiate athletics is net costing the state in public funds, vs. how much it costs to keep these programs going. Hint: most universities lose money on athletics.

I'd love to try to fix this, but given the politics here (hint: Rick Perry likes these policies, and his political party controls both houses of the state legislature), it's hard to see a workable path forward. It's not like this is going to be an honest debate about how to structure the state's higher education system (which we can and should have) - it's an ideological full-court press.

Think I'm exaggerating? The superintendent of the THECB, Raymund Paredes, is a close buddy of both Rick Perry and his pal Rick O'Connell, the guy who thinks that a bachelor's degree even in a technical field should be obtainable for $10000 total, period. You could do that, of course, but it would involve converting our colleges and universities essentially into community colleges or correspondence schools. I've yet to see any evidence that these guys have an appreciation for science or engineering at all. They want UT and TAMU to play good football, and they espouse populist rhetoric about wanting to cut costs, but they don't seem to want academic excellence at universities.

Lab habits + data management

2011-09-14T21:02:00.000-05:00

The reason I had been looking for that Sydney Harris cartoon is that I was putting together a guest lecture for our university's "Responsible Conduct of Research" course. I was speaking today about data management and retention, a topic I've come to know well over the last year through some university service work working on policies in that area. After speaking, it occurred to me that it's not a bad idea to summarize important points on this for the benefit of student readers of this blog. In brief:

Everything is data. Not just raw numbers or images, but also the final analyzed graphs, the software used to do the analysis, the descriptions of the instrument settings used to acquire the raw numbers - everything.
The data are the science. The data are the foundation for all the analysis, model-building, papers, arguments, further refinements, patents, etc. Protect the data!
If you didn't document it, you didn't do it.
Write down everything. Fill up notebooks. Annotate liberally, including false starts, what you were thinking when you set up the little sub-experiments or trials that go into any major research endeavor. I guarantee, you will never, ever in your life look back and say, "I regret that I was so thorough, and I wish I had written down less." After years of observation, I am convinced that good notebook skills genuinely reduce mean time to thesis completion in many cases. If you actually keep track of what you've been doing, and really write down your logic, you are less likely to go down blind alleys or have to repeat mistakes.
You may think that you own your data. You don't, technically. In an academic setting, the university has legal title to the data (that gives them the legal authority that they need to adjudicate disputes about access to data, including those that arise in the rare but unfortunate cases of research misconduct), while investigators are shepherds or custodians of the data. Both have their own responsibilities and rights. Some of those responsibilities are inherent in good science and engineering (e.g., the duty to do your best to make sure that the published results are accurate and correct, as much as possible), and others are imposed externally (e.g., federal funding agencies require preservation of data for some number of years beyond the end of an award).
Back everything up. In multiple ways. With the advent of scanners, digital cameras, cheap external hard drives, laptops, thumbdrives, "the cloud" (as long as it's better than this), etc., there is absolutely no excuse for not properly backing up data. To repeat, back everything up. No, seriously. Have a backup copy at an off-site location, as a sensible precaution against disaster (fire, hurricane, earthquake, zombie apocalypse).
Good habits are habits, and must be habituated. It took me more than 25 years to get in the habit of really flossing. Do yourself a favor, and get in the habit of properly caring for your data. Please.

Help finding a Syndey Harris cartoon

2011-09-12T15:44:00.000-05:00

I am trying to find a particular Syndey Harris physics cartoon, and google has let me down. The one I'm picturing has an obvious experimentalist at a workbench strewn with lab equipment. There's an angel on one shoulder, and a devil on the other. Anyone who has this cartoon, I'd be very grateful for a link to a scanned version! Thanks.

Single-molecule electric motor

2011-09-07T21:55:00.000-05:00

As a nano person, I feel like I'm practically obligated to comment on this paper, which has gotten a good deal of media attention. In this experiment, the authors have anchored a single small molecule down to a single-crystal copper surface, in such a way that the molecule can pivot about the single anchoring atom, rotating in the plane of the copper surface. Because of the surface atom arrangement and its interactions with the molecule, the molecule has six energetically equivalent ways that it can be oriented on the metal surface. It's experimentally impressive that the authors came up with a way to track the rotation of the molecule one discrete hop between orientations at a time. This is only do-able when the temperature is sufficiently low that thermally driven orientational diffusion is suppressed. When a current of electrons is properly directed at the molecule, the electrons can dump enough energy into the molecule (inelastically) to kick the molecule around rotationally. In that sense, this is an electric motor. (Of course, while the rotor is a single small molecule, the metal substrate and scanning tunneling microscope tip are macroscopic in size.) The requirements for this particular scheme to work include cryogenic temperatures, ultrahigh vacuum, and ultraclean surfaces. In that sense, talk in the press release about how this will be useful for pushing things around and so forth in, e.g., medical devices is a bit ridiculous. Still a nice experiment, though. I continue to find the whole problem of nanoscale systems driven out of thermal equilibrium (e.g., by the flow of "hot" electrons) to be fascinating - how is a steady state established, where does the energy go, where does irreversibility come into play, etc.

Playing with interfaces for optical fun and profit

2011-09-02T14:31:00.000-05:00

A team at Harvard has published in Science a fun and interesting result. When light passes from one medium to another, there are boundary conditions that have to be obeyed by the electromagnetic field (that is, light still has to obey Maxwell's equations, even when there's a discontinuity in the dielectric function somewhere). Because of those boundary conditions, we end up with the familiar rules of reflection and refraction. Going up a level in sophistication and worrying about multiple interfaces, we are used to having to keep track of the phase of the electromagnetic waves and how those phases are affected by the interfaces. In fact, we have gotten good at manipulating those phases, to produce gadgets like antireflection coatings and dielectric mirrors (and on a more sophisticated level, photonic band gap materials). What the Harvard team does is use plasmonic metal structures to pattern phase effects at a single interface. The result is that they can engineer some bizarre reflection and refraction properties when they properly stack the deck in terms of phases. Very cute. I must confess, though, that since Federico Capasso was once my boss's boss at Bell Labs, I'm more than a little disturbed by the photo accompanying the physorg article.

Supersymmetry, the Higgs boson, the LHC, and all that

2011-08-30T20:46:00.000-05:00

Lately there has been a big kerfluffle (technical term of art, there) in the blog-o-sphere about what the high energy physics experimentalists are finding, or not finding, at the LHC. See, for example, posts here and here, which reference newspaper articles and the like. Someone asked me what I thought about this the other day, and I thought it might be worth a post.

For non-experts (and in high energy matters, that's about the right level for me to be talking anyway), the main issues can be summarized as follows. There is a theoretical picture, the Standard Model of particle physics, that does an extremely good job (perhaps an unreasonably good job) of describing what appear to be the fundamental building blocks of matter (the quarks and leptons) and their interactions. Unfortunately, the Standard Model has several problems. First, it's not at all clear why many of the parameters in the model (e.g., the masses of the particles) have the values that they do. This may only be a problem with our world view, meaning the precise values of parameters may come essentially from random chance, in which case we'll just have to deal with it. However, it's hard to know that for sure. Moreover, there is an elegant (to some) theoretical idea called the Higgs mechanism that is thought to explain at the same time why particles have mass at all, and how the electroweak interaction has the strength and symmetry that it does. Unfortunately, that mechanism predicts at least one particle which hasn't been seen yet, the Higgs boson. Second, we know that the Standard Model is incomplete, because it doesn't cover gravitational interactions. Attempts to develop a truly complete "theory of everything" have, over the last couple of decades, become increasingly exotic, encompassing ideas like supersymmetry (which would require every particle to have a "superpartner" with the other kind of quantum statistics), extra dimensions (perhaps the universe really has more than 3 spatial dimensions), and flavors of string theory, multiverses, and whatnot. There is zero experimental evidence for any of those concepts so far, and a number of people are concerned that some of the ideas aren't even testable (or falsifiable) in the conventional science sense.

So, the LHC has been running for a while now, the detectors are working well, and data is coming in, and so far, no exotic stuff has been seen. No supersymmetric partners, no Higgs boson over the range of parameters examined, etc. Now, this is not scientifically unreasonable or worrisome. There are many possible scales for supersymmetric partners and we've only looked at a small fraction (though this verges into the issue of falsifiability - will theorists always claim that the superpartners are hiding out there just beyond the edge of what's measurable?). The experts running the LHC experiments knew ahead of time that the most likely mass range for the Higgs would require a *lot* of data before any strong statement can be made. Fine.

So what's the big deal? Why all the attention? It's partly because the LHC is expensive, but mostly it's because the hype surrounding the LHC and the proposed physics exotica has been absolutely out of control for years. If the CERN press office hadn't put out a steady stream of news releases promising that extra dimensions and superpartners and mini black holes and so forth were just around the corner, the reaction out there wouldn't be nearly so strong. The news backlash isn't rational scientifically, but it makes complete sense sociologically. In the mean time, the right thing to do is to sit back and wait patiently while the data comes in and is analyzed. The truth will out - that's the point of science. What will really be interesting from the history and philosophy of science perspective will be the reactions down the line to what is found.

great post by ZZ

2011-08-24T08:12:00.000-05:00

Before I go to teach class this morning, I wanted to link to this great post by ZapperZ about the grad student/research adviser relationship. Excellent.

Gating and "real" metals.

2011-08-20T12:15:00.000-05:00

Orientation week has kept me very busy - hence the paucity of posts. I did see something intriguing on the arxiv recently (several things, actually, but time is limited at the moment), though.

Suppose I want to make a capacitor out of two metal plates separated by empty space. If I apply a voltage, V, across the capacitor using a battery, the electrons in the two plates shift their positions slightly, producing a bit of excess charge density at the plate surfaces. One electrode ends up with an excess of electrons at the surface, so that it has a negative surface charge density. The other electrode ends up with a deficit of electrons at the surface, and the ion cores of the metal atoms lead to a positive surface charge density. The net charge on one plate is Q, and the capacitance is defined as C = Q/V.

So, how deep into the metal surfaces is the charge density altered from that in the bulk metal? The relevant distance is called the screening length, and it's set in large part by the density of mobile electrons. In a normal metal like copper or gold, which has a high density of mobile (conduction) electrons on the order of 10²² per cm³, the screening length is comparable to an atomic diameter! That's very short, and it tells you that it's extremely hard to alter the electronic properties of a piece of normal metal by capacitively messing about with its surface - you just don't mess with the electronic density in most of the material. (This is in contrast to the situation in semiconductors or graphene, by the way, when a capacitive "gate" electrode can change the number of mobile electrons by orders of magnitude.)

That's why this paper was surprising. The authors use ionic liquids (essentially a kind of salt that's molten at room temperature) to modulate the surface charge density of gold films by something like 10¹⁵ electrons per cm². The surprising thing is that they claim to see large (e.g., 10%) changes in the conductance of quite thick (40 nm) gold films as a result of this. This is weird. For example, the total number of electrons per cm² already in such a film is something like (6 x 10²²/cm³) x (4 x 10^-5 cm) = 2.4 x 10¹⁸ per cm². That means that the gating should only be changing the 2d electron density by something like a tenth of a percent. Moreover, only the top 0.1 nm of the Au should really be affected. The data are what they are, but boy this is odd. There's no doubt that these ionic liquids are an amazing enabling tool for pushing the frontiers of high charge densities in CM physics....

Topological insulator question

2011-08-14T13:07:00.000-05:00

I have a question, and I'm hoping one of my reader experts might be able to answer it for me. Let me set the stage. One reason 3d topological insulators are a hot topic these days is the idea that they have special 2d states that live at their surfaces. These surface states are supposed to be "topologically protected" - in lay terms, this means that they are very robust; something deep about their character means that true back-scattering is forbidden. What this means is, if an electron is in such a state traveling to the right, it is forbidden by symmetry for simple disorder (like a missing atom in the lattice) to scatter the electron into a state traveling to the left. Now, these surface states are also supposed to have some unusual properties when particle positions are swapped around. These unconventional statistics are supposed to be of great potential use for quantum computation. Of course, to do any experiments that are sensitive to these statistics, one needs to do quantum interference measurements using these states. The lore goes that since the states are topologically protected and therefore robust, this should be not too bad.

Here's my question. While topological protection suppresses 180 degree backscattering, it does not suppress (as far as I can tell) small angle scattering, and in the case of quantum decoherence, it's the small angle scattering that actually dominates. It looks to me like the coherence of these surface states shouldn't necessarily be any better than that in conventional materials. Am I wrong about this? If so, how? I've now seen multiple papers in the literature (here, here, and here, for example) that show weak antilocalization physics at work in such materials. In the last one in particular, it looks like the coherence lengths in these systems (a few hundred nanometers at 1 K) are not even as good as what one would see in a conventional metal film (e.g., high purity Ag or Au) at the same temperatures. That doesn't seem too protected or robust to me.... I know that the situation is likely to be much more exciting if superconductivity is induced in these systems. Are the normal state coherence properties just not that important?

DOE BES CMX PI mtg

2011-08-09T07:38:00.000-05:00

Went for the cryptic headline. I'm off for a Department of Energy Basic Energy Sciences Condensed Matter Experiment principal investigator meeting (the first of its kind, I believe) in the DC area. This should be really interesting, getting a chance to get a perspective on the variety of condensed matter and materials physics being done out there. This looks like it will be much more useful than a dog-and-pony show that I went to for one part of another agency a few years ago....

Evolution of blogger spam

2011-08-08T08:29:00.000-05:00

Over the last couple of weeks, new forms of spam comments have been appearing on blogger. One type takes a sentence or two from the post itself, and feeds them through a parser reminiscent of ELIZA, to produce a vaguely coherent statement in a comment. Another type that I've noticed grabs a sentence or two from an article that was linked in the original post. A third type combines these two, taking a sentence from a linked article, and chewing on it with the ELIZA-like parser. A few more years of this, and we'll have the spontaneous evolutionary development of generalized natural-language artificial intelligence from blogger spam....

Summer colloquium

2011-08-05T16:23:00.000-05:00

Every year at Rice in early August, the Rice Quantum Institute (old website) (shorthand: people who care about interdisciplinary science and engineering involving hbar) has its annual Summer Colloquium. Today is the twenty-fifth such event. It's a day-long miniconference, featuring oral presentations by grad students and posters, by both grad students and undergrad researchers from a couple of REU programs (this year, the RQI REU and the NanoJapan REU). It's a full day, with many talks. It's a friendly way for students to get more presentation experience, and a good way for faculty to learn what their colleagues are doing. I'd be curious to know if other institutions have similar things - my impression has been that this is comparatively unique, particularly its very broad interdisciplinary nature (e.g., talks on spectroscopy for pollution monitoring, topological insulators, plasmons, carbon nanotube composites, batteries) and combination of undergrads and grad students.

Plutonium: a case study in why CM physics is rich

2011-07-28T23:37:00.000-05:00

At the heart of condensed matter physics are two key concepts: the emergence of rich phenomena (including spontaneously occurring order - structural, magnetic, or otherwise) in the many-particle limit; and the critical role played by quantum mechanics in describing the many-body states of the system. I've tried to explain this before to lay persons by pointing out that while complicated electronic structure techniques can do an adequate job of describing the electronic and vibrational properties of a single water molecule at zero temperature, we still have a difficult time predicting really emergent properties, such as phase diagram of liquid, solid, and vapor water, or the viscosity or surface tension of liquid water.

Plutonium is an even more striking example, given that we cannot even understand its properties from first principle when we only have a single type of atom to worry about. The thermodynamic phase diagram of plutonium is very complicated, with seven different crystal structures known, depending on temperature and pressure. Moreover, as a resident of the actinide row of the periodic table, Pu has unpaired 5f electrons, though it is not magnetically ordered. At the same time, Pu is very heavy, with 94 total electrons, so that relativistic spin-orbit effects can't be neglected in trying to understand its structure. The most sophisticated electronic structure techniques out there can't handle this combination of circumstances. It's rather humbling that more than 70 years after its discovery/synthesis, we still can't understand this material, despite the many thousands of person-hours spent on it via various nations' nuclear weapons programs.

Einstein, thermodynamics, and elegance

2011-07-24T09:43:00.000-05:00

Recently, in the course of other writing I've been doing, I again came to the topic of what are called Einstein A and B coefficients, and it struck me again that this has to be one of the most elegant, clever physics arguments ever made. It's also conceptually simple enough that I think it can be explained to nonexperts, so I'm going to give it a shot.

Ninety-four years ago, one of the most shocking ideas in physics was the concept of the spontaneous, apparently random, breakdown of an atomic system. Radioactive decay is one example, but even light emission from an atom in an excited state will serve. Take ten hydrogen atoms, all in their first electronically excited state (electron kicked up into a 2p orbital from the 1s orbital). These will decay back into the 1s ground state (spitting out a photon) at some average rate, but each one will decay independently of the others, and most likely at a different moment in time. To people brought up in the Newtonian clockwork universe, this was shocking. How could truly identical atoms have individually differing emission times? Where does the randomness come from, and can we ever hope to calculate the rate of spontaneous emission?

Around this time (1917), Einstein made a typically brilliant argument: While we do not yet know [in 1917] how to calculate the rate at which the atoms transition from the ground state "a" to the excited state "b" when we shine light on them (the absorption rate), we can reason that the rate of atoms going from a to b should be proportional to the number of atoms in the ground state (N_a) and the amount of energy density in the light available at the right frequency (u(f)). That is, the rate of transitions "up" = B_ab N_a u(f), where B is some number that can at least be measured in experiments. [It turns out that people figured out how to calculate B using perturbation theory in quantum mechanics about ten years later.]. Einstein also figured that there should be an inverse process (stimulated emission), that causes transitions downward from b to a, with a rate = B_ba N_b u(f). However, there is also the spontaneous emission rate = A_baN_b, where he introduced the A coefficient.

Here is the brilliance. Einstein considered the case of thermal equilibrium between atoms and radiation in some cavity. In steady state, the rate of transitions from a to b must equal the rate of transitions from b to a - in steady state, no atoms are piling up in the ground or excited states. Moreover, from thermodynamics, in thermal equilibrium, the ratio of N_b to N_a should just be a Boltzmann factor, exp(-E_ab/k_BT), where E_ab is the energy difference between the two states, k_B is Boltzmann's constant, and T is the temperature. From this, Einstein shows that the two Bs were equal, was able to solve for the unknown A in terms of B (which can be measured and nowdays calculated), and to show that the energy density of the radiation (u(f,T)) is Planck's blackbody formula.

My feeble writing here doesn't do this justice. The point is, from basic thermodynamic reasoning, Einstein made it possible to derive an expression for the spontaneous emission rate of atoms, many years in advance of the theory (quantum electrodynamics) that allows one to calculate it directly. This is what people mean by the elegance of physics - in a few pages, from proper reasoning on fundamental grounds, Einstein was able to deduce relationships that had to exist between different physical parameters; and these parameters could be measured and tested experimentally. For more on this, here is a page at MIT that links to a great Physics Today article about the topic, and an English translation of Einstein's 1917 paper.

Slackers, coasters, and sherpas, oh my.

2011-07-21T08:45:00.001-05:00

This is mostly for my American readers - be forewarned.

I wrote last year about a plan put forward by Rick O'Donnell, a controversial "consultant" hired by the state of Texas (hint: Gov. Rick Perry, apparent 2012 presidential hopeful, wanted this guy.) to study the way public universities work in Texas. Specifically, O'Donnell came from a think tank that had very firm predetermined concept about higher education: Faculty are overpaid slackers that are ripping off students, and research is not of value in the educational environment. O'Donnell has written a report (pdf) about this topic, and he's shocked, shocked to find that he was absolutely right. By his metrics of number of students taught and research dollars brought in, he grouped faculty at UT and Texas A&M into "Dodgers, Coasters, Sherpas, Pioneers, and Stars". Pioneers are the people who bring in big grants and buy out of teaching. Stars are the people who bring in grants and teach large lecture classes. Sherpas are mostly instructors (he doesn't seem to differentiate between instructors and faculty) who lecture to large classes but don't bring in grants. Dodgers teach small classes and don't bring in grant money. Coasters teach small classes and bring in some grant money.

This is the exact incarnation of what I warned about in comments on my old post. This analysis basically declares that all social science and humanities faculty that teach upper division classes are worthless leeches (small classes, no grants) sponging off the university. People in the sciences and engineering who teach upper level classes aren't any better, unless they're bringing in multiple large research grants. Oh, and apparently the only metric for research and scholarship is money.

Nice. Perry, by the way, also appointed Barbara Cargill to run the state board of education. She's a biologist who wants evolution's perceived weaknesses to be emphasized in public schools, and she also was upset because the school board only has "six true conservative Christians" as members. I guess Jews, Muslims, Buddhists, Hindus, and atheists need not apply. Update: It looks like Texas has dodged creationism for another couple of years. Whew.

What is so hard about understanding high temperature superconductivity?

2011-07-20T21:12:00.000-05:00

As ZZ has pointed out, Nature is running a feature article on the history of high temperature superconductivity over the last 25 years. I remember blogging about this topic five years ago when Nature Physics ran an excellent special issue on the subject. At the time, I wrote a brief summary of the field, and I've touched on this topic a few times in the intervening years. Over that time, it's pretty clear that the most important event was the discovery of the iron-based high temperature superconductors. It showed that there are additional whole families of high temperature superconducting materials that are not all copper oxides.

Now is a reasonable time to ask again, what is so hard about this problem? Why don't we have a general theory of high temperature superconductivity? Here are my opinions, and I'd be happy for more from the readers.

First, be patient. Low-T superconductivity was discovered in 1911, and we didn't have a decent theory until 1957. By that metric, we shouldn't start getting annoyed until 2032. I'm not just being flippant here. The high-Tc materials are generally complicated (with a few exceptions) structurally, with large unit cells, and lots of disorder associated with chemical doping. This is very different than the situation in, e.g., lead or niobium.
Electron-electron interactions seem to be very important in describing the normal state of these materials. In the low-Tc superconductors, we really can get very far understanding the normal starting point. Aluminum is a classic metal, and you can do a pretty good job getting quantitative accuracy on its properties from the theory side even in single-particle, non-interacting treatments (basic band theory). In contrast, the high-Tc material normal states are tricky. Heck, the copper oxide parent compound is a Mott insulator - a system that single-particle band structure tells you should be a metal, but is in fact insulating because of the electron-electron repulsion!
Spin seems to be important, too. In the low-Tc systems, spin is unimportant in the normal state, and the electrons pair up so that each electron is paired with one of opposite spin, so that the net spin of the pair is zero, but that's about it. In high-Tc systems, on the other hand, very often the normal state involves magnetic order of some sort, and spin-spin interactions may well be important.
Sample quality has been a persistent challenge (particularly in the early days).
The analytical techniques that exist tend to be indirect or invasive, at least compared to the desired thought experiments. This is a persistent challenge in condensed matter physics. You can't just go and yank on a particular electron to see what else moves, in an effort to unravel the "glue" that holds pairs together (though the photoemission community might disagree). While the order parameter (describing the superconducting state) may vary microscopically in magnitude, sign, and phase, you can't just order up a gadget to measure, e.g., phase as a function of position within a sample. Instead, experimentalists are forced to be more baroque and more clever.
Computational methods are good, but not that good. Exact solutions of systems of large numbers of interacting electrons remain elusive and computationally extremely expensive. Properly dealing with strong electronic correlations, finite temperature, etc. are all challenges.

Still, it's a beguiling problem, and now is an exciting time - because of the iron compounds, there are probably more people working on novel superconductors than at any time since the heady days of the late '80s, and they're working with the benefit of all that experience and hindsight. Maybe I won't have to write something like this for the 30th high-Tc anniversary in 2016....

Updated look.

2011-07-18T12:55:00.000-05:00

I finally bit the bullet and updated the look of the blog. I'm still keeping it ad-free, though.

google+

2011-07-17T22:39:00.000-05:00

I have a nagging feeling that google+ could somehow be used to significantly increase readership of my blog, if only I was appropriately savvy. Anyone have any suggestions or thoughts on this? I don't crave the attention per se, but I'd be fibbing if I said I wasn't jealous of the readership numbers of the folks that blog at, e.g., scienceblogs, discovermagazine.com, or scientificamerican.com. Larger readership would undoubtedly motivate more writing, too, though that's not necessarily great for my time management....

It's all at the interface. Again.

2011-07-16T22:10:00.001-05:00

Over the last decade, there has been a great deal of exciting work in making electronically interesting systems at atomically sharp interfaces between different oxide materials (oxide heterostructures). Analogous efforts at semiconductor-dielectric interfaces have given us the conventional field-effect transistor, something like 10⁹ of which are being used to render this page for you. Likewise, heterointerfaces in compound semiconductor systems (especially the technologically relevant III-V materials like GaAs) have given us two Nobel Prizes and a great deal of quantum electronic fun. Oxides are much trickier beasts from the materials science side, making growth and interfacial control a major challenge. Moreover, with respect to basic science, transition metal oxides can be incredibly rich systems, because in many of them electron-electron interactions lead to competing electronic and magnetic phases, with consequences like the emergence of high temperature superconductivity.

A few years ago, this paper demonstrated that it was possible to get superconductivity at the interface between SrTiO₃ and LaAlO₃, two oxides that are both insulating if perfectly stoichiometric. Still, SrTiO₃ is known to superconduct if highly doped, and therefore this observation, while a great experiment, wasn't hugely shocking, given the existence of a high density electron gas at the STO/LAO interface. More recently, this paper showed that high temperature superconductivity could happen at the interface between a nominally insulating oxide and a metallic (but not superconducting) cuprate related to the high-Tc materials. This past week on the arxiv, a logical successor to these works appeared here. The authors use two nominally insulating oxides (STO again, and CaCuO₂. Because of imperfect stoichiometry at the interface (excess oxygen, apparently), there is a conducting layer at the interface, with a superconducting transition around 50 K (in one sample, though others all show transitions exceeding 25 K). Bearing in mind that this is a preprint (and therefore has not been refereed), it is still very exciting. We are finally approaching the ability to engineer complex materials (not just semiconductors) on the atomic layer level, and this should be an incredible playground for basic science and materials engineering. It'd be great to get plugged into a collaboration working in this area.

Science and the public

2011-07-14T15:03:00.000-05:00

I couldn't help but notice that one of my favorite producers of animated films, Aardman Animation, is coming out with a new movie (trailer here). I find it very interesting that the UK version of the movie is "The Pirates! In an Adventure with Scientists!", while the US version is "The Pirates! Band of Misfits!". The film is based on a book with the former title, by the way. I don't want to overanalyze this, but it's hard to escape the conclusion that some marketing drone decided, "scientist" is box-office poison, and that "misfit" was an acceptable and more marketable substitute in the US. Great. Wonderful. In case you're wondering, Charles Darwin shows up as a character in the book/movie. I imagine that the US ads won't be playing that up very much, or there will be protests. Sigh.

(I do have a science post I'll make shortly. I just couldn't let this pass w/o comment. And it's taking enormous self-restraint not to launch into extended political invective about the US, but there are many places where people can read that if they want to.)

Follow-up, and blogger drop-off

2011-07-07T10:03:00.000-05:00

Regarding the story mentioned here, Nature has published both a provocative and interesting article by Eugenie Reich about the larger issues raised, and an editorial. Sorry that these are behind a pay-wall. To summarize in a few sentences: Eugenie Reich points out that the misconduct investigation relevant to this discussion highlights important problems with the US Department of Energy's handling of such cases. To wit: There are issues of independence and chain of authority of the investigators, and lack of proper record keeping, documentation, etc. of investigation reports. The conclusion is that this is a powerful argument for the DOE to establish an Office of Research Integrity, like those in some other agencies. The editorial from Nature chastises the DOE along these lines. Interesting that the Nature editorial makes no mention at all of their own role in not publishing technical comments relevant to this particular matter.

In blogging news, there has been a drop-off in the number of active physical science bloggers. David Bacon's Quantum Pontiff has decohered. The Incoherent Ponderer has gone so far as to apparently delete his entire blog and blogger profile. Other blogs have not been updated in many months. It's likely that this is all part of a natural stabilization of blogging - people run out of things to say, and the novelty of blogging has trailed off. It will be interesting to see where this trend resolves. It'll be a shame to have fewer interesting voices to follow, though. (Clearly we should all switch to Twitter, since 140 characters should be more than sufficient to carry out detailed science discussions or popularizations for the lay audience. Ahem.)

Crowd-sourcing, video games, and the world's problems

2011-07-05T10:43:00.000-05:00

This past weekend, I caught a snippet of a rebroadcast of this NPR story about Jane McGonigal and the thesis of her recent book. In short, she points out that as a species we have spent literally millions of person-years playing World of Warcraft, an online game that involves teamwork and puzzle-solving (as well as all the usual fun silliness of videogames). Her point is that in the game environment, people have demonstrated great creativity as well as a willingness to keep coming back, over and over, to tackle challenging problems (in part because there is recognition by the players that problems are pitched at a level that is tricky but not insurmountable). She wants to harness this kind of intellectual output for good, rather than just have it as a social (or antisocial) outlet. She's not the first person to have this sort of idea, of course (see, e.g., Ender's Game, or the Timothy Zahn short story "The Challenge"), but the WoW numbers are truly eye-popping.

It would be great if there were certain scientific problems to which this could be applied. The overall concept seems easiest to adapt to logistics (e.g., coming up with clever ways of routing shipping containers or disaster relief supplies), since that's a puzzle-solving subdiscipline where the basic problems are at least accessible to lay-people. Trying this with meaty scientific challenges would be much more difficult, unless those challenges could be translated effectively into problems that don't require years and years of foreknowledge. Hmm. Still very thought-provoking.

The tyranny of the buried interface

2011-07-01T09:13:00.000-05:00

Time and again, a major impediment to research progress in condensed matter physics, electrical engineering, materials science, and physical chemistry is the need to understand what is happening in some system at a buried interface. For example, in organic photovoltaic devices, it is of great importance to learn more about what is happening at metal/organic semiconductor interfaces (charge transfer, interfacial dipole formation, Fermi level pinning) and organic/organic interfaces (exciton splitting at the interface between electron- and hole-transporting materials). Another example: in lithium ion batteries, at the interface between either the cathode or the anode and the electrolyte, after the first couple of charge and discharge cycles, there forms the "solid electrolyte interface" (SEI) layer. The SEI is nanoscale in thickness, stabilizes the electrode surface, establishes the energetic lineup between the electrolyte redox chemistry and the actual electrode surface, strongly affects the kinetics of the lithium ion transport, etc.

Unfortunately, probing buried interfaces in situ in functioning systems is extremely hard. There generally is no Star Trek scanner device that can nondestructively reveal atomic-scale details of buried 3d structures. Many of our best characterization approaches are surface-based, or require thinned down samples, and there are always difficult questions about how information gained in such investigations translates to the real situation of interest. This is not a new problem. From the early days of surface science and before, people have been worrying about, e.g., how to connect studies performed in UHV on single crystal surfaces with "real world" situations on polycrystalline surfaces with ambient contaminants. There are some macro-scale interface sensitive approaches (exploiting x-ray standing waves, or interfacial optical effects). Still, the more people working on developing better characterization tools toward this end, the better, even if it doesn't sound terribly exciting to the masses.

a recurring story

2011-06-23T09:08:00.001-05:00

Five years ago, there was a controversy in the pages of Nature regarding this paper from 1993, the first to claim atomic-resolution chemical analysis via scanning transmission electron microscopy. At issue was whether or not the data in the paper had been reprocessed (in response to referee concerns) in a legitimate or misrepresentative way, and whether the authors had been honest and forthcoming with the journal and the reviewers about the procedures they'd followed. The reason that matters came to a head more than 12 years after the original paper was the appearance of a preprint in the arxiv and subsequently submitted to Nature Physics, sharing two of the authors of the original paper, with further questions raised about the handling and analysis of data and images. This was all discussed clearly and succinctly by ZZ at the time. Nature allowed the authors to publish a corrigendum, a correction rather than a retraction, regarding the original '93 paper. This was sufficiently controversial that Nature felt the need to write an editorial explaining their decision. Oak Ridge did an investigation of the matter, and concluded that there was no fabrication or falsification of data; that report and a response by the authors are linked here. Judging from the appearance of this on the arxiv last night, it would appear that this isn't quite the end of things.

Pitch for a tv show

2011-06-15T21:52:00.000-05:00

Summer blogging has been and will continue to be light, as I try to get some professional writing done. In the meantime, though, I have to give my elevator pitch for the awesome new TV show that would be great fun. It's "Chopped" meets "Mythbusters" meets "Scrap Heap Challenge"/"Junkyard Wars". Start off with three teams. Give them a physics- or engineering-related task that they have to accomplish (e.g., write the opening crawl from Star Wars in one mm^2; weigh a single grain of salt), some number of tools that they have to use (e.g., a green laser pointer and an infrared corrected microscope objective), and access to a stocked "pantry" (including a PC, electronics components, etc.). Give them a time limit (4 hours, cleverly edited down to half an hour in broadcast). Points awarded for success at the task, time used, and elegance. I think it could be a hit, particularly if there are explanations (narrated by cool resident experts) delivered in a fun, accessible tone. It'd be fun, even if it did conjure up images of Guy Fleegman in Galaxy Quest.

Soliciting book or review article recommendations

2011-06-06T21:33:00.001-05:00

I am interested in reading good books or review articles on two particular topics, and I'm hoping that by "crowd-sourcing" to my readership, I might do better than wandering through the literature. First, I want to find an authoritative discussion of the physics behind the electrochemical potentials of battery materials - not the lore of decades of electrochemistry, but a real hashing out of the physics. Second, I would like to find a thorough, authoritative discussion of the physics behind catalysis. Again, I'm not interested in handwaves and parametrized empirical knowledge, but would prefer a physics-based discussion that explains, e.g., why Pd is good at splitting H₂, while Ti is not. Any help would be greatly appreciated.

Several items

2011-06-05T14:05:00.000-05:00

I returned late last week from Germany, where I spoke at a summer school. One fun part of the trip was a tour of the main experimental facility at the neighboring Max Planck Institute for the Chemical Physics of Solids. The facility was a large high-bay lab space, with 9 (!) dilution refrigerator apparatuses, as well as a 0.3K scanning tunneling microscope with 12 Tesla magnet. Very impressive infrastructure, and the place was neat as a pin - the very model of a lab. Note to self: figure out how to instill Germanic ultraprecise lab notebook habits in all incoming grad students...,

Other news this week that is interesting: the US National Academies have decided to make many of their books available for pdf download free of charge. I'm a particular fan of one or two of these. For example, with reference to recent discussions about helium as a resource, check this out.

There is also a great deal of attention being paid to a paper from this week's Science by the group of Aephriam Steinberg. The experiment sends single photons one at a time through a two-slit type apparatus. This is one of those experiments meant to blow the minds of undergrad physics majors taking quantum for the first time: you still build up an interference pattern from the slits, even though there's only one photon in there at a time. That means the photon must be interfering with itself(!). In the new work, the group uses optics techniques (that I freely admit I do not fully understand) to correlate, after the fact, the ("weakly" measured) momentum of the photon while in the apparatus with the (strongly measured) final position of the photon on a CCD. This does not violate the uncertainty relation, since it basically finds a quantum mechanical ensemble average of the momentum as a function of final position. Still, very neat, and discussed in some detail here and here.

I've liked Steinberg's work for years. This business about quantum measurement and post-selection is very fun to think about. For example, this comes up when considering the question, "how long does it take a quantum particle to tunnel through a classically forbidden region?". What you're basically asking is, given the successful measurement of a quantum particle at some position beyond the classically forbidden region, when did the particle, in the past, impinge upon that region in the first place? This is a very hard question to answer experimentally.

Recently in the arxiv

2011-05-27T21:40:00.000-05:00

As I get ready to head to Germany for my first ever experience lecturing at a Max Planck summer school, I wanted to point out very briefly three of a number of interesting papers that came through the arxiv this week.

arxiv:1105.4055 - Janssen et al., Graphene, universality of the quantum Hall effect and re-definition of the SI

This paper compares the quantization of the Hall resistance in two different two-dimensional electronic systems: a conventional 2d electron gas in a GaAs/AlGaAs structure, and graphene. The authors find that the Hall resistance is quantized in units of h/e² identically in the two systems to parts in 10¹¹. On the one hand, this is really amazing, since you're seeing essentially exact quantization in two different systems, and the whole basis for the quantum Hall effect relies in part on dirt - without disorder, you wouldn't see the quantum Hall physics. And yet, even though the materials differ and dirt plays an important role, you get precise quantization in terms of fundamental constants. This is the kind of emergent, exact phenomenon that shows the profound character of condensed matter physics.

arxiv:1105.4642 - Barends et al., Loss and decoherence due to stray infrared light in superconducting quantum circuits

As someone who struggled mightily in grad school to avoid the effects of infinitesimal amounts of rf noise leaking into his ultracold sample, this impressed me. The authors demonstrate that infrared radiation from the surroundings, even when those surroundings are at 4.2 K, can have marked, detectable impact on the coherence properties of superconducting quantum bits. They compare results with and without an absorbing radiation shield in the way, and the effects aren't small. Wild. Time to break out those 50 mK shields from our old nuclear demag cryostat....

arxiv:1105.4652 - Paik et al., How coherent are Josephson junctions?

Along these same lines, these authors have been able to demonstrate coherence times in superconducting qubits that stretching into the tens of microseconds scale. They do this via a new kind of cavity, essentially controlling the environmental dissipation. This isn't really my area, but I know enough to be impressed, and also to be surprised at the apparent lack of the usually ubiquitous 1/f noise problems (in the critical current) that often limit coherence in these kinds of devices. As they point out, these numbers are encouragingly close to the thresholds needed for quantum error correction to be realistic.

Nano for batteries

2011-05-20T21:15:00.000-05:00

Improved batteries would be of enormous benefit and utility in many sectors of technology. A factor of 10 improvement in battery capacity (with good charging rate, safety, etc.) would mean electric cars that get 1000 miles per charge, laptops that run for days w/o charging, electrical storage to help with the use of renewable energy, and a host of other changes. This rate of performance enhancement is completely commonplace in semiconductor electronics and magnetic data storage, yet batteries have lagged far, far behind.

There is real hope that nanostructured materials can help in this area. Three examples illustrate this well. Conventional lithium ion batteries have an anode (usually graphitic carbon, into which lithium ions may be intercalated) and a cathode (such as cobalt oxide), with an intervening electrolyte, and a separator barrier to prevent the two sides from shorting together. A reasonable figure of merit is the capacity of the electrodes, in units of mA-h/g. The materials described above, anode and cathode, have capacities on the order of 200-300 mA-h/g. It is known that silicon can take up even more lithium than carbon, with a possible capacity of more than 3000 mA-h/g (!). Complicating matters, Si swells dramatically when taking in Li, meaning that bulk single-crystal Si cracks and self-pulverizes when taken through a few charge/discharge cycles. However, Si nanowires have been observed to be much better behaved - they have large surface specific surface area, and have enough free surface to swell and shrink without destroying themselves - see here. Very recently, this paper has spectacular electron micrographs of the swelling of such nanowires.

A second example: nanostructured cobalt oxide particles, self-assembled using selectively modified virus proteins, have been put forward as high capacity Li ion battery cathodes. This approach has also been extended to iron phosphate cathode material.

A third example: dramatically improved charging rates may be possible using nanostructured electrode geometries, such as these inverse-opal shapes.

There is real hope that nanostructured materials may enable true breakthroughs in battery technology, even though batteries have been studied exhaustively for many decades. The ability to engineer materials at previously inaccessible scales may bear fruit soon.

Rice University clean room manager needed.

2011-05-16T11:09:00.000-05:00

Just in case anyone out there has or is a promising candidate, I wanted to point out that Rice University is looking for a new clean room facilities manager. (This is not a soft money position.) Here is the text of the advertisement:

Rice University is seeking a technical manager to oversee the operations of its clean room user facility and associated characterization equipment. This Class 100/1000 facility contains a suite of instruments, including a photolithography mask maker, a contact mask aligner, an e-beam evaporator, an RIE/PECVD system, and a collection of characterization tools. The manager’s responsibilities include oversight of this facility, training of undergraduate and graduate students and other users, and maintenance and upkeep of the equipment. Applicants must have a BS degree in a science or engineering discipline (PhD preferred but not required), and extensive experience with several of the relevant instruments or a related technical degree or diploma with an additional 2 years of the related experience (for a total of 7 years of related experience working with clean room instruments). Salary will commensurate with experience. The need to fill this position is immediate, and resumes will be examined as they arrive. Please visit http://cohesion.rice.edu/campusservices/humanresources/riceworks.cfm to apply for this listing. Rice University is an equal opportunity, affirmative action employer.

A university selling its soul

2011-05-13T11:46:00.001-05:00

I'll get back to physics shortly. These two articles (here and here) explain how, in exchange for $1.5M in donations, the Florida State economics department agreed to give the donors veto power over faculty hiring for the donor-supported positions. Moreover, the donors can withdraw the positions if they aren't happy with annual performance reviews of the professors. Wow. I know times are tight, but FSU has clearly decided that they're up for bid. I don't care whether the donors are right-wing or left-wing (hint: they're right wing) or centerist - a university that allows donors direct control over faculty hiring and evaluation is out of its mind. Gee, you think those professors are going to be free to do whatever research they want? Do you think there's going to be pressure on all of the faculty within the department to toe the line rather than risk angering the donors? What a mess. Well, at least it confirms that Texas doesn't have a monopoly on idiocy.

Update: blogger ate this post, and I had to reconstitute it from the cached version on bing (google blew this one all the way around). Clearly the Koch brothers are responsible :-)

Gravity Probe B

2011-05-05T21:18:00.000-05:00

Finally, after only 45 years from conception to publication of results, Gravity Probe B has announced (dramatic pause) that Einstein's General Theory of Relativity is consistent with their data. I had mentioned GPB ("The Project that Ate Stanford") once before. It was a fascinating, complex, multidisciplinary project that, thanks to its experimental design and extraordinarily long duration, had great impact on a large number of physics, materials science, and engineering careers. Still, I think they were in a bit of a no-win scenario, particularly once it became clear that there were problems with interpreting the data. Either they support general relativity, or people just wouldn't trust the results, given how much other evidence there is out there that GR is right, at least in the relatively weak field limit.

Nano for solar

2011-05-05T20:53:00.000-05:00

Sorry about the delay in this posting. Real life has been busy.

Solar energy is an obvious candidate for a long-term solution to many of our energy problems. The amount of power reaching the surface of the earth is on the order of 350 W/m². We could meet the world's projected energy needs in 2030 by covering around 250 km by 250 km with 10% efficient solar cells. Unfortunately, the total surface area of all photovoltaics ever manufactured is less than 0.1% of that. (This is why being able to produce photovoltaic cells by printing processes would be great. Hint: estimate the total area printed by the New York Times in a month.) There are a number of challenges involved in solar. Why might "nano" broadly defined be a big help? Let me give three examples from the large wealth of ideas out there.

1) Semiconductor nanocrystals as absorbers. Because of the beauty of quantum confinement, it is possible to make semiconductor nanocrystals out of a single material, and use different sizes to capture different parts of the solar spectrum. Moreover, there is evidence (after some controversy) that nanocrystals may enhance "multiexciton generation" (e.g., here and here). In a traditional solar cell, a photon with energy twice as large as the semiconductor band gap will generate an electron-hole pair (which must be ripped apart somehow), and inelastic processes will lead to the excess (above the band gap) energy being lost as heat. However, at some rate, instead you can generate two band-gap-energy pairs. The idea is that the rate of that process can be enhanced in nanocrystals, since conservation of "crystal momentum" can be relaxed in materials that are so surface-dominated.

2) Nanostructured materials for photoelectrochemical cells. There are a number of proposals for using electrolytes in solar applications, including dye-sensitized solar cells. In this case, one would like to use a high surface area anode, such as nanostructured TiO2 or some similar nanostructured material. Moreover, instead of using organic dyes as the absorbers and sources of photoexcited electrons, one could imagine again using semiconductor nanocrystals.

3) Plasmon-enhanced photovoltaics. One way to try to boost the efficiency of solar cells is to get the light to hang around the absorber material for longer. One compact way to do so is to use plasmonically active metal nanoparticles or nanostructures as optical antennas. The local fields near these structures can enhance scattering and local intensity in ways that tend to boost performance, though resistive losses in the metal may limit their effectiveness. It's worth pointing out that one can also use plasmonic antennas as sources of hot electrons, also interesting from the photovoltaic angle.

There are many more ideas out there - I haven't even mentioned anything about nanotubes or graphene. While the odds of any individual idea being a truly transformative breakthrough are small, there are probably more clever things being proposed in this area now that at any time ever before, thanks to our ability to manipulate matter on very small scales.

Nano and energy

2011-04-27T22:14:00.001-05:00

It might be fun to do a few posts on how nanoscale science can be used to the benefit of our energy concerns. First, let me specify what I mean when I say that there's an "energy problem". The fact is, average people enjoying first-world standards of living (e.g., US/Canada/Western Europe/Japan) have an enormous per capita energy consumption compared to, e.g., tribesmen in sub-Saharan Africa, or rural farmers in the hinterland of China. If the goal is to raise the standard of living of the 5-ish billion people not enjoying the high life, and to get everyone up to a high standard of living, then we've got a problem: there's no nice way to do so without incurring other enormous costs (e.g., burning enormous quantities of fossil fuels; building GW-scale power plants at very high rates, like several per day for the next 30 years). Either we're not going to raise that standard of living for those billions of people, or the energy costs for the top economic tier are going to have to fall, or we're headed for major upheaval (or possibly some of all of the above).

When I teach my second-semester nano class, I point this out, and if you want interesting quantitative references, check here. Broadly construed, nanotechnology and nanoscale science (and more broadly, condensed matter physics and materials science) can try to address several aspects of this challenge, though there are certainly no silver bullets. The areas that come to mind are: energy generation; energy storage; energy distribution; conservation or improved efficiency; and environmental remediation. In future posts, I'll try to summarize very briefly a few thoughts on this.

Public funding of science, and access to information

2011-04-23T07:47:00.000-05:00

On multiple blogs over the last few months, I've read comments from lay-persons (that is, nonscientists) that say, in essence, "As a citizen, I paid for this research, and therefore I should have access to all the data and all the software necessary to analyze that data." The implications are (1) research funded by the public should be publicly accessible; and (2) the researchers themselves sometimes/often? hold back information or misinterpret the results, perhaps because they are biased and have an agenda to further.

Now, as a pragmatist, there are a number of issues here. For example, making available raw columns of tab-delimited numerical data and, e.g., matlab code, won't give a nonscientist the technical know-how to do analysis properly, or to know what models to apply, etc. Things really get tricky if the "data" consists of physical samples (e.g., soil, or ice cores, or zebrafish).... Yes, scientists that are publicly funded have the responsibility to make their research results available to the public, and to explain those results and their analysis. As a practical matter, scientists are not obligated to make any interested citizen into an expert on their research.

While this is an interesting topic, I'd rather discuss a related issue: How much public funding triggers the need to make something publicly available? For example, suppose I used NSF funding to buy a coaxial cable for $5 as part of project A. Then, later on, I use that coax in project B, which is funded at the $100K level by a non-public source. I don't think any reasonable person would then argue that all of project B's results should become public domain because of 0.005% public support. When does the obligation kick in? Just an idle thought on a Saturday morning.

Friction, commensurability, and superlubricity

2011-04-19T22:05:00.000-05:00

In the limit of clean surfaces, friction has its origins in the microscopic, chemical interactions at the interface between the two objects in question. One of the more amazing (to me, anyway) consequences of this is the extremely important role played by commensurability between the surfaces. Let me explain with an example. Consider a gold crystal terminated at the (111) surface, and another gold crystal also terminated at the (111) surface. Now, if those two surfaces are brought into contact, with the right orientation so that they match up as if they were two adjacent layers of atoms inside a larger gold crystal, what will happen? The answer is, in the absence of adsorbed contaminants, the surfaces will stick. This is called "cold welding". In contrast, if you bring together two ultraclean surfaces that are incommensurate, they can slide past each other with essentially no friction. This is called "superlubricity". Here are two great examples (pdf of first one; pdf of second one) of this.

In this new paper, Liu et al. are able to do some very cute experiments in this regard, looking at the motion of thin graphite flakes (exfoliated from and) sliding on graphite pedestals. It's clear from the observations that graphite flakes shifted relative to the underlying graphite substrate can slide essentially frictionlessly over micron scales. Very neat and elegant, and surprising since there is not any rotation at work here to break commensurability. This is a very firm reminder that our macroscale physical intuition about materials and their interactions can fail badly at the nanoscale.

Playing chicken with the global economy

2011-04-12T22:27:00.002-05:00

I get it - we need to fix the structural problems associated with the US budget. However, don't these geniuses realize that threatening to default (let alone actually defaulting) on the US sovereign debt will severely undermine the dollar? It's like they actually want to have hyperinflation, so that they can claim it was all Obama's fault. Other countries don't have a "debt ceiling", you know. Update: seems I'm not alone in realizing that even talking about default is dangerous.

Choosing a postdoctoral position

2011-04-11T10:46:00.000-05:00

I had a request a while ago for a post about how to choose a postdoctoral position (from the point of view of a finishing-up grad student, I'm assuming). This is a tricky topic, precisely because it's somewhere between choosing a grad school (lots of good places to go, with guaranteed open positions every year) and getting a faculty job (many fewer open positions per year in a given field, and therefore a much restricted field of play; plus, a critical need to make some hard decisions that could be postponed or avoided in grad school). Moreover, different disciplines within the physical sciences have very different approaches on postdocs. In some fields like astronomy, externally funded fellowships sponsored by observatories/facilities/programs are standard practice, while condensed matter physics is much more principal-investigator-driven. So, I'll try to stick to general points.

I strongly suggest going somewhere that is not your graduate institution, unless there are strong extenuating circumstances. It's just intellectually healthier to get a broad exposure to what is out there, rather than to stay entirely comfortable.
This is also one of the relatively few points in your career when you can really shift gears, if you are so motivated. My doctorate was in ultralow temperature physics, but I decided to become a nano researcher, for example. More dramatically, this is often the point where many people get into interdisciplinary fields like biophysics. There are trade-offs, of course. If you do a postdoc in an area very close to your thesis work, you can often make rapid progress. On the other hand, most people who go on in research (industrial or academic) do not end up working on their thesis topic for the lion's share of their career, and this is a chance to broaden your skill set and knowledge base.
Word of mouth and self-motivation are essential to getting a good postdoc position, beyond posted ads. If you're finishing up in grad school, you are enough of a professional that you should be able to email or otherwise contact people whose work you find interesting and exciting, and ask whether they have any postdoctoral openings. You should make sure that these emails are reasonably detailed and that it's clear they're personalized - not a form letter being spammed to several hundred generic faculty members simultaneously. Your hit rate won't be high, but it's better than nothing.
Don't discount industry, though it's a narrowing field. There are still industrial postdoc positions, and if you've got an interest in industry more so than academia, then you should look at these possibilities. This includes places like Bell Labs (yes, they still exist), IBM, Intel, HP Labs, etc. It is a tragedy that there aren't more opportunities like this out there now.
You need to think about how a particular postdoc position is structured. Are you going to be acting as middle-management, helping to mentor a team of grad and undergrad students? Are you going to be leading a research project yourself? Is there a lot of lab-building or lab-moving? How long is the position, and how does it match up w/ the seasonal nature of academic hiring, if academia is what you want to do? Where have previous postdocs in that lab or group ended up?
How set are you on academia? If you are set on academia, what kind of academic position would make you happy? Go into the academic track with your eyes open! If you're looking beyond academia, what do you need out of a postdoc position (besides a paycheck)? Are there particular skills you want to learn?

None of this is particularly insightful, but it doesn't hurt to have this written down in one place. Suggestions for further things to consider are invited in the comments....

Designing a lab

2011-04-05T21:28:00.001-05:00

Designing a lab is not trivial, particularly if you have no experience in doing it before. My new lab (day 2 of the move....) was perhaps the ideal circumstance: a new building is being constructed, and you have a very free hand in determining the layout, the facilities, and so forth. In any realistic process you never get everything you want (e.g., this building does not have a building-wide deionized water system; I can't have unlimited space; there are restrictions based on cost and feasibility). The challenge is to end up with functional space - laid out intelligently, so that work flows well and you don't find yourself fighting with the building or yourselves. Sometimes this is not simple. In my original lab space, for example, that floor of the building was never designed with vibration-sensitive work in mind. The need to position certain pieces of equipment on the vibrationally quiet parts of the floor strongly influenced lab layout, rather than basic experimental logic.

Lab design ranges from the Big Picture (e.g., I have a couple of optics tables, so I should probably have a separate area with independently controlled lighting; I want isolation transformers to keep my sensitive measurement electronics off the power lines used for my big pumps.) to a zillion little details (e.g., where should every single electrical outlet and ethernet port be positioned? What about emergency power? Gas lines? What fittings are going to be on the chilled water lines?). Nothing is ever perfect, and there are always minor glitches (e.g., mislabeled circuit breakers). You also want to design for the future. If you think you're eventually going to need a gizmo that requires chilled water or a certain amount of 480V current, it's better to plan ahead, cost permitting.... The situation is definitely more constrained if you're moving into pre-existing space, particularly in an older building. Like many aspects of being a professor, this is something that no one ever sits down and teaches you. Rather, you're left to figure it out, hopefully with the help of a professional.

Moving the lab

2011-04-04T08:02:00.001-05:00

Today's the beginning of moving my lab into the new Brockman Hall for Physics here at Rice. As the week goes on, if I have time I'll write a bit about the process of lab design and the joys of moving equipment. It's exciting, but there's no question that I wish we could skip over the actual transition.

Blogger spam + McEuen novel

2011-03-27T16:13:00.000-05:00

Two unrelated topics. First, blogger needs to get their act together regarding comment spam. They have some attempt at automatic spam detection, but it's clear that in the last two or three weeks people have figured out how to evade their blocking algorithm. The spam comments quote some fragment of the original blog post or a previous comment, and then have a clickable username that links to some shady vendor website. Very annoying. I'd really rather not shift to a moderated comments approach, but I may have to if this keeps up.

Second, I was very surprised last weekend when reading the Wall Street Journal, and coming upon an article about Paul McEuen (author link, physicist link), who has written apparently a very successful novel. As one of my friends exclaimed upon hearing this news when I told her at the APS, come on Paul - you're again making the rest of us look like lazy underachievers! I'm going to have to get this on kindle....

March Meeting, further thoughts

2011-03-24T22:06:00.001-05:00

I had to cut my March Meeting a bit short this year, to get back to Rice in time for the dedication of our new Brockman Hall for Physics. Still, a few more thoughts from the APS meeting:

Michelle Simmons gave a terrific talk summarizing the work, over more than a decade, of her group at the University of New South Wales on their progress toward their eventual goal of building a quantum computer based on P donors in Si (the Kane approach). I knew of the work, but I'd never seen it all laid out like that, and it was impressive. There are very few people out there in the CM community with the fortitude to plan out and pursue steadily a coherent, goal-directed research program over a dozen years.
I also saw something I hadn't observed in a number of years: a speaker completely blowing off the 10 minute time limit on a contributed talk. When the yellow warning light clicked on, it was clear that the speaker was nowhere near the end. When the red light clicked on and started to blink, still no conclusion. The session chair stood up and loomed intimidatingly. No dice. Finally the speaker ended after a total of about 16 minutes. That takes nerve (and a lack of consideration for the others in the session....).
I chaired two sessions this year (note to self: only chair one session....), and in contrast to the previous point, really didn't have any bad talks at all in there. Very pleasant, generally. Most interestingly, the metal-insulator transition in vanadium oxide session was 100% experimental talks! Perhaps theorists have given up? (kidding.)

2011 APS March Meeting, first thoughts

2011-03-22T14:15:00.002-05:00

A few brief thoughts at the APS March Meeting (more later....) in Dallas:

First time I've ever been at a convention center with a graveyard adjacent to the building. Quite a time saver if there are really bad talks, I suppose.
Frank Wilczek still gives a terrific talk about the connection between superconductivity and high energy physics. Very droll, too. He clearly has a strong aesthetic desire for supersymmetry, but just as clearly acknowledges that all of this could go up in smoke, depending on what the LHC finds.
The APS's attempt at a mobile app (for iPad, iPhone, etc.) is so painfully slow and incomplete (no scheduling ability I can almost understand, but how can you not list the room numbers for the sessions?) that it's better to use wireless internet access to visit the APS meeting website instead.
Roland Wiesendanger also presents an outstanding talk. His group's accumulated work on spin-polarized STM is very impressive, and definitely made me feel an intense bout of "imaging envy" (in the sense that my group's work usually does not have beautiful 3d renders of data sets that grace the cover of glossy journals).
Lots of discussions with people about looming budget concerns, and separately the decline of science journalism. On some level, these topics are related....

Advice on choosing a graduate school

2011-03-13T21:05:00.000-05:00

This is my 500th post (!), and I realized, after spending a big part of the last two days talking with prospective graduate students, that I had never written down my generic unsolicited advice about picking a graduate school.

Always go someplace where there is more than one faculty member with whom you might want to work. Even if you are 100% certain that you want to work with Prof. Smith, and that the feeling is mutual, you never know what could happen, in terms of money, circumstances, etc. Moreover, in grad school you will learn a lot from your fellow students and other faculty. An institution with many interesting things happening will be a more stimulating intellectual environment, and that's not a small issue.
It's ok at the applicant stage not to know exactly what you want to do. While some prospective grad students are completely sure of their interests, that's more the exception than the rule.
If you get the opportunity to visit a school, you should go. A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students. Always talk to current graduate students if you get the chance - they're the ones who really know the score. A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.
I know that picking an advisor and thesis area are major decisions, but it's important to realize that those decisions do not define you for the whole rest of your career. I would guess (and if someone had real numbers on this, please post a comment) that the very large majority of science and engineering PhDs end up spending most of their careers working on topics and problems distinct from their theses. Your eventual employer is most likely going to be paying for your ability to think critically, structure big problems into manageable smaller ones, and knowing how to do research, rather than the particular detailed technical knowledge from your doctoral thesis. A personal anecdote: I did my graduate work on the ultralow temperature properties of amorphous insulators. I no longer work at ultralow temperatures, and I don't study glasses either; nonetheless, I learned a huge amount in grad school about the process of research that I apply all the time.
You should not go to grad school because you're not sure what else to do with yourself. You should not go into research if you will only be satisfied by a Nobel Prize. In both of those cases, you are likely to be unhappy during grad school.
I know grad student stipends are low, believe me. However, it's a bad idea to make a grad school decision based on a financial difference of a few hundred or a thousand dollars a year. Different places have vastly different costs of living. Pick a place for the right reasons.
Likewise, while everyone wants a pleasant environment, picking a grad school largely based on the weather is silly.
Pursue external fellowships if given the opportunity. It's always nice to have your own money and not be tied strongly to the funding constraints of the faculty, if possible.
Be mindful of how departments and programs are run. Is the program well organized? What is a reasonable timetable for progress? How are advisors selected, and when does that happen? Who sets the stipends? What are TA duties and expectations like? Are there qualifying exams? Know what you're getting into!
It's fine to try to communicate with professors at all stages of the process. We'd much rather have you ask questions than the alternative. If you don't get a quick response to an email, it's almost certainly due to busy-ness, and not a deeply meaningful decision by the faculty member.

There is no question that far more information is now available to would-be graduate students than at any time in the past. Use it! Look at departmental web pages, look at individual faculty member web pages. Make an informed decision. Good luck!

Blogging scarcity - tidbits.

2011-03-09T11:23:00.001-06:00

My blogging has been sparse of late because of several colliding deadlines and constraints (NSF report due; review article due; APS meeting coming up; impending travel; visits of prospective graduate students; the ever-present book; teaching; impending move of my whole lab to the new Brockman Hall for Physics). This doesn't mean that there aren't interesting things going on out there in condensed matter physics (and physic in general) - just that I've been extraordinarily busy.

To tide you over, here are a handful of interesting links.

This is an amazing video made entirely from shots of Saturn and its moons taken by the Cassini spacecraft. It looks like something out of Hollywood, but is a zillion times more fascinating because it's real - no cgi here.

This older preprint (I'll revise the link when the paper comes out in PRL next week) puts forward the argument that the pseudospin degree of freedom of electrons in graphene does actually correspond to a real half-integer angular momentum. Surprising - I need to think about this more.

This experiment is extremely slick. The authors are able to use the magnetic field gradient from a sharp magnetic scanned probe tip to interact w/ individual nitrogen vacancy centers in diamond (which have an unpaired electron spin). This is basically magnetic resonance imaging of single electron spins.

This paper shows a clear implementation of an idea that is increasingly popular: using the plasmon properties of metal nanostructures to enhance solar energy harvesting. Essentially the evanescent optical fields from the metal nanoparticles trap the light near the interface where, in this case, the photochemistry is happening.

Of gaps and pseudogaps

2011-02-26T13:02:00.001-06:00

ZapperZ's recent post about new work on the pseudogap in high temperature superconductors has made me think about how to try to explain something like this to scientifically literate nonspecialists. Here's an attempt, starting from almost a high school chemistry angle. Chemists (and spectroscopists) like energy level diagrams. You know - like this one - where a horizontal line at a certain height indicates the existence of a particular (electronic) energy level for a system at some energy. The higher up the line, the higher the energy. In extended solid state systems, there are usually many, many levels. That means that an energy level diagram would have zillions of horizontal lines. These tend to group into bands, regions of energy with many energy levels, separated by gaps, regions of energy with no levels.

Let's take the simplest situation first, where the energies of those levels don't depend on how many electrons we actually have. This is equivalent to turning off the electron-electron interaction. The arrangement of atoms gives us some distribution of levels, and we just start filling it up (from the bottom up, if we care about the lowest energy states of the system; remember, electrons can be spin-up or spin-down, meaning that each (spatial state) level can in principle hold two electrons). There's some highest occupied level, and some lowest unoccupied level. We care about whether the highest occupied level is right up against an energy gap, because that drastically affects many things we can measure. If our filled up system is gapped, that means that the energetically cheapest (electronic) excitation of that system is the gap energy. Having gaps also restricts what processes can happen, since any quantum mechanical process has to take the system from some initial state to some final state. If there's no final state available that satisfies energy conservation, for example, the process can't happen. This means we can map out the gaps in the system by various spectroscopy experiments (e.g., photoemission; tunneling).

So, what happens in systems where the electron-electron interaction does matter a lot? In that case, you should think of the energy levels as rearranging and redistributing themselves depending on how many electrons are in the system. This all has to happen self-consistently. One particularly famous example of what can happen is the Mott insulating state. (Strictly speaking, I'm going to describe a version of this related to the Hubbard model.) Suppose there are N real-space sites, and N electrons to place in there. In the noninteracting case, the highest occupied level would not be near a gap - it would be in the middle of a band. Because the electrons can shuffle around in space without any particular cost to doubly occupying a site, the system would be a metal. However, suppose it costs an energy U to park two electrons on any site. The lowest energy state of the whole system would be each of the N sites occupied by one electron, with an energy gap of U separating that ground state from the first excited state. So, in the presence of strong interactions, at exactly "half-filling", you can end up with a gap. Even without this lattice site picture, in the presence of disorder, it's possible to see signs of the formation of a gap near the highest occupied level (for experts, in the weak disorder limit, this is the Altshuler-Aronov reduction in the density of states; in the strong disorder limit, it's the Efros-Shklovskii Coulomb gap).

Another kind of gap exists in the superconducting state. There is an energy gap between the superconducting ground state and the low lying excitations. In the high temperature superconductors, that gap is a bit weird, since there actually are low-lying excitations that correspond to electrons with very specific amounts of momentum ("nodal quasiparticles").

A pseudogap is more subtle. There isn't a "hard" gap, with zero states in it. Instead, the number of states near the highest occupied level is depressed relative to noninteracting expectations. That reduction and how it varies as a function of energy can tell you a lot about the underlying physics. One complicated aspect of high temperature superconductors is the existence of such a pseudogap well above the superconducting transition temperature. In conventional superconductors (e.g., lead), this doesn't exist. So, the question has been lingering for 25 years now, is the pseudogap the sign of incipient superconductivity (i.e., electrons are already pairing up, but they lack the special coherence required for actual superconductivity), or is it a sign of something else, perhaps something competing with superconductivity? That's still a huge question out there, complicated by the fact that doping the high-Tc materials to be superconductors adds disorder to the problem.

This is why micro/nanofab with new material systems is hard.

2011-02-21T22:15:00.000-06:00

Whenever I read a super-enthusiastic news story about how devices based on new material XYZ are the greatest thing ever and are going to be an eventual replacement for silicon-based electronics, I immediately think that the latter clause is likely not true. People have gotten very spoiled by silicon (and to a lesser degree, III-V compound semiconductors like GaAs), and no wonder: it's at the heart of modern technology, and it seems like we are always coaxing new tricks out of it. Of course, that's because there have been millions of person-years worth of research on Si. Any new material system (be it graphene, metal oxide heterostructures, or whatever) starts out behind the eight ball by comparison. This paper on the arxiv this evening is an example of why this business is hard. It's about Bi₂Se₃, one of the materials classified as "topological insulators". These materials are meant to be bulk insulators (well, at low enough temperature; this one is actually a fairly small band gap semiconductor), with special "topologically protected" surface states. One problem is, very often the material ends up doped via defects, making the bulk relatively conductive. Another problem, as studied in this paper, is that exposure to air, even for a very brief time, dopes the material further, and creates a surface oxide layer that seems to hurt the surface states. This sort of problem crops up with many materials. It's truly impressive that we've learned how to deal with these issues in Si (where oxygen is not a dopant, but does lead to a surface oxide layer very quickly). This kind of work is very important and absolutely needs to be done well....

You could, but would you want to?

2011-02-15T10:08:00.000-06:00

Texas governor Rick Perry has proposed (as a deliberately provocative target) that the state's (public) universities should be set up so that a student can get a bachelor's degree for $10,000 total (including the cost of books). Hey, I'm all for moon shot-type challenges, but there is something to be said for thinking hard about what you're suggesting. This plan (which would set costs per student cheaper than nearly all community colleges, by the way) is not well thought-out at all, which is completely unsurprising. To do this, the handwave argument is that professors should maximize online content for distance learning, and papers could be graded by graduate students or (apparently very cheaply hired) instructors. Even then, it's not clear that you could pull this off. Let me put it this way: I can argue that the world would benefit greatly from a solar electric car that costs $1,000, but that doesn't mean that one you'd want to own can actually be produced in an economically sustainable way at that price. This is classic Perry, though.

Battle hymn of the Tiger Professor

2011-02-13T21:12:00.000-06:00

Like Amy Chua, I'm choosing to be deliberately provocative in what I write below, though unlike her I don't have a book to sell. I recently heard a talk where a well reputed science educator (not naming names) argued that those of us teaching undergraduates need to adapt to the learning habits of "millennials". That is, these are a group of people who have literally grown up with google (a thought that makes me feel very old, since I went to grad school w/ Sergei Brin) - they are used to having knowledge (in the form of facts) at their fingertips in a fraction of a second. They are used to nearly continuous social networking, instantaneous communication, and constant multitasking (or, as a more stodgy person might put it, complete distraction, attention deficit behavior, and a chronic inability to concentrate). This academic argued that we need to make science education mimic real research if we want to produce researchers and get students jazzed about science. Moreover, this academic argued that making students listen to lectures and do problem sets was (a) ineffective, since that's not how they were geared to learn, and (b) somewhere between useless and abusive, being slavishly ruled by a culture of "covering material" without actually educating. Somehow we should be more in tune with how Millennials learn, and appeal to that, rather than being stodgy fogies who force dull, repetitious "exercises at the end of the chapter" work.

While appealing to students' learning modalities has its place, I contend that this concept simply will not work well in some introductory, foundational classes in the sciences, math, and engineering. Physical science (chemistry, physics) and math are inherently hierarchical. You simply cannot learn more advanced material without mastery of the underpinnings. Moreover, in the case of physics (with which I am most familiar), we're not just teaching facts (which can indeed be looked up easily on the internet); we're supposedly teaching analytical skills - how to think like a physicist; how to take a physical situation and translate it into math that enables us to solve for what we care about in terms of what we know. Getting good at this simply requires practice. To take the Amy Chua analogy, hard work is necessary and playdates are not. There literally is no substitute for doing problems and getting used to thinking this way. While open-ended reasoning exercises can be fun and useful (and could be a great addition to the standard curriculum, or perhaps a way to run a lab class to be more like real research), at some point students actually do need to become proficient in basic problem-solving skills. I really don't like the underlying assumption that this educator was making: that the twitter/facebook/short-attention-span approach is unavoidable and possibly superior to focused hard work. Hey, I'm part of the distractable culture as much as anyone in the 21st century, but you'll have to work hard to convince me that it's the right way to teach foundational knowledge in physics, math, and chemistry.

Science and the nation

2011-02-09T19:06:00.000-06:00

(The US, that is.) More people need to read this.

Triboelectricity and enduring mysteries of physics

2011-02-06T21:34:00.000-06:00

This past week I hosted Seth Putterman for a physics colloquium here at Rice, and one of the things he talked about is some of his group's work related to triboelectricity, or the generation of charge separation by friction/rubbing. When you think about it, it's quite amazing that we have no first-principles explanation of a phenomenon we're all shown literally as children (rub a balloon on your hair and it builds up enough "static" charge that it will stick to a plaster wall, unless you live in a very humid place like Houston). The amount of charge that may be moved is on the order of 10¹² electrons per square cm, and the resulting potential differences can measure in the tens of kilovolts (!), leading to remarkable observations like the generation of x-rays from peeling tape, or UV and x-ray emission from a mercury meniscus moving along a glass surface. In fact, there's still some disagreement about whether the charge moving in some triboelectric experiments is electrons or ions! Wild stuff.

Now that's an impressive capability.

2011-01-30T10:52:00.000-06:00

The Bad Astronomer periodically makes posts that show just how cool some astro phenomenon or astro observational capability can be. In keeping with this idea, I find this paper to be just damned impressive. (Apologies for the subscription-only link.) The investigators at Oxford University have one of the best and fanciest transmission electron microscopes (TEM) in the world. In TEM, a highly focused (on the atomic scale!) beam of electrons is fired through a very thin (under 100 nm thick) sample, and the transmitted electrons are analyzed as the beam is scanned over the sample surface. By using very clever electron optics techniques (aberration correction) and the right choice of samples, the investigators have been able to watch the motion of single atoms and few-atom clusters (of praesodymium, which has a big atomic number and therefore interacts strongly with the electron beam) within a carbon nanotube. They can study the formation of 1d crystals this way. Very impressive imaging tool. I want one :-)

Not even wrong.

2011-01-29T13:55:00.000-06:00

No, I'm not talking about Peter Woit's website or Wolfgang Pauli. Instead, I mean this article, which shows that Allstate Insurance apparently thinks that it's meaningful to look at car accident risk as a function of the astrological sign of the driver. Astrology? A major company using astrology? We're supposed to believe that there is a statistically meaningful correlation between the time of the year you're born and your driving ability? This is why there is a crying need for math and science literacy.

Cold fusion (err, low energy nuclear reactions) yet again.

2011-01-22T15:13:00.000-06:00

I'm starting to know how Phil Plait must feel every time he has to write yet another article about how Betelgeuse is not about to explode. (Though my readership is about 0.01% of the Bad Astronomer's)

Once again, there is a claim receiving attention from various media sources (here, here, here) that someone has demonstrated some gadget that produces so much "excess heat" that the conjectured source of the energy is some kind of nuclear reaction taking place in a condensed matter environment. This time, it's two Italian researchers, and they have demonstrated (in some very restricted way, more on this below) a device that they say uses a reaction involving nickel and ordinary hydrogen. The claim is that for a steady state input power of 400 watts, they can produce around 12 kW steady state of power in the form of heat. The device when running supposedly takes in room temperature water at some rate and outputs dry steam, and doing the enthalpy balance and water flow rate is how one gets the 12 kW figure. Crucially, the claim is that this whole process only consumes a tiny amount of hydrogen (far too little for some kind of chemical combustion to be the source of all the heat). The conjectured nuclear reaction is some pathway from 62Ni + p -> 63Cu. No big radiation produced, though of course the demo doesn't really allow proper measurements. Don't even bother reading the would-be theoretical "explanation" - it's ridiculously bad physics, and completely beside the point. What's really of interest is the experimental question.

As always in these cases, there are HUGE problems with all of this. The would-be paper is "published" in an online journal run by one of the claimants. The claimants won't let independent people examine the apparatus. They also don't do the completely obvious demonstration - setting up a version that runs in closed cycle (that is, take some of that 12 kW worth of steam flow, and generate the 400 W of electrical power needed to keep the apparatus running, and just let the system run continuously). If the process really is nuclear in origin, and the hydrogen accounting is correct, it should be possible to run such a system continuously for months or longer. The claimants say that they've been using a 10 kW version of such a unit to heat a factory in Italy for the past year, but they conveniently don't show that to anyone.

The burden of proof is on these people - if they've really done this, the world will beat a path to their door, and that would be great. I'm not buying my nickel futures yet, however. Once again there will be people out there who claim that evil scientists are suppressing these unorthodox geniuses; this is such a ridiculous mischaracterization of science that it still ticks me off every time I read it. Of course I wish this were a genuine discovery - it would be world-changing and reveal enormous new physics. However, so far no version of this kind of low energy nuclear reaction business has passed the bar of reasonable reproducibility in controlled circumstances. (See here for a past discussion concerning the palladium variety and its reproducibility. Read the comments there before posting angrily below that I don't understand the situation, or that I haven't looked at this, or that I'm otherwise hugely ignorant on the subject.) That's not the establishment being oppressive, it's the way good science works. Extraordinary claims require extraordinary evidence. The self-sustaining demo I described above with independent verification and measurements would go a long way. I'm not holding my breath.

Various and sundry

2011-01-18T20:29:00.000-06:00

Here are a number of links that may be of interest:

Back in December, Steven Blau at Physics Today wrote an interesting blog post about the arrogance of physicists. For some reason I just came across this today. Prof. Stone's comment on the post is, I think, right on the mark, and reminds me of this xkcd comic.

Here is a series of four blog posts (one, two, three, four) from Mike Mayberry at Intel, to give you a sense of some of the research directions they're pursuing as we near the possible end of scaling for conventional Si-based FETs. Very interesting stuff on the challenges of integrating other materials (like III-V compound semiconductors) with Si.

Veering into humor, here is a video made by Adam Ruben, whom I know through the alumni network of the Princeton Band. It's called "The Grad Student Rap", and it's part of the promotion for his book, Surviving Your Stupid, Stupid Decision to go to Graduate School.

This just in: a Nobel in medicine does not imply knowledge of basic physics.

2011-01-13T22:28:00.001-06:00

Having read something about this online, I had to see for myself. Take a look at this paper. One of the 2008 Nobel laureates for medicine is the lead author, and he claims that simply having certain kinds of DNA in water (1) creates electromagnetic waves at very low frequencies, like 7 Hz; (2) those waves are sufficiently strong that a simple pickup coil of copper wire can be used to detect them inductively; and (3) somehow those waves continue to self-propagate in a weird way so that repeated dilution of the solution preserves the "imprint" of those waves. Wow. The science here is so unbelievably bad, it's hard to imagine that this is serious. A pick-up coil?! No serious discussion of the magnitude of the effect, and whether it's even remotely credible that detectable inductive signals could be produced? Silly numerology demonstrating a complete lack of understanding of quantum mechanics? Impressive. Can we make a deal? Medicine laureates won't make crazy, misinformed claims about physics (which then naturally get picked up by the media, who love to report "the controversy", as if there is no such thing as a right or wrong answer to a scientific question), and physics laureates won't make crazy, misinformed claims about biology. Please?

Blast from the past

2011-01-13T10:42:00.000-06:00

Yesterday I received a very nice and welcome email from a faculty member who had been one of my best classroom instructors in graduate school. This email was, effectively, a reply to an email that I had sent him regarding Stanford's graduate physics curriculum. The amusing bit is that I had sent him that email 14 years ago, when I was a senior grad student representative to Stanford's physics graduate committee. At the time, there had been ongoing discussions about what topics should be in the first-year graduate curriculum, particularly the "mechanics" sequence, and my opinion had been asked for. It's interesting to look back now as a faculty member at what I'd suggested at the time. Here are the bullet point topics I'd suggested. Remember that Stanford is on the quarter system, meaning that there are three ten-week quarters during the regular academic year.

For "Mechanics of Particles" (basically graduate mechanics and dynamics), I'd said:

- Brief review of variational calculus
- Lagrangians and Hamiltonians, action principle
- Canonical transformations, phase space
- Symmetries and conservation laws (Noether's thm?)
- Normal modes, harmonic oscillator review
- Rigid body motion (numerical work?)
- Orbital mechanics review
- Classical perturbation theory (w/ orbits, rigid body dynamics, anharmonic oscillator)
- Action-angle variables
- Poisson brackets, symplectic structure (*definitions of 1-forms, tangent spaces, tangent bundles?)
- Chaos, nonlinear dynamics, ergodicity
- Brief review of Einstein summation convention
- Special relativity w/ Einstein summation convention, space-time diagrams

For "Continuum mechanics" (fairly unique, I now realize - many departments offer no such course), my suggestions reflected my undergrad engineering background to some degree. I now realize that what I list below is considerably too much for a 10 week course:

- Mechanics of solids:
+ Continuum mechanics version of Hooke's law; stress, strain, tension, compression, shear, bulk modulus, a few numbers about strength of materials, Young's modulus, shear modulus
+ Lagrangian/Hamiltonian densities, more variational calculus
+ *Flexure of beams, bending moments, areal moments of inertia (why I-beams are stiffer than rods of the same cross-sectional area)
+ *Torsion of members, polar "moments of inertia"
+ *Dynamics of beams: the wave equation, longitudinal and transverse sound, natural frequencies of cantilevers
+ Acoustics, idea of acoustic impedance and mismatch
- Fluid statics
+ Hydrostatics, Archimedes' principle, buoyancy
+ *Surface tension, capillary action, wetting
- Fluid mechanics
+ Euler and Lagrange pictures
+ "Convective derivatives", transport of momentum and energy
+ The energy equation, the momentum equation, the continuity equation, the Navier-Stokes equation
+ Inviscid, incompressible flow:
- Bernoulli's Eqn.
- Potential theory
- *Vorticity, circulation, Magnus' law, "lift"
+ Viscous, incompressible flow:
- Definition of viscosity, comparison w/ shear modulus, definition of Newtonian fluid
- Stoke's law
- Intro to dimensional analysis, Reynolds' number
- Laminar flow, parabolic velocity profile in a round pipe
- Turbulent flow, mention engineering approach to these problems (Moody chart, friction factor, Bernoulli w/ losses)
- Froud number, hydraulic jumps (example of a "shock" discontinuity that you can demonstrate in a sink)
+ Compressible flow
- Mention of shockwaves, scaling

For "Statistical Mechanics", the main challenge was dealing with the divergent backgrounds of incoming students - some people had very strong undergrad preparation in statistical and thermal physics, others much less so. This is an issue in graduate quantum mechanics to an even greater degree. Now that I've taught undergrad stat mech several times, I think what I listed below could use some additional advanced topics:

- Definition of entropy, why it's a log
- The equal prob. postulate/ergodic thm.
- The Boltzmann factor and the partition fn., Fermi and Dirac distributions
- *Mention of Feynman diagram methods, saddle-point integration to get Z in complicated systems
- The canonical and grand canonical ensembles, the chemical potential
- "Natural" variables, Legendre transforms, thermodynamic potentials, *the idea of a constrained maximization of S, the Maxwell relations, the "thermodynamic square"
- Gases
+ Ideal classical
+ Van der Waals, virial coefficients
+ Fermi gas at zero and finite T
+ Ideal Bose gas, BEC, phonons & photons *(incl. laser discussion!)
- Liquids - diagrammatic methods of treating interactions?
- Solids
+ Phonons
+ Concept of long-range order
- *Correlation functions, *connection w/ susceptibilities
- *Correlations and fluctuations, *how they're measured!
- Theories of phase transitions
+ Concept of order parameter
+ Ginsberg-Landau theory, diff. betw. 1st and 2nd order, extensions to include fluctuations
+ 1st order: Van der Waals reprise, Clausius-Clapeyron
+ Mean-field theory, example of magnetism
+ Ising model in 1-d
+ Renormalization group to solve Ising model, critical behavior, correlation length ideas
- *Transport
+ *Boltzmann equation
+ *Noise in transport: fluctuation/dissipation thm

It was definitely interesting to me to see how my thinking on this stuff has evolved now that I have to teach it.

Friction - sometimes electrons matter!

2011-01-09T20:49:00.001-06:00

While I don't do any research on the subject myself, over the last few years I've become more interested in the origins of friction, a subject about which almost no physics progress was made between from around 1650 to 1950. Since the development of the tools of surface science (ultrahigh vacuum, for example) and scanned probe microscopy, however, people have learned much about where friction comes from.

We all have an intuitive grasp of what friction is, and in freshman physics (or even high school), we learn that we can model friction as a (shear) force between two surfaces as they slide (or attempt to slide) relative to one another. That force is modeled as proportional to the normal force between the surfaces, with the surface-dependent friction coefficient as the proportionality constant. The force is further traditionally modeled as being independent of the contact area between the two surfaces, and independent of the relative speeds of the two surfaces (except for the distinction between static friction - with no relative motion - and kinetic or sliding friction). That approach does a very good job at describing many many experiments on friction between macroscopic objects.

The problem is, as many famous scientists (e.g., Coulomb) discovered, it's very difficult to come up with a microscopic model of the interaction between surfaces that has these properties. One of the essential difficulties is rather deep: friction has to result in real dissipation. Energy has to be transferred from macroscopic degrees of freedom (the motion of a hockey puck relative to the ice) into microscopic degrees of freedom (the relative vibrational motions of the atoms in the hockey puck, and similar motions of the atoms in the ice - heat, in short.). That transfer of energy from macroscopic coordinates to microscopic motions or coordinates is irreversible in the same sense that the motion of water in a pond is irreversible after a stone is tossed in. (Yes, it's physically conceivable from the point of view of Newton's laws that all the little bits of water at the edge of the pond could jiggle just right so as to send coordinated ripples inward toward the center of the pond, spitting the stone back out. However, that's incredibly unlikely, given all of the possible microscopic states of the water, so from the standpoint of macroscopic thermodynamics, the water rippling process is irreversible.)

There has been some beautiful work on friction at the nanoscale, and much of it has focused on chemical interactions between surfaces, as well as vibrations (phonons) as the relevant microscopic degrees of freedom. However, in the case of metals, there are other excitations where the energy could end up: electrons! That's one defining characteristic of a metal, the existence of possible electronic excitations of (almost) arbitrarily low energy. How can you tell if the energy is ending up in the electrons? Well, you'd really like to do an experiment where none of the vibrational properties are changed, but that allows you to compare between with-electrons and without-electrons. Amazingly, it is possible to do something close to that by working with a metal that is superconducting! Above the superconducting transition temperature, T_c, the metal has plenty of low energy electronic excitations. Below T_c, however, in the superconducting state, electronic excitations are forbidden below some threshold energy (this "gap" in the excitation spectrum is one key reason why superconductors have no electrical resistance). In this new paper (sorry about not having an arxiv version to link), the investigators have demonstrated that the (noncontact) friction between a metal tip and a niobium film drops dramatically once the niobium becomes superconducting. This argues that electronic dissipation is responsible for much of the friction in this case (in the normal state). I should point out that previous work with lead films had hinted at similar physics. The new experiment is very clear and benefits from technique developments in the meantime.

Spin-orbit coupling

2011-01-06T22:09:00.000-06:00

Happy new year! I want to write a little about what physicists call spin-orbit interactions. It turns out that there is a deep connection between electric and magnetic fields that can be made somewhat obvious by considering a thought experiment. (For a great discussion of this, see the textbook by Purcell.) Imagine a line of stationary positive charges. From our perspective (at rest relative to the line of charges), there is no current, so one should see an electric field pointed radially outward from the line of charges, and a positive charge placed next to the line of charges should respond accordingly, being pushed radially outward. Now consider viewing this from a reference frame moving parallel to the line of charges. From our point of view in that frame, we see a current, and therefore there should be a magnetic field associated with that current (as well as an electric field from the net positive charge). In special relativity, one can figure out how electric and magnetic fields transform into and out of each other when changing reference frames.

This shift of point of view is the way that spin-orbit coupling is usually explained in undergrad quantum mechanics. Consider a hydrogen atom. The electron zipping around the proton has a spin degree of freedom, and a corresponding magnetic moment. From the point of view of the (classically) moving electron, the proton is essentially a current producing a magnetic field, which will tend to align the electron magnetic moment. This couples the spin of the electron to the orbital motion of the electron; hence the name "spin-orbit coupling"; and it is technically a relativistic effect which tends to be bigger in heavier atoms.

Why should you care? Well, spin-orbit coupling can be important in solids, too, since one can think of their electronic states as being built out of atomic orbitals. As ZapperZ points out, a recent paper shows that these kinds of relativistic corrections are not necessarily tiny in ordinary, everyday solids. In fact, it appears that it is essential to worry about such relativistic effects in order to understand why the electrochemical redox potentials of an ordinary car battery are what they are!

Behold the power of good lab notebook practice!

2010-12-29T16:53:00.000-06:00

I was just able to help out my postdoc by pulling an old Bell Labs notebook from 11.5 years ago off my bookshelf and showing him a schematic of an electrical measurement technique. This is an object lesson in why it is a good idea to keep a clear, complete lab notebook! I try very hard to impress upon undergrad and graduate students alike that it's critically important to keep good notes, even (perhaps especially) in these days of electronic data acquisition and analysis. I've never once looked back and regretted how much time I spent writing things down, or how much paper I used - good record keeping has saved my bacon (and lots of time) on multiple occasions. Unfortunately, with rare exceptions, students come in to the university (at the undergrad or grad levels) and seem determined to write as little as possible down using as few sheets of paper as they can manage. Somewhere along the way (before grad school, though my thesis advisor was outstanding about this), it got pounded into my brain: if you didn't document it, you didn't do it. Perhaps we should make a facebook-like or twitter-like application that would sucker student researchers into obsessively updating their work status....

Statistical mechanics: still work to be done!

2010-12-28T21:36:00.000-06:00

Statistical mechanics, the physics of many-particle systems, is a profound intellectual achievement. A statistical approach to systems with many degrees of freedom makes perfect sense. It's ridiculous to think about solving Newton's laws (or the Schroedinger equation, for that matter) for all the gas molecules in this room. Apart from being computationally intractable, it would be silly for the vast majority of issues we care about, since the macroscopic properties of the air in the room are approximately the same now as they were when you began reading this sentence. Instead of worrying about every molecule and their interactions, we characterize the macroscopic properties of the air by a small number of parameters (the pressure, temperature, and density). The remarkable achievement of statistical physics is that it places this on a firm footing, showing how one can go from the microscopic degrees of freedom, through a statistical analysis, and out the other side with the macroscopic parameters.

However, there are still some tricky bits, and even equilibrium statistical mechanics continues to be an active topic of research. For example, one key foundation of statistical mechanics is the idea of replacing time-averages with energy-averages. When we teach stat mech to undergrads, we usually say something like, a system explores all of the microscopic states available to it (energetically, and within other constraints) with equal probability. That is, if there are five ways to arrange the system's microscopic degrees of freedom, and all five of those ways have the same energy, then the system in equilibrium is equally likely to be found in any of those five "microstates". How does this actually work, though? Should we think of the system somehow bopping around (making transitions) between these microstates? What guarantees that it would spend, on average, equal time in each? These sorts of issues are the topic of papers like this one, from the University of Tokyo. Study of these issues in classical statistical mechanics led to the development of ergodic theory (which I have always found opaque, even when described by the best in the business). In the quantum limit, when one must worry about physics like entanglement, this is still a topic of active research, which is pretty cool.

Science's Breakthrough of the Year for 2010

2010-12-20T15:06:00.003-06:00

Science Magazine has named the work of a team at UCSB directed by Andrew Cleland and John Martinis as their scientific breakthrough of the year for 2010. Their achievement: the demonstration of a "quantum machine". I'm writing about this for two reasons. First, it is extremely cool stuff that has a nano+condensed matter focus. Second, this article and this one in the media have so many things wrong with them that I don't even know where to begin, and upon reading them I felt compelled to try to give a better explanation of this impressive work.

One of the main points of quantum mechanics is that systems tend to take in or emit energy in "quanta" (chunks of a certain size) rather than in any old amount. This quantization is the reason for the observation of spectral lines, and mathematically is rather analogous to the fact that a guitar string can ring at a discrete set of harmonics and not any arbitrary frequency. The idea that a quantum system at low energies can have a very small number of states each corresponding to a certain specific energy is familiar (in slightly different language) to every high school chemistry student who has seen s, p, and d orbitals and talked about the Bohr model of the atom. The quantization of energy shows up not just in the case of electronic transitions (that we've discussed so far), but also in mechanical motion. Vibrations in quantum mechanics are quantized - in quantum mechanics, a perfect ball-on-a-spring mechanical oscillator with some mechanical frequency can only emit or absorb energy in amounts of size hf, where h is Planck's constant. Furthermore, there is some lowest energy allowed state of the oscillator called the "ground state". Again, this is all old news, and such vibrational quantization is clear as a bell in many spectroscopy techniques (infrared absorption; Raman spectroscopy).

The first remarkable thing done by the UCSB team is to manufacture a mechanical resonator containing millions of atoms, and to put that whole object into its quantum ground state (by cooling it so that the thermal energy scale is much smaller than hf for that resonator). In fact, that's the comparatively easy part. The second (and really) remarkable thing that the UCSB team did was to confirm experimentally that the resonator really was in its ground state, and to deliberately add and take away single quanta of energy from the resonator. This is very challenging to do, because quantum states can be quite delicate - it's very easy to have your measurement setup mess with the quantum system you're trying to study!

What is the point? Well, on the basic science side, it's of fundamental interest to understand just how complicated many particle systems behave when they are placed in highly quantum situations. That's where much of the "spookiness" of quantum physics lurks. On the practical side, the tools developed to do these kinds of experiments are one way that people like Martinis hope to build quantum computers. I strongly encourage you to watch the video on the Science webpage (should be free access w/ registration); it's a thorough discussion of this impressive achievement.

Taking temperatures at the molecular scale

2010-12-14T16:59:00.000-06:00

As discussed in my previous post, temperature may be associated with how energy is distributed among microscopic degrees of freedom (like the vibrational motion of atoms in a solid, or how electrons in a metal are placed into the allowed electronic energy levels). Moreover, it takes time for energy to be transferred (via "inelastic" processes) among and between the microscopic degrees of freedom, and during that time electrons can actually move pretty far, on the nano scale of things. This means that if energy is pumped into the microscopic degrees of freedom somehow, it is possible to drive those vibrations and electronic distributions way out of their thermal equilibrium configurations.

So, how can you tell if you've done that? With macroscopic objects, you can think about still describing the nonequilibrium situation with an effective temperature, and measuring that temperature with a thermometer. For example, when cooking a pot roast in the oven (this example has a special place in the hearts of many Stanford graduate physics alumni), the roast is out of thermal equilibrium but in an approximate steady state. The outside of the roast may be brown, crisp, and at 350 F, while the inside of the pot roast may be pink, rare, and 135 F. You could find these effective temperatures (effective because strictly speaking temperature is an equilibrium parameter) by sticking a probe thermometer at different points on the roast, and as long as the thermometer is small (little heat capacity compared to the roast), you can measure the temperature distribution.

What about nanoscale systems? How can you look at the effective temperature or how the energy is distributed in microscopic degrees of freedom, since you can't stick in a thermometer? For electrons, one approach is to use tunneling (see here and here), which is a topic for another time. In our newest paper, we use a different technique, Raman spectroscopy.

In Raman, incoming light hits a system under study, and comes out with either less energy than it had before (Stokes process, dumping energy into the system) or more energy than it had before (anti-Stokes process, taking energy out of the system). By comparing how much anti-Stokes Raman you get vs. how much Stokes, you can back out info about how much energy was already in the system, and therefore an effective temperature (if that's how you want to describe the energy distribution). You can imagine doing this while pushing current through a nanosystem, and watching how things heat up. This has been done with the vibrational modes of nanotubes and graphene, as well as large ensembles of molecules. In our case, through cool optical antenna physics, we are able to do Raman spectroscopy on nanoscale junctions, ideally involving one or two molecules. We see that sometimes the energy input to molecular vibrations from the Raman process itself is enough to drive some of those vibrations very hard, up to enormous effective temperatures. Further, we can clearly see vibrations get driven as current is ramped up through the junction, and we also see evidence (from Raman scattering off the electrons in the metal) that the electrons themselves can get warm at high current densities. This is about as close as one can get to interrogating the energy distributions at the single nanometer scale in an electrically active junction, and that kind of information is important if we are worried about how dissipation happens in nanoelectronic systems.

Temperature, thermal equilibrium, and nanoscale systems

2010-12-13T21:09:00.000-06:00

In preparation for a post about a new paper from my group, I realized that it will be easier to explain why the result is cool if I first write a bit about temperature and thermal equilibrium in nanoscale systems. I've tried to write about temperature before, and in hindsight I think I could have done better. We all have a reasonably good intuition for what temperature means on the macroscopic scale: temperature tells us which way heat flows when two systems are brought into "thermal contact". A cool coin brought into contact with my warm hand will get warmer (its temperature will increase) as my hand cools down (its temperature will locally decrease). Thermal contact here means that the two objects can exchange energy with each other via microscopic degrees of freedom, such as the vibrational jiggling of the atoms in a solid, or the particular energy levels occupied by the electrons in a metal. (This is in contrast to energy in macroscopic degrees of freedom, such as the kinetic energy of the overall motion of the coin, or the potential energy of the coin in the gravitational field of the earth.)

We can turn that around, and try to use temperature as a single number to describe how much energy is distributed in the (microscopic) degrees of freedom. This is not always a good strategy. In the coin I was using as an example, you can conceive of many ways to distribute vibrational energy. Number all the atoms in the coin, and have the even numbered atoms moving to the right and the odd numbered atoms moving to the left at some speed at a given instant. That certainly would have a bunch of energy tied up in vibrational motion. However, that weird and highly artificial arrangement of atomic motion is not what one would expect in thermal equilibrium. Likewise, you could imagine looking at all the electronic energy levels possible for the electrons in the coin, and popping every third electron each up to some high unoccupied energy level. That distribution of energy in the electrons is allowed, but not the sort of thing that would be common in thermal equilibrium. There are certain vibrational and electronic distributions of energy that are expected in thermal equilibrium (when the system has sat long enough that it has reached steady-state as far as its statistical properties are concerned).

How long does it take a system to reach thermal equilibrium? That depends on the system, and this is where nanoscale systems can be particularly interesting. For example, there is some characteristic timescale for electrons to scatter off each other and redistribute energy. If you could directly dump in electrons with an energy 1 eV (one electron volt) above the highest occupied electronic level of a piece of metal, it would take time, probably tens of femtoseconds, before those electrons redistributed their energy by sharing it with the other electrons. During that time period, those energetic electrons can actually travel rather far. A typical (classical) electron velocity in a metal is around 10⁶ m/s, meaning that the electrons could travel tens of nanometers before losing their energy to their surroundings. The scattering processes that transfer energy from electrons into the vibrations of the atoms can be considerably slower than that!

The take-home messages:

1) It takes time for electrons and vibrations arrive at a thermal distribution of energy described by a single temperature number.

2) During that time, electrons and vibrations can have energy distributed in a way that can be complicated and very different from thermal distributions.

3) Electrons can travel quite far during that time, meaning that it's comparatively easy for nanoscale systems to have very non-thermal energy distributions, if driven somehow out of thermal equilibrium.

More tomorrow.

NSF grants and "wasteful spending"

2010-12-11T14:34:00.000-06:00

Hat tip to David Bacon for highlighting this. Republican whip Eric Cantor has apparently decided that the best way to start cutting government spending is to have the general public search through NSF awards and highlight "wasteful" grants that are a poor use of taxpayer dollars.

Look, I like the idea of cutting government spending, but I just spent two days in Washington DC sitting around a table with a dozen other PhD scientists and engineers arguing about which 12% of a large group of NSF proposals were worth trying to fund. I'm sure Cantor would brand me as an elitist for what I'm about to write, but there is NO WAY that the lay public is capable of making a reasoned critical judgment about the relative merits of 98% of NSF grants - they simply don't have the needed contextual information. Bear in mind, too, that the DOD budget is ONE HUNDRED TIMES larger than the NSF budget. Is NSF really the poster child of government waste? Seriously?

The tyranny of reciprocal space

2010-12-07T22:56:00.001-06:00

I was again thinking about why it can be difficult to explain some solid-state physics ideas to the lay public, and I think part of the problem is what I call the tyranny of reciprocal space. Here's an attempt to explain the issue in accessible language. If you want to describe where the atoms are in a crystalline solid and you're not a condensed matter physicist, you'd either draw a picture, or say in words that the atoms are, for example, arranged in a periodic way in space (e.g., "stacked like cannonballs", "arranged on a square grid", etc.). Basically, you'd describe their layout in what a condensed matter physicist would call real space. However, physicists look at this and realize that you could be much more compact in your description. For example, for a 1d chain of atoms a distance a apart from each other, a condensed matter physicist might describe the chain by a "wavevector" k = 2 \pi/a instead. This k describes a spatial frequency; a wave (quantum matter has wavelike properties) described by cos kr would go through a complete period (peak of wave to peak of wave, say) and start repeating itself over a distance a. Because k has units of 1/length, this wavevector way of describing spatially periodic things is often called reciprocal space. A given point in reciprocal space (k_x, k_y, k_z) implies particular spatial periodicities in the x, y, and z directions.

Why would condensed matter physicists do this - purely to be cryptic? No, not just that. It turns out that a particle's momentum (classically, the product of mass and velocity) in quantum mechanics is proportional to k for the wavelike description of the particle. Larger k (shorter spatial periodicity), higher momentum. Moreover, trying to describe the interaction of, e.g., a wave-like electron with the atoms in a periodic lattice is done very neatly by worrying about the wavevector of the electron and the wavevectors describing the lattice's periodicity. The math is very nice and elegant. I'm always blown away when scattering experts (those who use x-rays or neutrons as probes of material structure) can glance at some insanely complex diffraction pattern, and immediately identify particular peaks with obscure (to me) points in reciprocal space, thus establishing the symmetry of some underlying lattice.

The problem is, from the point of view of the lay public (and even most other branches of physics), essentially no one thinks in reciprocal space. One of the hardest things you (as a condensed matter physicist) can do to an audience in a general (public or colloquium) talk is to start throwing around reciprocal space without some preamble or roadmap. It just shuts down many nonexperts' ability to follow the talk, no matter how pretty the viewgraphs are. Extreme caution should be used in talking about reciprocal space to a general audience! Far better to have some real-space description for people to hang onto.

A seasonal abstract

2010-12-03T22:56:00.000-06:00

On the anomalous combustion of oleic and linoleic acid mixtures

J. Maccabeus et al., Hebrew University, Jerusalem, Judea

Olive-derived oils, composed primarily of oleic and linoleic fatty acids, have long been used as fuels, with well characterized combustion rates. We report an observation of anomalously slow combustion of such a mixture, with a burn rate suppressed relative to the standard expectations by more than a factor of eight. Candidate explanations for these unexpectedly slow exothermic reaction kinetics are considered, including the possibility of supernatural agencies intervening to alter the local passage of time in the vicinity of the combustion vessel.

(Happy Hanukkah, everyone.)
(Come on, admit it, this is at least as credible as either this or this.)

Writing exams.

2010-11-29T22:33:00.000-06:00

Writing (or perhaps I should say "creating", for the benefit of UK/Canada/Australia/NZ grammarians) good exams is not a trivial task. You want very much to test certain concepts, and you don't want the exam to measure thing you consider comparatively unimportant. For example, the first exam I ever took in college was in honors mechanics; out of a possible 30 points, the mean was a 9 (!), and I got a 6 (!!). Apart from being a real wake-up call about how hard I would have to apply myself to succeed academically, that test was a classic example of an exam that did not do its job. The reason the scores were so low is that the test was considerably too long for the time allotted. Rather than measuring knowledge of mechanics or problem solving ability, the test largely measured people's speed of work - not an unimportant indicator (brilliant, well-prepared people do often work relatively quickly), but surely not what the instructor cared most about, since there usually isn't a need for raw speed in real physics or engineering.

Ideally, the exam will have enough "dynamic range" that you can get a good idea of the spread of knowledge in the students. If the test is too easy, you end up with a grade distribution that is very top-heavy, and you can't distinguish between the good and the excellent. If the test is too difficult, the distribution is soul-crushingly bottom-heavy (leading to great angst among the students), and again you can't tell between those who really don't know what's going on and those who just slipped up. Along these lines, you also need the test to be comparatively straightforward to take (step-by-step multipart problems, where there are still paths forward even if one part is wrong) and to grade.

Finally, in an ideal world, you'd actually like students to learn something from the test, not just have it act purely as a hurdle to be overcome. This last goal is almost impossible to achieve in classes so large that multiple choice exams are the only real option. It is where exam writing can be educational for the instructor as well, though - nothing quite like starting out to write a problem, only to realize partway through that the situation is more subtle than you'd first thought! Ahh well. Back to working on my test questions.

Memristors - how fundamental, and how useful?

2010-11-18T21:18:00.000-06:00

You may have heard about an electronic device called a memristor, a term originally coined by Leon Chua back in 1971, and billed as the "missing fourth fundamental circuit element". It's worth taking a look at what that means, and whether memristors are fundamental in the physics sense that resistors, capacitors, and inductors are. Note that this is an entirely separate question from whether such devices and their relatives are technologically useful!

In a resistor, electronic current flows in phase with the voltage drop across the resistor (assuming the voltage is cycled in an ac fashion). In the dc limit, current flows in steady state proportional to the voltage, and power is dissipated. In a capacitor, in contrast, the flow of current builds up charge (in the usual parallel plate concept, charge on the plates) that leads to the formation of an electric field between conducting parts, and hence a voltage difference. The current leads the voltage (current is proportional to the rate of change of the voltage); when a constant voltage is specified, the current decreases to zero once that voltage is achieved, and energy is stored in the electric field of the capacitor. In an inductor, the voltage leads the current - the voltage across an inductor, through Faraday's law, is proportional to the rate at which the current is changing. Note that in a standard inductor (usually drawn as a coil of wire), the magnetic flux through the inductor is proportional to the current (flux = L I, where L is the inductance). That means that if a certain current is specified through the inductor, the voltage drops to zero (in the ideal, zero-resistance case), and there is energy stored in the magnetic field of the inductor. Notice that there is a duality between the inductor and capacitor cases (current and voltage swapping roles; energy stored in either electric or magnetic field).

Prof. Chua said that one could think of things a bit differently, and consider a circuit element where the magnetic flux (remember, in an inductor this would be proportional to the time integral of the voltage) is proportional to the charge that has passed through the device (the time integral of the current (rather than the current itself in an inductor)). No one has actually made such a device, in terms of magnetic flux. However, what people have made are any number of devices where the relationship between current and voltage depends on the past history of the current flow through the device. One special case of this is the gadget marketed by HP as a memristor, consisting of two metal electrodes separated by a titanium oxide film. In that particular example, at sufficiently high bias voltage, the flow of current through the device performs electrochemistry on the titanium oxide, either reducing it to titanium metal, or oxidizing it further, depending on the polarity of the flow. The result is that the resistance (the proportionality between voltage and current; in the memristor language, the proportionality between the time integral of the voltage and the time integral of the current) depends on how much charge has flowed through the device. Voila, a memristor.

I would maintain that this is conceptually very different and less fundamental than the resistor, capacitor, or inductor elements. The resistor is the simplest possible relationship between current and voltage; the capacitor and inductor have a dual relationship and each involve energy storage in electromagnetic fields. The memristor does not have a deep connection to electromagnetism - it is one particular example of the general "mem"device, which has a complex electrical impedance that depends on the current/voltage history of the device. Indeed, my friend Max di Ventra has, with a colleague, written a review of the general case, which can be said to include "memcapacitors" and "meminductors". The various memgizmos are certainly fun to think about, and in their simplest implementation have great potential for certain applications, such as nonvolatile memory.

Great moments in consumer electronics

2010-11-15T22:06:00.000-06:00

It's been an extremely busy time of the semester, and there appears to be no end in sight. There will be more physics posts soon, but in the meantime, I have a question for those of you out there that have Nintendo Wii consoles. (The Wii is a great example of micromachining technology, by the way, since the controller contains a 3-axis MEMS accelerometer, and the Wii Motion Plus also contains a micromachined gyroscope.) Apparently, if there is a power glitch, it is necessary to "reset your AC adapter" in order to power on the console. The AC adapter looks for all the world like an ordinary "brick" power supply, which I would think should contain a transformer, some diodes, capacitors, and probably voltage regulators. Resetting it involves unplugging it from both ends (the Wii and the power strip), letting it sit for two solid minutes, and then plugging it back directly into a wall outlet (not a power strip). What the heck did Nintendo put in this thing, and why does that procedure work, when plugging it back into a power strip does not?! Does Nintendo rely on poorly conditioned power to keep the adapter happy? Is this all some scheme so that they can make sure you're not trying to use a gray-market adapter? This is so odd that it seemed like the only natural way to try to get to the bottom of it (without following my physicist's inclination of ripping the adapter apart) was to ask the internet.

Paul Barbara

2010-11-10T08:34:00.000-06:00

I was shocked and saddened to learn of the death of Paul Barbara, a tremendous physical chemist and National Academy of Sciences member at the University of Texas. Prof. Barbara's research focused largely on electron transfer and single-molecule spectroscopy, and I met him originally because of a mutual interest in organic semiconductors. He was very smart, funny, and a class act all the way, happy to talk science with me even when I was a brand new assistant professor just getting into our field of mutual interest. He will be missed.