Work functions - a challenge of molecular-scale electronics

This past week I was fortunate enough to attend this workshop at Trinity College, Dublin, all about the physics of atomic- and molecular-scale electronics.  It was a great meeting, and I feel like I really learned several new things (some of which I may elaborate upon in future posts).  One topic that comes up persistently when looking at this subject is the concept of the work function, defined typically as the minimum amount of energy it takes to kick an electron completely out of a material (so that it can go "all the way to infinity", rather than being bound to the material somehow).  As Einstein and others pointed out when trying to understand the photoelectric effect, each material has an intrinsic work function that can be measured, in principle, using photoemission.  You can hit a material surface with ultraviolet light and measure the energy of the electrons that get kicked out (for example, by slowing them down with an electric field and seeing how long it takes them to arrive at a detector).  Alternately, with a fancy tunable light source like a synchrotron, you can dial around the energy of the incident light and see when electrons start getting kicked out.   As you might imagine, if you are trying to understand electronic transport, where an electron has to leave one electrode, traverse through a system such as a molecule, and end up back in another electrode, the work function is important to know.

One problem with work functions is, they are extremely sensitive to the atomic-scale details of a surface.  For example, different crystallographic faces of even the same material (e.g., gold) can have work functions that differ by a couple of hundred millielectronvolts (meV).  Remember, the thermal energy scale at room temperature is 25 meV or so, so these are not small differences.  Moreover, anything that messes with the electronic cloud that spills a little out of the surface of materials at the atomic scale can alter the work function.  Adsorbed impurities on metal surfaces can change the effective work function by more than 1 eV (!).  To see how tricky this gets, imagine chemically assembling a layer of covalently bound molecules on a metal surface.  There is some charge transfer where the molecule chemically bonds to the metal, leading to an electric dipole moment and a corresponding change in work function.  The molecule itself can also polarize or be inherently polar based on its structure.  In the end, ordinary photoemission measures just the total of all of these effects.  Finally, ponder what then happens if the other end of the molecules is also tethered chemically to a piece of metal.  How big are all the dipole shifts?  What is the actual energy landscape "seen" by an electron going from one metal to the other, and is there any way to measure it experimentally, let alone compute it reliably from quantum chemistry methods?  Really understanding the details is difficult yet ultimately essential for progress here.

Catalysis seems like magic.

In our most recent paper, we found that we could dope a particularly interesting material, vanadium dioxide, with atomic hydrogen, via "catalytic spillover". By getting hydrogen in there in interstitial sites, we could dramatically alter the electrical properties of the material, allowing us to stabilize its unusual metallic state down to low temperatures. The funkiest part of this to me is the catalysis part. The metal electrodes that we use for electronic measurements have enough catalytic activity that they can split hydrogen molecules into atomic hydrogen at an appreciable rate even under very modest conditions (e.g., not much warmer than the boiling point of water). This paper (sorry it is subscription only) shows an elegant experimental demonstration of this, where gold is exposed to H2 and D2 gas and HD molecules are then detected. I would love to understand the physics at work here better. Any recommendations for a physics-based discussion would be appreciated - I know there is enormous empirical and phenomenological knowledge about this stuff, but something closer to an underlying physics description would be excellent.

 

Vanity journals: you've got to be kidding me.

I just received the following email:

Dear Pro. ,
Considering your research in related areas, we cordially invite you to submit a paper to Modern Internet of Things (MIOT).

The Journal of Modern Internet of Things (MIOT) is published in English, and is a peer reviewed free-access journal which provides rapid publications and a forum for researchers, research results, and knowledge on Internet of Things. It serves the objective of international academic exchange.

Wow!  I feel so honored, given my vast research experience connected to "Internet of Things". 

The publisher should be shamed over this.  This is absurd, and while amusing, shows that there is something deeply sick about some parts of academic publishing.

The unreasonable clarity of E. M. Purcell

Edward Purcell was one of the great physicists of the 20th century.  He won the Nobel Prize in physics for his (independent) discovery of nuclear magnetic resonance, and was justifiably known for the extraordinarily clarity of his writing.  He went on to author the incredibly good second volume of the Berkeley Physics Course (soon to be re-issued in updated form by Cambridge University Press), and late in life became interested in biophysics, writing the evocative "Life at Low Reynolds Number" (pdf).   

Purcell is also known for the Purcell Factor, a really neat bit of physics.  As I mentioned previously, Einstein showed through a brilliant thermodynamic argument that it's possible to infer the spontaneous transition rate for an emitter in an excited state dropping down to the ground state and spitting out a photon.  The spontaneous emission rate is related to the stimulated rate and the absorption rate.  Both of the latter two may be calculated using "Fermi's Golden Rule", which explains (with some specific caveats that I won't list here) that the rate of a quantum mechanical radiative transition for electrons (for example) is proportional to (among other things) the density of states (number of states per unit energy per unit volume) of the electrons and the density of states of the photons.  The density of states for photons in 3d can be calculated readily, and is quadratic in frequency.  

Purcell had the insight that in a cavity, the number of states available for photons is not quadratic in frequency anymore.  Instead, a cavity on resonance has a photon density of states that is proportional to the "quality factor", Q,  of the cavity, and inversely proportional to the size of the cavity.  The better the cavity and the smaller the cavity, the higher the density of states at the cavity resonance frequency, and off-resonance the photon density of states approaches zero.  This means that the spontaneous emission rate of atoms, a property that seems like it should be fundamental, can actually be tuned by the local environment of the radiating system.  The Purcell factor is the ratio of the spontaneous emission rate with the cavity to that in free space.

While I was doing some writing today, I decided to look up the original citation for this idea.  Remarkably, the "paper" turned out to be just an abstract!  See here, page 681, abstract B10.  That one paragraph explains the essential result better than most textbooks, and it's been cited a couple of thousand times.  This takes over as my new favorite piece of clear, brief physics writing by a famous scientist, displacing my long-time favorite, Nyquist's derivation of thermal noise.  Anyone who can be both an outstanding scientist and a clear writer gets bonus points in my view.

Models and how physics works

Thanks to ZapperZ for bringing this to my attention. This paper is about to appear in Phys Rev Letters, and argues that the Lorentz force law (as written to apply to magnetic materials, not isolated point charges) is incompatible with Special Relativity. The argument includes a simple thought experiment. In one reference frame, you have a point charge and a little piece of magnetic material. Because the magnet is neutral (and for now we ignore any dielectric polarization of the magnet), there is no net force on the charge or the magnet, and no net torque on the magnet either. Now consider the situation when viewed from a frame moving along a line perpendicular to the line between the magnet and the charge. In the moving frame, the charge seems to be moving, so that produces a current. However (and this is the essential bit!), in first year physics, we model permanent magnetization as a collection of current loops. If we then consider what those current loops look like in the moving frame, the result involves an electric dipole moment, meaning that the charge should now exert a net torque on the magnet when all is said and done. Since observers in the two frames of reference disagree on whether a torque exists, there is a problem! Now, the author points out that there is a way to fix this, and it involves modifying the Lorentz force law in terms of how it treats magnetization, M (and electric polarization, P). This modification was already suggested by Einstein and a coauthor back in 1908.

I think (and invite comments one way or the other) that the real issue here is that our traditional way to model magnetization is unphysical at the semiclassical level. You really shouldn't be able to have a current loop that persists, classically. A charge moving in a loop is accelerating all the time, and should therefore radiate. By postulating no radiation and permanent current loops, we are already inserting something fishy in terms of our treatment of energy and momentum in electrodynamics right at the beginning. The argument by the author of the paper seems right to me, though I do wonder (as did a commenter in ZZ's post) whether this all would have been much more clear if it had been written out in four-vector/covariant notation rather that conventional 3-vectors.

This raises a valuable point about models in physics, though. Our model of M as resulting from current loops is extremely useful for many situations, even though it is a wee bit unphysical. We only run into trouble when we push the model beyond where it should ever have been expected to be valid. The general public doesn't always understand this distinction - that something can be a little wrong in some sense yet still be useful. Science journalists and scientists trying to reach the public need to keep this in mind. Simplistically declaring something to be wrong, period, is often neither accurate nor helpful.

 

Heat flow at the mesoscale

When we teach about thermal physics at the macroscopic scale, we talk in terms of the thermal conductivity, k.  For the 1d problem of a homogeneous rod of cross sectional area A and length L, the rate that energy flows from one end of the rod to the other is given by (kA/L)(T_h-T_c), where T_h and T_c are the temperatures of the hot and cold ends of the rod, respectively.  Built into this approach is the tacit assumption that the phonons, the quantized vibrational modes of the lattice that carry what we consider to be the thermal energy of the atoms in the solid, move in a diffusive way.  That is, if a phonon is launched, it bounces many times in a random walk sort of motion before it traverses across our region of interest.  Phonons can scatter off disorder in the lattice, or mobile charge carriers (or even each other, if the vibrations aren't perfectly harmonic).  

However, phonon motion doesn't have to be diffusive!   If phonons don't scatter while propagating a certain length scale, their motion is said to be "ballistic".  In this paper, the authors have done a very clever experiment to look at whether there is a significant contribution of ballistic phonons to heat transport in silicon at room temperature on scales considerably longer than the "textbook" mean free path for phonon scattering under those conditions, about 40 nm.  The authors use the interference pattern between two "pump" lasers to produce a (sin^2) intensity pattern (and thus, because of absorption and the electron-lattice coupling, a (sin^2) pattern of elevated temperature) in a suspended Si membrane.  The change in local temperature leads to a small change in local index of refraction.  A low intensity "probe" laser can diffract off the grating pattern set up by the temperature variation.  Depending on how long one waits between pump and probe, the temperature pattern can wash itself out due to phonon transport.  So, by varying the delay between pump and probe and looking at the strength of the diffracted probe signal, they can monitor the time evolution of the temperature profile.  By changing the pitch of the initial interferogram, they can look at thermal transport over different length scales.   They find that there are significant deviations from the expectations of diffusive phonon transport (originally worked out by Klaus Fuchs, among others) up to micron scales, which is pretty darn cool, and important for understanding heat flow in, e.g., computer chips.   Very elegantly done.

Persistent currents and an impressive experiment

A long while ago, I brought up the topic of persistent currents in normal metal rings.  Please click the link to get the context.  The point is, even in a normal metal (as opposed to a superconductor), if you consider a metal ring small enough that the electrons remain quantum mechanically coherent in going about the ring, the electronic wavefunction must remain single-valued.  That means that the quantum mechanical phase accumulated by an electron diffusing around the ring back to its starting point (to speak in a semiclassical way) has to add up to an integer multiple of 2 pi. Since magnetic flux through the ring tweaks the accumulated phase (via the Aharonov-Bohm effect), a persistent current develops in the ring to make sure that the total phase (that from the electron motion and that from the resulting Aharonov-Bohm contribution) add up to a multiple of 2 pi.  As I'd discussed before, these currents and the magnetic fields they produce tend to be quite small and difficult to detect.

To make matters worse, when an electron scatters off static disorder in a solid, it acquires a phase shift that depends on that particular scattering site.  What this really means is, if you consider an ensemble of nominally identical metal rings, you'll actually get some distribution of persistent currents, because each ring has its own particular configuration of disorder.  Now Jack Harris' group at Yale has done a beautiful measurement, looking at many individual rings and examining the statistics of these persistent currents in the ensemble.  They place each ring at the end of a floppy cantilever.  In the presence of a magnetic field, the magnetic dipole moment from the persistent current exerts a torque on the cantilever, and the results can be detected optically via interferometry.  The experiment requires low temperatures, precision fabrication, and very clean technique.  Very nice.

Academic science researchers and economics

This article in the NY Times is rather provocative in several ways. First, it raises the question of whether there is a dramatic rise taking place in the number of journal article retractions (spread across all disciplines). The answer is, it's really not clear, given the enormous increase in the number of published articles. Moreover, it's certainly much easier for people to find, read, and compare articles than ever before. Google Scholar, for example, can see through most pay-walls enough to search for words and phrases, making it far easier than ever before to test for plagiarism. Moving on, the article then looks at whether the culture of academic science research is, for lack of a better word, ailing. There are some choice quotes:

[L]abs continue to have an incentive to take on lots of graduate students to produce more research. “I refer to it as a pyramid scheme,” said Paula Stephan, a Georgia State University economist and author of “How Economics Shapes Science,” published in January by Harvard University Press.

In such an environment, a high-profile paper can mean the difference between a career in science or leaving the field. “It’s becoming the price of admission,” Dr. Fang said.

The scramble isn’t over once young scientists get a job. “Everyone feels nervous even when they’re successful,” he continued. “They ask, ‘Will this be the beginning of the decline?’ ”

...

“What people do is they count papers, and they look at the prestige of the journal in which the research is published, and they see how many grant dollars scientists have, and if they don’t have funding, they don’t get promoted,” Dr. Fang said. “It’s not about the quality of the research.”

Dr. Ness likens scientists today to small-business owners, rather than people trying to satisfy their curiosity about how the world works. “You’re marketing and selling to other scientists,” she said. “To the degree you can market and sell your products better, you’re creating the revenue stream to fund your enterprise.”

I don't want to quote any more for fear of running afoul of fair use. Read the article. This does hit some of the insecurities felt by any reasonable US faculty science or engineering researcher. I would dispute the pyramid scheme comment because it's based on a false premise, that every doctoral student is looking to become a professor and is crushed if they don't get a faculty position. The prestige paper comments are more worrisomely accurate.

Getting the most out of an experimental technique

This post is a mini-summary of a Perspectives piece I wrote for ACS Nano.  One conceptually simple way to measure the electronic properties of materials at the atomic scale is to use a "break junction".  Imagine taking a metal needle touching a metal surface, and slowly lifting up on the needle.  At some point, the needle will come out of contact with the surface.  As it does so, at the last instant, the contact between the two will take place only at the atomic scale.  If you hook up one end of a battery to the needle and the other through an ammeter to the metal surface to measure the flow of current, you can measure the electrical conduction throughout this process.  Thanks to the availability of high speed electronics these days, it is possible to record conductance, G, vs. time data throughout the process.  A standard analytic approach is then to compile a histogram of all the data points, counting how many times each value of G is measured.  As explained here, the most stable junction configurations naturally have more data points, and this will lead to peaks in the conductance histogram at the values of conductance corresponding to those configurations.   Molecules may be incorporated into such junctions (as I've written about here).  Since it's possible to set up a system to make and break junctions repeatedly and rapidly in an automated way, this approach has proven very fruitful and revealing.

Of course, only looking at the histograms is wasteful.  You actually have an enormous amount of additional information contained in the G vs. t traces.  For instance, you can check to see if the occurrence of a "plateau" in G vs. t at one conductance level always (or never!) correlates with a similar plateau at a different conductance value.  These kinds of cross-correlations are best represented in two-dimensional histograms of various types.  Makk et al. have written a very clear and tutorial paper about how this works in practice, and what kinds of things one can learn from such analyses.  It's definitely worth a read if you work on this stuff, and it's also a great lesson in how as much of your data as possible.

DOI numbers, Web of Science, and article numbers

Two recurring complaints about bibliographies and citations for papers and proposals:

• Most people really like DOI, a system meant to assure that reference materials like journal articles get an effectively permanent web address, something that will "always" point to that article.  It's become very very popular, and every online journal that I know provides a doi reference for each article.  It shows up in every Web of Science reference these days, too, if it exists.  So, why can't Web of Science make those doi numbers a clickable link?  That is, instead of forcing me to copy and paste the doi into a browser URL line with "http://dx.doi.org/" stuck in front, why not just make the doi itself a link to that?  I mean, why would anyone just want the doi without the link??  Is this some weird bs rule about Web of Science not wanting to have direct links?
• How come Physical Review handles bibliographic information so badly when it comes to article numbers?   A number of years ago, Phys Rev switched from old-fashioned page numbers for articles to 6-digit article numbers.  Unfortunately, when you try to export bibliographic information for reference management software, for many Phys Rev articles, the automatic response is to stick the article number (which replaced the page number for all practical purposes) in some completely random field, and instead list the page numbers as either blank or the oh-so-useful "1-4" for a four-page article.  Can someone please fix this?  

Both of these are trivial, silly things, but I'd be willing to be that hundreds of person-hours (at least) are lost per year dealing with the latter one.

Commitment and conflicts

One of the various hats I wear right now is chair of Rice's university committee on research, and one topic that has come up lately (in the context of the US government's new regs about conflict of interest) is the discussion of "commitment". Conflict of interest is comparatively simple to explain to people - everyone grasps the idea that financial or other compensation that may give the appearance of affecting your scholarly objectivity is potentially a conflict of interest. Commitment is a more challenging concept. Most universities expect their science and engineering faculty in particular to spend some of their time doing things that are not immediately, directly connected to their simplest academic duties (teaching courses, supervising research students and postdocs, performing university service). For example, technical consulting isn't that unusual. Likewise, there are other broadly defined academic duties that can come up (serving on advisory or editorial boards; professional society work) that can enhance the academic mission of the university in a higher order way. However, it's clear that there have to be limits of some kind on these auxiliary activities - we would all agree that someone who does so much alternative work that they can't teach their classes or adequately do their normal job is having problems with time allocation. The general question is, how should a university manage these situations - how are they identified, how are they mitigated, and what are the consequences if someone is knowingly going over the line (e.g., spending three working days per week running the day to day operations of a startup company rather than doing their academic job)? Things get particularly complicated when you factor in disciplines that basically demand external work (architecture, business school), and the increasingly common practice of special appointments at foreign universities. If anyone has suggestions of universities with what they think are especially good approaches (or lousy ones, for that matter) to this issue, please post in the comments.

An open letter to Neil deGrasse Tyson

Hello, Dr. Tyson. First, let me say that I'm a huge fan. You do the scientific community a tremendous service by being such an approachable, clear spokesman, maintaining scientific accuracy while also entertaining the public. Astronomy is a great side interest of mine (like many scientists and engineers), and I really wanted to be an astronaut for a while (until my eyes were demonstrably lousy); that's why on some gut level I enjoyed your call for a renewed vigor in space exploration.

However, my brain's response to your call is, is this really the best strategy? Much as I'd love to one day walk on the moon or Mars, I can't help but be deeply skeptical of NASA's ability to allocate resources. Right now their annual budget is about $17B, more than twice that of the NSF, and more than three times that of the DOE Office of Science. While the achievements of the robotic spacecraft missions are truly amazing, much of the rest of NASA seems very dysfunctional. I'll admit, my impression colored by my thesis advisor's experience on the Columbia accident investigation board, my knowledge of the ISS (hint: the Soyuz "lifeboats" where the ISS crew shelters in case of debris impact? They're actually the most debris-vulnerable part of the ISS.), and the fact that NASA has employees that do things like this and this at some rate.

If taxpayers are going to be persuaded to invest another $17B/yr in federally funded research, I think a much more compelling case needs to be made that NASA is the place for that investment, given the alternatives. Yes, NASA's history and subject matter are inspiring, but you need to convince me that NASA as an agency will really get value out of that investment, given that their recent leadership has been singularly unimpressive.

PS - If you ever need a sub to go onto Colbert in your stead, please call.

Several items

My apologies to my readers for low blogging rate recently. Multiple papers, proposals, teaching, travel, etc. have all contributed to this slow-down. Here are a few brief items to consider:

• The (nearly) final details have come out regarding the OPERA experiment.  Goodbye, superluminal neutrinos - we hardly knew ye.    Would've been fun!
• It would appear that one can correlate political affiliation in the US with the somewhat ill-defined concept of "trust in science".  Much as it's tempting to make a wry comment here, I suspect that some of this is due to the very disparate nature of those self-identifying as "conservative" these days.  Either way, this is a problem, though.  Science (in the sense of careful, rigorous testing of hypotheses that allege predictive power) is an incredibly useful way to look at much of the world, and I would hope that this would be appreciated by the vast majority of people out there.
• Someone has advanced the idea that Mitt Romney is a quantum object.  Clearly we should put him through some sort of interferometer to test this idea.  Alternately, he should interlace his fingers and make a loop with his arms - we can then thread magnetic flux through him and see if his response about the individual mandate for healthcare oscillates as the magnetic field is swept.
• Visiting NSF is always enlightening.  I really hadn't appreciated before the quantitative problem that they face in proposal evaluation and administration:  the number of proposals that are submitted has more than doubled in the last few years, while their staffing has remained unchanged.  Even apart from overall resource problems (e.g., the runaway positive feedback cycle, when people realize that the odds of funding are bad, so they submit more proposals, making the odds of funding worse), just the challenge of properly handling all the paperwork is becoming incredibly difficult.
• April Fools is always fun on the web.  This is one of my favorites.

Responsibilities, rational and otherwise

Professors have many responsibilities - to their students and postdocs, to their departments and colleagues, to their university, to the scientific community, and to the public. When on a doctoral committee, for example, a professor's duty is to make sure that the candidate's thesis is rigorous and careful, and that the student actually knows what they're talking about. Obviously primary responsibility for supervision of the student lies with the advisor(s), but the committee members are not window dressing; they're supposed to serve a valuable role in upholding the quality of the work.

I have a colleague at another institution (names and circumstances have been changed here; I'll say no more about specifics) who really had to put his foot down several years ago, as a committee member, to make sure that a student (the last one of a just-retired professor) didn't hand in a thesis sufficiently fringe that it bordered on pseudoscience. It was pretty clear that the advisor would have been willing to let this slide (!) for the sake of getting the last student out the door. My colleague (junior faculty at the time) had to push hard to make sure that this got resolved. Eventually the student did complete an acceptable thesis (on a much more mainstream topic) and got the degree. This colleague just recently came across the former student again, and was disappointed and sad to see that the fringe aspects of science are back in what he's doing. My colleague is now feeling (irrational) guilt about this (that the former student is now credentialed and pushing this stuff), even though the actual thesis was fine in the end. This does raise the question, though: how much of a gatekeeper should a committee member be?

Paranormal activity edition

Two items, oddly about parapsychology (as a means to raise points about science and the public).  First, this article from The Guardian last week is both unsurprising and disappointing.  It is not at all surprising that careful attempts to reproduce almost-certainly-spurious results implying precognitive phenomena have shown that those effects apparently to not really exist.  What is worth pondering and discussion, however, is the fact that the authors who tried to check the original results had such a hard time publishing their work, because the major journals dismiss attempts to reproduce controversial results as unoriginal or derivative.  This is a problem.  Sure, you don't want to take up premiere journal space with lots of confirmations or repetitions of previous work.  However, if a journal is willing to hype controversial results to boost circulation, then surely there is some burden on them to follow up on whether those extraordinary claims withstand the test of time.  

Second, this morning's Dear Abby column (yes, I still read a newspaper on Sundays) had a letter from a woman seeking advice about how to use her "psychic gifts".   It's very depressing that the response said "Many people have psychic abilities to a greater or lesser degree than you do, and those "vibes" can be invaluable."  Really?  Many people have psychic abilities?  How's this for advice:  if you really have psychic abilities, go to the James Randi Foundation and take their Million Dollar Challenge.  Once you pass, you can use the money to make peoples' lives better.  I know it's stupid to get annoyed by this, just as it's pointless to complain about the horoscopes that run in the paper.  Still, if someone has an audience as large as Dear Abby, they should think a little bit about spreading this silliness.

Tidbits

Some interesting and thought-provoking things have come up in the last week or so. For instance, here is an article from the IEEE that discusses the decline in science and engineering jobs in the US. Figure 2 is particularly thought-provoking, showing that the number of US undergrad STEM degrees is very strongly correlated with the number of non-medical US federal research dollars spent, from 1955-2000. My personal take is, if you really want Americans to become scientists, engineers, and more broadly supportive of technical education, you need to create a culture where those professions are (more) respected and valued, not viewed as nerdy, geeky, asocial, elitist, or otherwise unacceptable.

On this same theme, there was this op-ed in the New York Times about why so few American political figures are scientists. Accurate (in my opinion) and depressing. I'm not saying we should live in a society run by technocrats, but surely we can be better than this. As a culture, do we really need more lawyers and undergrad "business" majors?

On a more technical note, the ICARUS collaboration, another group in Gran Sasso in Italy working with neutrinos produced by CERN, has announced (paper here) that their measurements show neutrinos traveling at a speed consistent w/ c. Not surprising, and only truly independent measurements can really pin down the issues w/ the OPERA work.

Here is a beautiful new paper by the Manoharan group at Stanford. By arranging spatially ordered arrays of CO molecules on a copper surface, they can manipulate surface states in a way that produces dispersion relations (the relationship between energy and momentum for electrons) with the same kinds of features seen in graphene. While I haven't had a chance to read this in detail yet, it is very slick, and makes explicit the connection between real-space distortions of the graphene structure and how these are mathematically equivalent to electric and magnetic fields for the charge carriers confined to that 2d environment. It's also a great demonstration of how the motion of charge carriers in a condensed matter environment depends on the potential energy's distribution as a function of position, rather than the details. Here, the electrons are not carbon p electrons feeling the "chickenwire" potential energy of the carbon atom lattice in graphene. Rather, the electrons are those that live in the copper surface state, and they feel a designer "chickenwire" potential energy due to the arrangement of CO molecules on the copper surface. However, the net effect is the same. Very pretty. (Still makes me wonder a bit about the details, though.... At the end of the day, electrons have to scatter out of that surface state and into the bulk for the STM measurement to work, and yet that process has to be sufficiently weak that it doesn't screw up the surface state much. Very fortunate that the numbers happen to work!)

Finally, here is a cool, fun project, using nanofab tools to make art (too small to see with the unaided eye). Sameer Walavalkar did his PhD with the well known nano group of Axel Sherer at CalTech. This kind of creative outlet is another way to do outreach, and it's a heck of a lot cooler than many other approaches.

Mini update

I am out on a brief break, but I wold be remiss if I didn't point out this exciting result. The investigators have managed to make a light emitting diode with greater than 100% electrical efficiency when operated just right. The trick is, the LED gets the energy for the "extra" photons from the temperature difference between the LED and it's surroundings. Basically it's a combination LED and heat engine. Very clever. I wonder if there are some entropy restrictions that come into play, particularly if the final photon state is, e.g., the macroscopically occupied state of a laser cavity.

NSF - proposal compliance

This is for everyone out there who submits to proposals to the Division of Materials Research, and more broadly, to the National Science Foundation. Here's some context for those who don't know the story. The NSF has a Grant Proposal Guide that spells out, in detail, the proper content and formatting for proposals. You can understand why they do this, particularly with regard to things like font size. There's a 15 page limit on the "Project Description" part of a proposal, and if they didn't specify a font size and margins, there would be people trying to game the system by submitting proposals in 6-pt unreadable font with 1cm margins. Historically, NSF has erred on the side of latitude about the minutiae, however. For example, they have never really been aggressive about policing whether the bibliographic references are perfectly formatted.

That's why this news came as a surprise: As part of a new policy, starting this past fall, DMR is taking basically a zero-tolerance approach regarding compliance with the Grant Proposal Guide. That means, for example, that any letter of collaboration included with a proposal can only say, in effect, "I agree to do the tasks listed in the Project Description". Anything more (e.g., context about what the collaborator's expertise is, or mentioning that this continues an existing collaboration) is no longer allowed, and would be cause for either deletion of the letter or outright rejection of the proposal without review. This new policy also means, and this is scary, that your references have to be perfectly formatted - leaving out titles, or leaving out the second page number, or using "et al." instead of long author lists - all of these can lead to a proposal being rejected without review. I heard this first hand from a program officer. Imagine spending weeks writing a proposal, and having it get bounced because you used the wrong setting in bibTeX or EndNote.

We can have a vigorous discussion in the comments about whether this policy makes much sense. In the meantime, though, I think it's very important that people be aware of this change. The bottom line: Scrupulously follow the Grant Proposal Guide. Cross every "t" and dot every "i".

Please spread this information - if one division of NSF is doing this, you can bet that it will spread, and you don't want to be the one whose proposal gets bounced.

March Meeting last day and wrap-up

Not too much to report from the final day of the March Meeting. Lots of good conversations with colleagues, though I never did get a chance to sit down with a couple of folks I'd wanted to see. Ahh well.

I split most of my time between two invited sessions. The first of these was on the unusual properties of the nu=5/2 fractional quantum Hall state. This may sound very narrow and esoteric, but it is actually quite profound. A good review of the whole topic in more generality is here. At a very particular value of perpendicular magnetic field (related to the number of charge carriers per square centimeter), the electrons in a 2d layer in GaAs/AlGaAs semiconductor structures apparently condense into a really weird state. The lowest energy excitations of this state, its quasiparticles, have very strange properties. First, they have an effective electronic charge of 1/4 e. Second, when two of these fractionally charged quasiparticles are moved around each other to swap positions, the whole quantum mechanical state of the system changes (to another state with the same energy as the original), in a way much more complex than just picking up a phase factor (which would be -1 if the quasiparticles acted like ordinary electrons). Somehow the detailed history of winding the particles around each other is supposedly encoded in the many-body state itself. Quasiparticles with this bizarre property are said to obey "non-Abelian statistics". To date, there has not been an experimental "smoking gun" demonstrating these weird properties unambiguously. My postdoc mentor, Bob Willett, gave a very data-heavy talk showing persuasive evidence for consistency with a number of the relevant theory predictions in this system. Following him, Woowon Kang of the University of Chicago showed other data that also looks consistent with some of these ideas (though I'm no expert).

The other invited session dealt with the theory behind the transport of electrons and ions in nanoscale systems. Unfortunately I missed the beginning (since I was seeing the other talks above), but I did get to hear a neat discussion by Kirk Bevan of McGill University about the physics of electromigration. Electromigration is the mechanism by which flowing electrons can scatter off defects and grain boundaries, dumping momentum into atoms and pushing them around.

Final suggestions for the APS:

1) Don't have the small rooms arranged so that getting to seats in the front requires blocking the projector. The result of that is that the front 6 rows or so remain almost completely empty, while people pile up in the back of the rooms.

2) Would it really be that hard to have wireless internet access that doesn't suck? Are there no convention centers that can really support this?

3) Having a big bio presence at the meeting and then scheduling it directly opposite the Biophysical Society meeting seems odd.

4) Every year, there is an electronic letter-writing or petition campaign to support federal funding of research. That's fine and dandy, but is there any way we could try to get some representative Congress-critters to come hear a session, perhaps one of the fun, general invited sessions, or one about industrially relevant research? Remember, next year in Baltimore is quite close to DC....

March Meeting day 3 (for me)

Yesterday I spent a fair bit of time seeing specialized talks related to my group's research. In the contributed session in the morning, I saw a couple of talks by the theory group of Kevin Ingersent at the University of Florida. When describing electronic transport through a molecule, there are two basic theory approaches. One way to tackle this problem is to try to do realistic quantum chemistry calculations about specific molecular orbitals and how a molecule couples electronically to metal electrodes. A complementary tactic is to construct a mathematical model that you think contains the essential physics (e.g., treat the molecule as a "dot" with two electronic levels, each coupled to generic conduction electrons in the leads; then add in a single, local harmonic vibrational mode with some coupling between the level populations and the amplitude of the vibration, etc.). These two schemes correspond well with approaches to bulk materials: realistic electronic structure calculations vs. construction of model Hamiltonians. Ingersent's group takes the latter approach, and it looks like there is even more rich physics buried in single-impurity junctions than I'd previously appreciated.

In the same session, there were some nice experimental talks from Latha Venkataraman's group at Columbia. Recently, her students have seen that it's possible to create comparatively good contacts between molecules and metals, with a single quantum channel being transmitted through the system with a transmission of about 90%. This is in contrast to the more common situation, where transmission is more like 0.1%. She's also started doing single-molecule measurements of thermopower and Seebeck coefficient, where you apply a temperature gradient across a molecule and look at the resulting voltage difference that shows up. Cool data, though thermal transport at these scales is very challenging.

Later in the day I heard some nice invited talks. Jean-Marc Triscone gave a nice presentation of the properties of the two-dimensional electron gas that shows up at the interface between strontium titanate and lanthanum aluminate (STO/LAO). This field of oxide heterostructures has become very popular, and includes all sorts of rich physics, including coexistent superconductivity and magnetism. Any topic that gets to talk about a "polarization catastrophe" has to be good.

In another invited session, Cyrus Hirbijehedin talked in detail about Kondo physics in single magnetic atoms on very thin insulating layers, as probed by STM. Dan Ralph gave an extremely clear talk (via iphone from Cornell, due to inclement weather) on Kondo physics in single-molecule junctions, with their particular experimental twist of being able to stretch or squish the junctions in situ. Very neat. There are some lingering technical points in such structures that need further examination by the community.

I did not, unfortunately, see Leo Kouwenhoven's ballyhooed (here and here) talk about Majorana fermions. I need to read more about the particular work before I can offer any intelligent commentary.

March Meeting day 2

Today was probably the maximum crowd at the APS meeting. Saw a number of talks and had lots of conversations. One particularly interesting talk was about variations on this result. In condensed matter we've become used to the idea of "photonic band gap" or "photonic crystal" materials, systems where a spatially periodic pattern of dielectric contrast (e.g., glass vs. air) results in optical properties that mimic the electronic properties of crystals: bands (energy ranges) where light can propagate freely, and (photonic) band gaps, where light is reflected and can't propagate. This talk was about weird "hyperuniform" disordered dielectric structures that nonetheless have a complete photonic band gap (in all directions) in 2d, and the fact that by introducing voids and defects in these systems, it's possible to make very selective and directional waveguides. I need to read more about the math behind this. The experiment uses microwaves rather than visible light, meaning that it's possible to build such structures by hand using sapphire plates and rods on the centimeter scale.

The Buckley Prize session was also very good, though I missed talks in the middle. Very crowded, particularly for Charlie Kane's talk. That one would have been fun if it'd been a full hour - it felt like he had to abbreviate some of the discussion to fit into the 30+6 minute slot.

Another highlight this afternoon was the big Kavli session about the mesoscale. The lead talk was from Bob Laughlin, who is always entertaining. He focused on the big open question of whether there are laws that emerge in biology. By his definition, a law is a quantitative relationship between measured parameters that always holds. Some laws are (apparently) fundamental, like the force between two point charges in vacuum. Others are emergent, like the relationship between stress and strain in elastic media, or the Navier-Stokes equations that govern hydrodynamics. In the emergent situation, emergent laws hold when the system is sufficiently macroscopic. Laughlin's big question is, are there universal quantitative relationships that emerge in biological systems (beyond the trivial ones already mentioned, like elasticity being useful for describing cell membranes)? He says that there are hints all over, but it's very hard to do the definitive experiments because biology is just so complicated and our experimental tools are comparatively invasive and crude. When asked to describe biologists in one word, he said "frustrated".

The talk also put forward two definitions of what condensed matter physicists do, both of which are very good. Via Laughlin, Michael Fisher says: "Our job [as condensed matter physicists] is to discover and understand the phases of matter and the transitions between them." Laughlin himself says: "Our job [as condensed matter physicists] is to discover the emergent laws of nature, and hand them to engineers so that they can be put to use." Good stuff. On a lighter note, I realized partway through the talk that every now and then it's entertaining to imagine Laughlin's words as if said by William Shatner. "We trust the emergent laws of rigidity and hydrodynamics...to make an airplane...that can go up to 35000 ft...and not...explode!"

March Meeting day 1

As I am every year, I was again somewhat shocked by how many people come to this meeting. The attendance just keeps going up, and unfortunately that usually correlates with a decrease in the utility of the average talk. For PIs, the meeting is more about interacting with each other, vendors, potential hires (postdoc or faculty) than it is about actually learning things from talks.

I did actually learn a few things in talks yesterday, though, even without going to sessions on the super hot topics (graphene, topological insulators).

I went to the first half of an interesting session about presenting science to the public, something that I think is very important. The first talk was about a theater group collaborating with MIT, with a history of developing plays based on physics (including Einstein's Dreams by Alan Lightman). Here, free of charge, are my suggestions for other play possibilities: The birth of Silicon Valley, emphasizing that in many ways it stems from the fact that Shockley was such a micromanager that no one could work with him; the genesis of the hydrogen bomb, emphasizing that Teller's difficult personality delayed the development of his much-loved "super" by at least two years, because very few people could work with him. (Sense a theme?). The second talk was by Odd Todd Rosenberg, an animator who works with Robert Krulwich of ABC and NPR science fame. The cartoons were funny and informative, though the discussion raised the recurring issue of oversimplification and inaccuracy in broadly popular marketing of science.

I also went to a very strong session all about vanadium dioxide, which was quite informative. Sounds like many people in the strong correlations game are starting to look very hard at ionic liquids for gating. Here the major challenge is whether people are inadvertently (or advertently) doing chemistry on their structures.

There was also a very fun talk by Hongkun Park of Harvard, who discussed "quantum plasmonics" - trying to couple individual emitters and absorbers to plasmonic waveguides. For example, you can have a GaAs nanowire touching a silver nanowire. When biased appropriately the GaAs emits photons which couple strongly into guided plasmon modes of the Ag wire, propagate as plasmons, and are then generate photocurrent at a junction with a Ge nanowire at the other end of the silver wire. This is a "dark plasmonic" circuit, with light being generated, propagated, and detected on the subwavelength scale. Cool stuff.

March Meeting program and app

For an organization so deeply ingrained with technology, the American Physical Society has some surprising issues. For example, as I type this, I'm downloading the complete program to the March Meeting at the blazing pace of 5 kB/s, and that's not limited by my connection. For another example, once again the APS has released a mobile app that contains the whole program and is supposed to be searchable. Unfortunately, the user interface on the app (at least the ipad version) is dreadful, unintuitive, and creepingly slow. Geez.

Job interview humor.

Thanks, Wolff, for pointing me to this.

Coming up this week, some commentary from the March Meeting of the APS.

superluminal neutrinos - not (quite) dead yet.

When historians of science look back on the whole OPERA superluminal neutrino discussion, one way or the other, there are going to be a number of lessons to draw from the experience about how science and science journalism function in the early 21st century.

Yesterday, with "BREAKING NEWS" headlines, Science magazine proclaimed: "Error Undoes Faster-Than-Light Neutrino Results". In that article, "according to sources familiar with the experiment", the whole timing discrepancy for the neutrinos is traced to a bad fiber optic connection to a GPS receiver. The claim from that article is that "After tightening the connection then remeasuring the time it takes data to travel the length of the fiber, researchers found that the data arrive 60 nanoseconds earlier than assumed." Since that's the critical amount by which the neutrinos allegedly arrived too soon for special relativity, that would seem to be the end of the story. Embarrassing for OPERA, but case closed, right? T

Wrong. First, on its face, this seems weird - no one is quoted by name, and the idea that a loose fiber coupling could contribute 60 ns in timing is pretty odd (since that would correspond to something like 18 m of free space optical path). At minimum, the description above must be garbled.

Moreover, the actual email from the CERN director does not say this at all. Rather, it says that OPERA has identified two outstanding issues, one involving an oscillator that provides timestamps for the GPS synch, and an optical fiber connector that brings the GPS signal to the OPERA master clock. The former issue could make the neutrino timing problem worse, in fact. Moreover, the message says explicitly that they are going to take new measurements in May to check these issues. That seems to flatly contradict the news article claiming that they've already done tests. For a detailed discussion, see Matt Strassler's excellent blog here.

Bottom line: as I've said before, the superluminal neutrino result is almost certainly wrong, but the jury is still out on how and why, despite what the Science news blurb says. Believe me, if they knew for sure how this stood, they'd end it with a definitive statement, not stretch this out 'til May.

UPDATE:  Prof. Strassler has the best write-up of this, based on detailed reporting from the European press.   Because of two different, subtle technical flaws, the uncertainty in the OPERA results is bigger than the 60 ns timing discrepancy, meaning (1) the result is not in contradiction w/ special relativity (big surprise), and (2) they need to run with fresh data and the problems rectified to make any more definitive statement.

Symmetry, crystals, and "time crystals"

"Spontaneous symmetry breaking" is a profound idea from condensed matter physics that has been adopted with a vengeance by the high energy physics community. Let me give an example. The laws of physics are, as far as we know, "translationally invariant" - they don't depend on our location in space, and if we move a little bit nothing happens to those laws. That's translational symmetry (and it turns out it's deeply connected to conservation of momentum, but that's another story). However, if we consider atoms moving about in space and worry about what configurations they like to adopt, the atoms tend spontaneously to adopt a lower symmetry state. For example, atoms will often arrange themselves into a regular spatial periodicity that we call a crystal structure. Now instead of space being continuously translationally invariant, the arrangement of atoms has a discrete translational symmetry - the arrangement of atoms reproduces itself if translated by integer multiples of a particular lattice parameter.

This week, two papers (here and here) appeared, from (the polymath) Frank Wilczek's group at MIT. Wilczek is one of the rare theorists who moves seamlessly between condensed matter and high energy theory. In these papers, the idea of "time crystals" is discussed. As science fiction-y as this sounds, it's real, but it is an application of this idea of spontaneous symmetry breaking, not some exotic Doctor Who concept. In analog with the above discussion, as far as we know, the laws of physics are also invariant under translations in time. The idea in these papers is that dynamical systems may spontaneously break that continuous translational invariance, and exhibit discrete time translation invariance instead. That means that dynamical systems may spontaneously take on periodicity in time! I need to read the second paper in detail soon - the quantum versions of these ideas seems very deep....

Meme contest

There's a new meme floating around the web these days. Here's an example:

I'd like to propose a contest for a version of this for "Physicist", or even better, "Condensed Matter Physicist", "Nanotechnologist", or something along those lines. Put up some nominations (links in the comments), and I'll have voting to pick a winner.

Gaming the system

Two semi-related topics have come to mind lately.  First, this post by the FSP caught my attention, regarding "citation circles", where a sub-community within a scientific discipline agree to cite each others' work.  I've heard of such things, and there's nothing inherently wrong there as long as the citations are relevant and don't consciously omit other equally relevant papers.  Still, I never considered this practice to have too much impact.  Back when I was in grad school, I'd heard of something that is equally fine ethically, and perhaps more important to progress in the long term:  the timely reviewing circle - a group of scientists who agree to respond promptly to invitations to review one anothers' work.  This is not a matter of conspiring to give positive reviews, but an agreement to get manuscripts through the review process quickly.  Imagine if certain "Letters" journals were actually speedy!

The second topic is the idea of trying to influence the selection of referees.  Of course, under many circumstances you as an author can suggest possible referees for scientific papers or grant proposals.  Let's call that a first-order influence.  It's a way of making sure that the editors can get the paper out to technically knowledgeable people in a specialty.  (I've been told by multiple editors that authors who try to suggest "friendly" referees often do themselves more harm than good, because those suggested reviewers are often more harsh than randomly selected peers.)  Recently I learned about a "higher-order" approach:  avoiding citing the work of potentially hostile reviewers, under the assumption that the editor/program officer often gets referee ideas from the reference lists.  I am very skeptical that this approach could matter in a statistically significant way.

Science and politics - 2 items

I really got a kick out of this video, and this one (some overlap), of President Obama at the White House Science Fair.  See here for a press description, and here for the White House's own page on the matter.  Note that Bill Nye was there - how cool is that?  It's great to see a President who uses a bit of the prestige of the office to shine a spotlight on the importance of science education.   

On an unrelated note, on the way home from work today I heard this story on NPR, about the challenges of nonprofits, specifically charities, that fund science research.  There is a tendency for those charities to shy away from politically controversial topics (e.g., human embryonic stem cells) because charities don't want to risk alienating any potential donor.  It is an interesting set of issues.  Charities are of course not obligated to fund any specific thing.  My sense is that one clear ethical line is that charities should be open and honest about what they do and don't support.  No hidden agendas.  If you don't want to fund something, just say so clearly, so that there's no "buyer's remorse" from donors who feel mislead.

Media relations at universities

Many people online have heard about and commented on this sad story.  An assistant prof in molecular biology at Case Western published a paper in the "journal" Life that purported to explain essentially all of creation in terms of "gyres", some hand-wavy vortex-like entities.  As the always provocative PZ Myers points out, the paper itself is basically word salad - it sounds disturbingly like the writings of someone having real mental health issues.  The fact that it got published shows what a sham some journals are.  I suspect that many of my academic peers have gotten email invitations to serve on the editorial boards of pay-to-publish journals.  Several members of the board at Life have apparently resigned over this mess.  However, this sad affair does raise some points worthy of consideration:

1) How did the media services office at CWRU actually end up putting out a big press release about this?  Do they simply have no judgment whatsoever about content?  I mean, could any professor ask them to put out a press release about anything, and it wouldn't be filtered at all before going out to the media?

2) Do aggregators like Eurekalert and Physorg serve as a positive influence overall?  Yes, they help get science news stories out to the wider media, but don't they have some responsibility to make sure that they aren't just a conduit for junk?  Surely they weren't originally intended just to be redistributors of unedited press releases.

3) What are the responsibilities of academic authors, department chairs, deans, etc. when it comes to press releases?  Lord knows, I would not want to have to get permission from a higher-up at my university to speak my mind or point out a cool new result.  However, it doesn't necessarily do anyone a lot of good if people put out press releases and have a media blitz for every little result, let alone the occasional whacko idea.  While universities generally like media mentions of their researchers, CWRU can't be happy about this situation.

If I could do this with electrons....

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

This is damned peculiar....

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

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.

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

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

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)

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?