String theory (!) and "bad metals"

I saw a remarkable talk today by Hong Liu from MIT, about quantum gravity and what it has to say about high temperature superconductivity.  Yes, you read that correctly.  It was (at least for a nonexpert) a reasonably accessible look at a genuinely useful physics result to come from string theory.  I doubt I can do it justice, so I'll just give the bare-bones idea.  Within string theory, Maldacena (and others following) showed that there is a duality (that is, a precise mathematical correspondence) between some [quantum theories of gravity in some volume of d+1 dimensions] and some [quantum field theories w/o gravity on the d-dimensional boundary of that volume].  This sounds esoteric - what could it be good for?  Well, we know what we think the classical limit of quantum gravity should be:  Einstein's general relativity, and we know a decent number of solutions to the Einstein equations.  The duality means that it is possible to take what could be a very painful interacting many-body quantum mechanics problem (say, the quantum field theory approach to dealing with a large number of interacting electrons), and instead of solving it directly, we could convert it into a (mathematically equivalent) general relativity problem that might be much simpler with a known solution.  People have already used this approach to make predictions about the strongly-interacting quark-gluon plasma produced at RHIC, for example.


I'd known about this basic idea, but I always assumed that it would be of very limited utility in general.  After all, there are a whole lot of possible hard many-body problems in solid state physics, and it seemed like we'd have to be very lucky for the duals of those problems to turn out to be easy to find or solve.  Well, perhaps I was wrong.  Prof. Liu showed an example (or at least the results), in which a particular general relativity solution (an extremal charged blackhole) turns out to give deep insights into a long-standing issue in the strongly-correlated electron community.  Some conducting materials are said to be "bad metals".  While they conduct electricity moderately well, and their conductivity improves as temperature goes down (one definition of metal), the way that the conductivity improves is weird.  Copper, a good metal, has an electrical resistance that scales like T² at low temperatures.  This is well understood, and is a consequence of the fact that the low-energy excitations of the electrons in copper act basically like noninteracting electrons.  A bad metal, in contrast, has a resistance that scales like T, which implies that the low energy excitations in the bad metal are very complex, rather than electron-like.  Well, looking at the dual to the extremal black hole problem actually seems to explain the properties of this funny metallic state.  A version of Prof. Liu's talk is online at the KITP.  Wild stuff!  It's amazing to me that we're so fortunate that this particular correspondence exists.

Climate change talk

This afternoon we were fortunate enough to have our annual Rorschach Lecture, delivered by Ralph Cicerone, president of the US National Academy of Sciences.  The subject was climate change and its interaction with energy policy, and unsurprisingly to anyone who isn't willfully ignorant, this was a scary talk.  The atmospheric CO₂ data, the satellite-based measurements of accelerating Greenland and Antarctic ice loss, the amazing pace at which China is building coal-fire power plants (roughly 1 GW of electric generating capacity from coal coming on line every 10 days), are all very sobering.  The planet doesn't care, of course, but it sure looks like the human species had better get its act together, and the only way that's going to happen is if we come up with an energy approach that is cheap compared to coal (that includes the possibility of making coal more expensive, of course, but how do you persuade China and India not to burn their cheap, abundant coal?). 

Ahh, Air China

Posting from International Check-in at Beijing International Airport....

I was actually supposed to get home last night, but Air China had other plans.  At least I have quite the story out of it.  I'd originally booked a 2 hour 45 min layover in Beijing, figuring that would be plenty of time.  However, our Hangzhou-Beijing flight was delayed 2 hours.  Then, the pilot made two go-arounds at Beijing, very bumpy (cue the airsick bags and retching noises from fellow passengers), each time getting w/in about 30 feet of the ground, before giving up (due to high winds, I guess), and we diverted to Tianjin.  In Tianjin they kept us on the plane on the tarmac out at the end of their runway for close to 4 hours.  At least the AC worked there.  They ran out of water, and then orange juice.  Finally, they refueled and flew the plane back to Beijing, arriving only 8 hours late.  At least I wasn't alone (two other americans on the flight in the same situation as me), and Air China did, after some convincing, spring for a hotel for the night.

Clearly the simplest possible explanation for this is that I'm destined to make some universe-shattering discovery in the future, the echoes of which are rippling backward in time to try to prevent my return to the US. 

Conference observations so far

This is a nice gathering of people, and the organizers have done a very good job.  More discussion would be nice - the program is very dense.  A few (not very serious) observations:

• I used to think that I was the only condensed matter physicist not working on graphene.  Now I realize I'm the only condensed matter physicist not working on graphene, iron pnictide superconductors, or topological insulators. 
• Chinese ring tones are different than US or European ringtones.
• One speaker inadvertently stumbled on a great, subtle psychological trick:  he used a font for most of his talk that is identical to the font (some Helvetica variant) used by the Nature publishing group for their titles and subtitles.  That font makes everything seem important :-).  He blew this aura of profundity it at the end, though, by switching to comic sans.
• The Chinese groups that have been charging on the iron pnictides must have enormous resources in terms of people and equipment - the rate at which they are cranking out material and data is remarkable.  US materials growers seem very undersupported by comparison.
  
• Laser-based angle-resolved photoemission, in its appropriate regime, is damned impressive.
  

In China this week

I'm off tomorrow for a week-long trip to China, to go to this workshop.  I've never been to China before, so this should be an interesting experience!  I may try to blog a little, but I don't know how internet access will work during the conference.  Hopefully the trip will go more smoothly than the travel arrangements beforehand.  If I ever hear Expedia's "on hold" music again, I may snap.

Update: The trip in was long but problem-free. Blogger access only works through VPN, thanks to the Great Firewall....

Fiber and CCDs

As you've all no doubt read by now, the 2009 Nobel in Physics was awarded to Charles K. Kao, for the development of truly low loss fiber optics (a technology that you're all using right now, unless the internet backbone in your country consists of smoke signals or semaphore flags), and Willard Boyle + George Smith for the invention of the CCD (charge-coupled device, which is the basis for all digital cameras, and has revolutionized spectroscopy).  

The CCD portion makes a tremendous amount of sense.  CCDs work by using local gates on a doped semiconductor wafer to capture charge generated by the absorption of light.  The charge is then shifted to an amplifier and the resulting voltage pulses are converted into a digital signal that can be interpreted by a computer.  The description given in the supporting document (pdf) on the Nobel website is very good.  CCDs have revolutionized astronomy and spectroscopy as well as photography, and the physics that must be understood and controlled in order to get these things to work well is quite rich (not just the charge generation process, but the solid state physics of screening, transport, and carrier trapping).

The fiber optic portion is more tricky, since many people have worked on the development of fiber optic communications.  Still, Kao had the insight that the real limitation on light propagation in fiber came from particular types of impurities, understood the physics of those impurities, guided a program toward clean material, and had the vision to see where this could all lead.  

Certainly there will be grumbling from some that these are <sneer>engineering</sneer> accomplishments rather than essential physics, as if having a practical impact with your science that leads to technology and helps society is somehow dirty, second-rate, or a sign of intellectual inferiority.  That is a terrible attitude, and I'm not just saying that because my bachelor's degree is in engineering.  Trust me:  some engineers have just as much raw intellectual horsepower as high energy theoretical physicists.  Finding intellectual fulfillment in engineering is not some corruption of pure science - it's just how some very smart people prefer to spend their time.  Oh, by the way, the actual will of Alfred Nobel refers to accomplishments that "shall have conferred the greatest benefit on mankind", and specifically mentions "the person who shall have made the most important discovery or invention [my emphasis] within the field of physics".

Finally, this provides yet another data point on just how transformative Bell Labs (and other remarkable industrial R&D labs, including IBM, GE, and others) really was in the physical sciences.  The withering of long-term industrial research will be felt for a long, long time to come.


 

Single atoms in semiconductors

One last post before the obligatory Nobel post tomorrow.

Recently, there has been progress in examining the electronic transport properties of individual dopant atoms in semiconductors.  There are several motivations for this.  First and probably foremost, with increasing miniaturization we are rapidly approaching the limit when the active channel in semiconductor devices will contain, statistically, only a small number of dopants; it makes sense to figure out how these systems work and whether they have any intrinsically useful properties.  Second, these systems are the ultimate small-size limit of quantum dots, even smaller than single-molecule transistors.  Third, since the host materials are extremely well-studied, and quantum chemistry calculations can handle the relevant volumes of material, there is the possibility of realistic, detailed theoretical treatments.  This paper is a great example of treating an individual phosphorus donor in Si as a quantum dot.  This other paper looks at a single arsenic donor, and can see Kondo physics involving the unpaired electron on the donor site interacting with the (valley degenerate) Si conduction electrons.  Very cool stuff!

The return of the embarassing news story.

As mentioned previously, the news story about NSF upper level staff surfing for porn while on the job is back.  This would be funny if it weren't so pathetic and sad.  Obviously this is inappropriate behavior, and NSF clearly needs to get their IT staff up to snuff, since it's certainly possible in a corporate environment to detect and stop this kind of activity.  Still, it seems unfair to single out NSF like this.  I'd be surprised if this didn't go on in all large, computer-heavy organizations at some rate.

First principles vs. toy models

One of the hot topics at the workshop I attended was the proper role of "first principles" calculations in trying to understand electronic conduction at the atomic and molecular scale.  In this business, there tend to be two approaches.  The first, which I call for lack of a better term the "toy model" paradigm, constructs models that are highly idealized and minimalistic, and you hope that they contain the essential physics needed to describe real systems.  An example of such a model would be the single-level Anderson-Holstein model of transport through a molecule.  Instead of worrying about all of the detailed electronic levels of a molecule and the many-electron physics there, you would concentrate on a single electronic level that can either be empty, singly occupied, or doubly occupied.  Instead of worrying about the detailed band structure of the electrodes, you would treat them as ideal electronic reservoirs, and there would be some couplings that allows electrons to hop between the level and the reservoirs.  Instead of considering all of the possible molecular vibrations, you would assume a single characteristic vibrational mode that "lives" on the molecule, and there would be some additional energy cost for having that vibration excited while there is an electron occupying the level.  While this sounds complicated, it is still a comparatively idealized situation that can be described by a handful of characteristic energies, and it contains rich physics.

On the other hand, one can consider trying to model a specific molecule in detail, worrying about the precise electronic and vibrational levels appropriate for exactly that molecule bonded in a particular configuration to a specific kind of metal electrode surface.  While this sounds in some ways like it's what you "really" ought to do, this "first principles" approach is fraught with challenges.  For example, just solving for the electronic levels of the molecule and their relative alignment with the electronic levels in the electrodes is extremely difficult in general.  While there are impressive techniques that can work well in certain situations (e.g., density functional theory), very often the circumstances where those methods work best (quasi-equilibrium, far away from resonances, in situations where electron correlation effects are minimal) are often not too interesting. 

It's interesting to watch the gradual convergence of these approaches.  As computing power grows and increasingly sophisticated treatments are developed, it looks like first-principles calculations are getting better.  One direction that seems popular now, as our condensed matter seminar speaker yesterday pointed out, is using such calculations as guidelines for correctly estimating the parameters that should be fed into the essential physics toy models.  Interesting times are on the horizon.

AAAS and advertising

I've received three pieces of fundraising advertising from AAAS in the last two days via US Mail.  This makes me wonder about a few things.  First, in this day and age, why can't they get a mailing database set up that can tell that Douglas Natelson and Dr. Douglas Natelson at the same address are actually the same person? Second, do they really think that I pay a lot of attention to bulk-mailed fundraising appeals? Third, how much money are they spending, how much energy is consumed, and how much pollution is generated in sending out these tree-killing mailings, when they claim to be environmentally conscious and already have my email address as a subscriber to Science? Fourth, this many appeals in one week smacks of desperation - is there something we should know? 

Curve fitting

Very often in experimental physics, we're interested in comparing some data to a physical model that may involve a number of unknown parameters, and we want to find the set of parameters that gives the best fit.  Typically "best fit" means minimizing a "cost" function, often the sum of the squares of the deviations between the model and the data.  The challenge is that many models can be very complicated, with nonlinear dependences on the parameters.  This often means that finding the optimal parameters can be very difficult - the cost function in parameter-space can have lots of shallow, local minima, for example.  The cost function may also be extremely sensitive to some parameters (the "stiff" ones) and comparatively insensitive to others (the "sloppy" ones).  In arxiv:0909.3884, James Sethna and Cornell colleagues take a look at this dilemma using the tools of differential geometry, and they propose an improvement to standard techniques based on geodesics on the relevant hypersurface in parameter space.  This looks really cool (if mathematical!), and I wish they'd included an example of an actual minimization problem that they'd done with this (instead of leaving it for an "in preparation" reference).  Any prospect for real improvements in nonlinear fitting is exciting.

Ahh, KLM.

Stuck in Schipol, forced to fly back to Houston via BRE-AMS-DET-IAH, since mechanical difficulties cancelled my early BRE-AMS flight (thus causing me to miss my AMS-IAH direct flight).  The other AMS-IAH direct flight on their schedule is really just a psychological torture device, since it's really a charter that's 100% business class and un-bookable except as a cash purchase (which would set me back $4K on top of everything I've already paid).

Could be worse.  There was another guy on the original BRE-AMS flight that got involuntarily rebooked through Paris.  After hanging out at the Bremen airport for four hours, he got to have his BRE-Paris flight also cancelled due to mechanical difficulties.

At least the workshop was extremely good.

Draconian ISP.

The ISP (netopsie) for my hotel here in Bremen, Germany has apparently decided to block access to all "blogspot.com" domains.  If I try to view my blog, I get redirected to a page that says "Banned Site.  You are seeing this error because what you attempted to access appears to contain, or is labeled as containing, material that has been deemed inappropriate."  Ironically, I can post new entries since that is done from a blogger.com page.   I can't view the blog, however, or see comments.  Idiots.  Makes me wonder what they find objectionable on blogs in particular, or whether they are complete puritans and block lots of stuff.

This week in cond-mat

Three quick blurbs from the arxiv this week.  I'm going to a workshop in Germany next week and have a bunch to do in the meantime, so blogging will likely be light.

arxiv:0909.0628 - Bocquet and Charlaix, Nanofluidics, from bulk to interfaces
This paper is an outstanding overview of fluids confined to the nanoscale.  I will definitely be referring to this the next time I teach my graduate course that touches on this topic.  Two of the central questions that comes up when thinking about fluids at the nanoscale are, when do large-scale assumptions about hydrodynamics (e.g., that fluid right at the walls of a container is at rest relative to the walls, even when the fluid away from the walls is flowing - the so-called "no slip" boundary condition) break down, and when does the continuum picture of the fluid (i.e., that fluid may be modeled as a homogeneous medium with some density, rather than a collection of strongly coupled particles) fall apart?  This article looks at these issues in detail, with many useful references.

arxiv:0909.0951 - Saikin et al., On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters
This one is of interest to me because of its relevance to some of the research done in my group.  Raman scattering is inelastic light scattering, where light can lose (or gain) energy to a molecule by exciting (or de-exciting) molecular vibrations.  It's been known for more than 30 years that the Raman scattering process can be greatly (many orders of magnitude) enhanced on nanostructured metal surfaces.  This happens for two reasons.  First, nanostructured metals support local plasmon modes, so that the metal acts like a little optical antenna, helping the molecule to "receive" (and "transmit") light.  This is called electromagnetic enhancement.  Second, there can be additional enhancing effects due to resonances involving charge transfer between the molecule and the nearby metal.  This latter effect is called chemical enhancement, and this paper takes a detailed look at how this can arise, considering specific configurations of molecules on Ag clusters.  It is very challenging to do calculations like this and get realistic results!

arxiv:0909.1205 - Martineau et al, High crystalline quality single crystal CVD diamond
I picked this one because (a) the fact that it is possible to grow high quality single crystal diamond by chemical vapor deposition is just plain cool, as well as of great technological potential; and (b) the x-ray topographs in this paper showing crystallographic defects in the crystals are very pretty.

If you're reading this, you're probably pretty net-savvy.

Perhaps this feature has always been available, but I just noticed the other night that Google Analytics can tell me stats about what kind of web browsers people use to access this page.  Far and away the number one browser used was Firefox (57%), followed by Safari (16%), Internet Explorer (15%), and Chrome (8%).  Interestingly, the breakdown for those accessing my group webpage was quite different, with IE having more like 30% of the total.  Very educational.  No one using lynx, though.

Two nanoscience tidbits

Since nearly everyone else in the science blogging world has touched on this (see here, here, here, here, and here, to name a few), I might as well do so, also.  Leo Gross and coworkers at IBM Zurich have used an atomic force microscope to do something incredibly impressive:  They have been able to image the bonding orbitals in individual pentacene molecules with better than atomic resolution, using the very short-range forces that contribute to the "hard core repulsion" between atoms.  Atoms tend to be attracted to each other on nanometer scales, even in the absence of strong chemical interactions, due to the van der Waals interaction, which comes from the fluctuating motion of their electron clouds.  At very short distances, though, atoms effectively repel each other extremely strongly, both from the Coulomb interaction (electrons don't like each other overly much) and the effects of the Pauli Exclusion Principle.  Gross and colleagues accomplished this feat by working in ultrahigh vacuum (around 10^-15 of atmospheric pressure) and at 5 K, and by deliberately attaching a single CO molecule to their conducting atomic force microscope (AFM) tip.  It's a heck of a technical achievement for AFM.  Atomic resolution has been demonstrated before, but this kind of sensitivity is remarkable.  (FWIW, I once heard one of the major coauthors, Gerhard Meyer, speak at a meeting about the same group's ultrahigh resolution STM work.  He seemed very low key about their obviously impressive achievements - amazingly so.  I hope he got excited about this!)

Also, a group at Berkeley has made a laser based on a CdS nanowire, and like the result mentioned last week, this gadget uses plasmons (this time in a Ag film) to act as an effective cavity.  Clearly using the extreme confinement of some plasmon modes to do photonics is going to be a growth industry.

Industrial R&D

I've felt for a long time that the current business climate, which punishes rather than rewards long-term research investments by companies, is misguided. When most stock is owned and traded by institutional investors and large funds who don't have any interest in holding particular companies for the long term, and when executive compensation massively overvalues year-over-year growth (because we all know that 40% annual growth in cell phone sales is sustainable forever, right? There's no such thing as market saturation, is there?), you end up where long-term investment is viewed by company boards as a misuse of resources. This article in Business Week makes some interesting arguments on ways to try and fix this. Unfortunately I think most of these ideas are not very compelling or likely. Norm Augustine had an interesting suggestion: scale the capital gains tax rate inversely with the amount of time one owns a stock. If someone holds a stock less than a year, tax the capital gains at 90%. If they own the stock 10 years or more, tax the capital gains at nearly 0%. Interpolate appropriately. The idea here is to set up a system that incentivizes long-term investment, which in turn is more likely to support industrial research. Hard to see how such an overhaul would ever get passed in Congress, though. I imagine the financial industry would crush it like a bug, since anything that slows down trading is viewed as interference in the free market, or, more cynically, interference in their enormous transaction fee profits.

How we fund grad students

As new grad students flood onto campuses across the US, I just got around to reading this piece in Science from a few of weeks ago about Roald Hoffman's idea for changing the way we support grad students in the sciences and engineering. Most S&E grad students in the US are supported by a mix of teaching assistantships (TAs), research assistanceships (RAs), and fellowships. A typical S&E grad student at an American university shows up and is supported during their their first year by a mix of university funds and pay for teaching. They then often make the transition to being supported as an RA by research funds obtained by their advisor through research grants. (Some remain as TAs - this is more common at large, public institutions with large undergraduate teaching needs.) Some relatively small fraction of S&E grad students are supported instead by fellowships, awarded competitively by agencies like NSF, DOE, DOD, NIH, etc. or by private foundations such as the Hertz Foundation.

Prof. Hoffman suggests that we should move to a system where all grad student support is fellowship-based. The idea is that this will (a) fund only the best students; (b) allow students much greater independence since an advisor will no longer be able to say "You have to do boring experiment #23 because that's what the grant that's paying your salary says we're going to do"; (c) result in better mentoring b/c faculty will no longer view students as "hands". Now, there's basically no way to see how such a drastic change in the system would ever happen, but it's worth looking at the idea.

As someone lucky enough to have a fellowship in grad school, I understand the appeal from the student side. Independence is great - it means that you and your advisor are freed from the stress of worrying that your grant won't get renewed when you're in year 3 of your program. It means that you are a free agent.

However, I think Hoffman's idea would be a disaster, for two main research-related reasons (not to mention the challenge of how you'd handle TA duties at large places that suddenly had many fewer grad students). First, there is little doubt that this would skew an already tilted system even further in favor of the top, say, 20 institutions in the country. Right now it's possible for good researchers at second tier universities to write grants, hire students, and do research. Imagine instead if the only source of student support were competitive external fellowships. It's all well and good to talk about overproduction of PhDs, and say that drastically reducing the number of grad students would be good for employment and salaries. There is a point to that. However, you would effectively end research as an enterprise at many second and third-tier schools, and there are a fair number of really good programs that would go away. Second, since federally funded fellowships would presumably only go to US citizens, this idea would drastically reduce international PhD students in S&E. That, too, would be a mess. Some of our best students are international students, and whether or not they stay in the US after their degrees, training these people is a valuable service that the current US system provides.

It is worth considering other funding schemes, though. I know that in the UK students are supported through their PhD, rather than on a schedule set by external grant deadlines. Perhaps some of my UK readers could comment on the pluses and minuses of this approach.

plasmons instead of cavities

Sorry for the delay in posts. The beginning of the new academic year is a hectic time.

This paper is a very exciting new result. Unfortunately there does not appear to be a publicly accessible version available. Ordinarily, lasing (that is, light amplification by the stimulated emission of radiation) requires a few things. One needs a "gain medium", some kind of optically active system that has (at least one) radiative transition. In this paper, the medium is a dielectric oxide containing dye molecules known to fluoresce at a wavelength of 520 nm. This medium needs to be pumped somehow, so that there are more optically active systems in the excited state than in the ground state. This is called "population inversion". (It is possible to get lasing without inversion, but that's a very special case....) Finally, one generally needs a cavity - an optical resonator of high enough quality that an emitted photon stays around long enough to stimulate the emission of many more photons. The cavity has to be somewhat leaky, so that the laser light can get out. However, if the cavity is too leaky, the optical gain from stimulated emission in the pumped medium can't outpace the cavity losses. The usual approach is to have a rather high quality cavity, made using either dielectric mirrors, total internal reflection, or some other conventional reflectors.

In this paper, however, the authors take a different tactic. They use the near-field from the plasmon resonance of the gold core (not coincidentally, at around 520 nm wavelength) of Au-core-dielectric-shell nanoparticles. Plasmon resonances are often quite lossy, and this is no exception - the Q of the plasmon resonance is around 14. However, the enhanced near field is so large, and the effective mode volume (confinement) is so small, that gain still outpaces loss. When the dye is optically pumped, it is possible to make these nanoparticles lase. This paper is likely to spawn a great deal of further work! It's cool, and there are many clear directions to pursue now that this has been demonstrated.

This week in cond-mat

Where did summer go? Several interesting things on the arxiv recently. Here are two from this past week that caught my eye.

arxiv:0806.3547 - Katz et al., Uncollapsing of a quantum state in a superconducting phase qubit
This paper first appeared on the arxiv last year, and it made it onto this week's mailings because the authors uploaded the final, published version (PRL 101, 200401 (2008)). This experiment is important as a technical development in the quantum computing community, since the ability to restore some measure of purity to a quantum state after that state gets entangled with some environmental degrees of freedom could be very useful. It is also a great example of why simplistic thought experiments about wavefunction collapse are misleading. A better way to think about this experiment is in (an imperfect) analogy to spin echo in nuclear magnetic or electron spin resonance. In a spin echo experiment, an ensemble of spins is set precessing, and the evidence of their coherent precession gets smeared out as a function of time as the spins "dephase" (get out of sync because of perturbing interactions with other degrees of freedom). However, in these echo experiments, a properly defined external perturbation (a pulse of microwaves) can flip all of these spins around, so that the ones originally going ahead of the pack are put in the back, and the slow ones are put in the front. The spins rephase, or become coherent in their motion again. The authors do something rather analogous here using superconducting devices. Nice!

arxiv:0908.1126 - N. P. Armitage, Electrodynamics of correlated electron systems
I'm not promoting this just because Peter sometimes comments on this blog. This is a great set of lecture notes from a 2008 summer school at Boulder. These notes provide a very good, pedagogical overview of how electromagnetic radiation interacts with the electronic systems of real materials, and how one can use measurements ranging from the THz (mm-wave) to the ultraviolet to infer details of the electronic properties. These sorts of reviews are a wonderful feature of the arxiv.

LHC and the hazards of Big Science

This article in the NY Times about the LHC's current problems was interesting. To be fair, the LHC is an incredibly complex undertaking. Making high quality superconducting joints between magnets is a complex business, involving spot-welding annoying materials like niobium-titanium alloys. Testing is a real pain, since room temperature measurements can't always identify bad joints. Still, they clearly didn't design an optimal testing and commissioning regimen. I'm sure they'll get these problems licked, and great science will eventually come out of the machine - it's just a question of how long that'll take. I do wonder, though, if stories like this are, in part, a consequence of their own publicity machine, which has been hammering the general public relentlessly for years about how the LHC is going to unlock the secrets of the universe.

This situation is a prime hazard of Big Science. One thing I definitely like about condensed matter and AMO physics, for example, is that you are often (though not always) in control of your own destiny. Progress is generally not dependent on 1000 other people and 500 vendors and suppliers, nor do you have to hope that some launch schedule isn't screwed up by a hailstorm. The general public needs to know that really good science can be done on a much smaller scale. While the LHC outreach effort is meant to inspire young people into pursuing physics, situations like these delays and the accompanying reporting probably frighten away more people from the field than they attract. If a layperson ends up with the impression that all physics is hugely expensive, and even then doesn't work right, that's not a good thing.

Chemistry vs. Physics blogging

Interesting. The pseudonymous Kyle Finchsigmate at The Chem Blog just gave some stats about his blogging. He gets something like 6000 unique visitors a day, while I get about 150. Admittedly, we have rather different styles (pseudonymity makes it easy to write with more, umm, gusto, and to slam lousy papers openly, both of which probably make his blog have more broad appeal), and there are a lot more chemists out there than condensed matter physicists. Still, the factor of 40 is a bit intimidating.

More musing about phase transitions

Everyone has seen phase transitions - water freezing and water boiling, for example. These are both examples of "first-order" phase transitions, meaning that there is some kind of "latent heat" associated with the transition. That is, it takes a certain amount of energy to convert 1 g of solid ice into 1 g of liquid water while the temperature remains constant. The heat energy is "latent" because as it goes into the material, it's not raising the temperature - instead it's changing the entropy, by making many more microscopic states available to the atoms than were available before. In our ice-water example, at 0 C there are a certain number of microscopic states available to the water molecules in solid ice, including states where the molecules are slightly displaced from their equilibrium positions in the ice crystal and rattling around. In liquid water at the same temperature, there are many more possible microscopic states available, since the water molecules can, e.g., rotate all over the place, which they could not do in the solid state. (This kind of transition is "first order" because the entropy, which can be thought of as the first derivative of some thermodynamic potential, is discontinuous at the transition.) Because this kind of phase transition requires an input or output of energy to convert material between phases, there really aren't big fluctuations near the transition - you don't see pieces of ice bopping in and out of existence spontaneously inside a glass of icewater.

There are other kinds of phase transitions. A major class of much interest to physicists is that of "second-order" transitions. If one goes to high enough pressure and temperature, the liquid-gas transition becomes second order, right at the critical point where the distinction between liquid and gas vanishes. A second order transition is continuous - that is, while there is a change in the collective properties of the system (e.g., in the ferro- to paramagnetic transition, you can think of the electron spins as many little compass needles; in the ferromagnetic phase the needles all point the same direction, while in the paramagnetic phase they don't), the number of microscopic states available doesn't change across the transition. However, the rate at which microstates become available with changes in energy is different on the two sides of the transition. In second order transitions, you can get big fluctuations in the order of the system near the transition. Understanding these fluctuations ("critical phenomena") was a major achievement of late 20th century theoretical physics.

Here's an analogy to help with the distinction: as you ride a bicycle along a road, the horizontal distance you travel is analogous to increasing the energy available to one of our systems, and the height of the road corresponds to the number of microscopic states available to the system. If you pedal along and come to a vertical cliff, and the road continues on above your head somewhere, that's a bit like the 1st order transition. With a little bit of energy available, you can't easily go back and forth up and down the cliff face. On the other hand, if you are pedaling along and come to a change in the slope of the road, that's a bit like the 2nd order case. Now with a little bit of energy available, you can imagine rolling back and forth over that kink in the road. This analogy is far from perfect, but maybe it'll provide a little help in thinking about these distinctions. One challenge in trying to discuss this stuff with the lay public is that most people only have everyday experience with first-order transitions, and it's hard to explain the subtle distinction between 1st and 2nd order.

The Anacapa Society

Hat tip to Arjendu for pointing this out. The Anacapa Society is a national society that promotes and encourages research in computational and theoretical physics at primarily undergrad institutions. They've had a good relationship with the KITP at UCSB, and have just signed an agreement that gives them a real home at Amherst College. (I've had a soft spot for Amherst since back in the day when I was struggling to decide whether to do the tier-1 research route vs. the undergrad education trajectory.) The nice thing about promoting this kind of research is that, particularly on the computational side of things, well-prepared undergrads at smaller institutions can make real contributions to science without necessarily the expensive infrastructure required for some hardcore experimental areas.

Cute optics demo

This youtube video is something that I'll have to remember for a future demo. It shows that cellophane tape makes (1-side) frosted glass appear to be transparent. Quite striking! The reason this works is pretty straightforward from the physics perspective. Frosted glass looks whitish because its surface has been covered (by sandblasting or something analogous) with little irregularities that have a typical size scale comparable to the wavelengths of visible light. Because of the different in index of refraction between glass and air, these little irregularities diffusely scatter light, and they do a pretty equitable job across the visible spectrum. (This is why clouds are white, too, by the way.) By coating the glass intimately with a polymer layer (with an index of refraction closer to the glass than that of the air), one is effectively smoothing out the irregularities to a large degree. As far as I know, this is essentially the same physics behind why wet fabrics often appear darker than dry fabrics. Some of the apparent lightness of the dry material is due to diffuse scattering by ~ wavelength-sized stray threads and fibers. A wetting liquid acts as an index-matching medium, effectively smoothing out those inhomogeneities and reducing that diffuse scattering.

Phase transitions and "mean field theory"

One truly remarkable feature of statistical physics (and condensed matter physics in particular) is the emergence of phase transitions. When dealing with large numbers of particles one often finds that, as a function of some parameter like temperature or pressure, the whole collection of particles can undergo a change of state. For example, as liquid water is warmed through 100 C at atmospheric pressure, it boils into a vapor phase of much lower density, even though it is still made up of the same water molecules as before. Understanding how and why phase transitions take place has kept many physicists occupied for a long time.

Of particular interest is understanding how microscopic interactions (e.g., polar attraction between individual water molecules) connect to the phase behavior. A classic toy model of this is used to examine magnetism. It's a comparatively simple statistical physics problem to understand how a single magnetic spin (in real life, something like one of the d electrons in iron) interacts with an external magnetic field. The energy of a magnetic moment is lowered if the magnetic moment aligns with a magnetic field - this is why it's energetically favorable for a compass needle to point north. So, one does the statistical physics problem of a single spin in a magnetic field, and there's a competition between this alignment energy on the one hand, and thermal fluctuations on the other. At large enough fields and low enough temperatures, the spin is highly likely to align with the field. Now, in a ferromagnet (think for now about a magnetic insulator, where the electrons aren't free to move around), there is some temperature, the Curie temperature, below which the spins spontaneously decide to align with each other, even without an external field. Going from the nonmagnetic to the aligned (ferromagnetic) state is a phase transition. A toy model for this is to go back to the single spin treatment, and instead of thinking about the spin interacting with an externally applied magnetic field, say that the spin is interacting with an average (or "mean") magnetic field that is generated by its neighbors. This is an example of a "mean field theory", and may be solved self-consistently to find out, in this model, the Curie temperature and how the magnetization behaves near there.

Mean field theories are nice, but it is comparatively rare that real systems are well described in detail by mean field treatments. For example, in the magnetism example the magnetization (spontaneous alignment of the spins, in appropriate units) goes like (1-T/T_C)^1/2 at temperatures just below T_C. This is not the case for real ferromagnets - the exponent is different. Because of the nature of the approximations made in mean field theory, it is expected to be best in higher dimensionality (that is, when there are lots of neighbors!). Here's a question for experts: what real phase transitions are well described by mean field theory? I can only think of two examples: superconductivity (where the superconducting gap scales like (1-T/T_C)^1/2 near the transition, just as mean field theory predicts) and a transition between liquid crystal phases. Any others?

The elevator message

I had a conversation today that made me think about the following. These days we're told countless times that it's essential for a scientist to have an "elevator message". That is, we need to be able to describe what we're doing in a pitch accessible to a lay person ideally in something like a single sentence. Some people have a comparatively easy time of this. They can say "I'm trying to cure cancer", or "I'm trying to solve the energy crisis", and have that be a reasonable description of their overarching research goals. Condensed matter physicists in general often have trouble with this, and tend to fall back on things like "My work will eventually enable faster computers" or "...better sensors". I'm all in favor of brief, accessible descriptions of what scientists do, but there are times when I think the elevator message idea is misguided. Not every good research program can be summed up in one sentence.

In the case of my group, we are trying to understand the (electronic, magnetic, and optical) properties of matter on the smallest scales, with an eye toward eventually engineering these properties to do useful things. It's basic research. Sometimes we can test existing theoretical ideas or address long-standing questions; sometimes, because we're working in previously unexplored regimes, we find surprises, and that can be really fun. I know that this italicized section is more sophisticated and therefore less pithy than "it'll give us faster computers". Still, I feel like this longer description does a much better job of capturing what we're actually doing. Our work is much more like puzzle-solving and exploring than it is a focused one-goal pursuit. I don't think that this means I lack vision, but I'm sure others would disagree.

On a separate note: Thanks, Arjendu, for pointing me to this, Microsoft Research's hosting of a series of Feynman lectures at Cornell in 1964. Very cool, even if I had to install MS's plug-in for the video.

We need more papers like this.

Somehow I had missed this paper when it came out on the arxiv last November, but I came across it the other day while looking for something else in the literature. It's all about the challenges and hazards of trying to measure magnetization of either tiny samples or those with extremely small magnetic responses. Some of the cautions are rather obvious (e.g., don't handle samples with steel tools, since even tiny amounts of steel contamination will give detectable magnetic signals), and others are much more subtle (e.g., magnetic signatures from Kapton tape (due to dust! I learned about this one first hand a few years ago.) and deformed plastic straws (commonly used as sample holders in a popular brand of magnetometer)). Papers like this are incredibly valuable, and usually hard to publish. Still, I much prefer this style, writing a substantive, cautionary paper that is informative and helpful, to the obvious alternative of writing aggressive comments in response to papers that look suspect to you. The paper is so good that I'm even willing to forgive them their choice of font.

Figures and permissions - Why, AAAS?

Perhaps someone out there can enlighten me. For review articles, if you want to reproduce a figure from someone's published work, you are required to get permission from the copyright holder (e.g., APS for Physical Review, ACS for Nano Letters, etc.). As far as I can tell, the professional societies (APS, ACS) are cool about this, and won't charge you for permission. Even Nature, a for-profit magazine, does not charge for this if all you're doing is using a figure here and there. However, Science, run by the non-profit AAAS, wants to charge $31.25 per figure for permission to reproduce that figure in a review article. Why is Science doing this? Is this some attempt to recoup publication costs? Anyone got an explanation?

arxiv failure

It would appear that the arxiv is having some issues. Bizarrely, this seems to affect cond-mat, but not (for example) astr-ph. In cond-mat, asking for "recent" papers points you to October, 2008. Asking for "new" papers gets you things like:

New submissions for Wed, 8 Jul 09

Error with 0907.1092

Error with 0907.1096

Error with 0907.1111

Very odd. Hopefully this will be fixed soon. Come to think of it, this is the first problem I've seen like this in a decade of reading cond-mat.

This week in cond-mat

There have been a number of exciting (to me, anyway) papers on the arxiv this past week. One in particular, though, seems like a neat illustration of a physical principal that crops up a lot in condensed matter physics.

arxiv:0906.5206 - Tanda et al., Aharonov-Bohm Effect at liquid-nitrogen temperature: Frohlich superconducting quantum device

There are several examples in condensed matter physics of "special" (I'll explain what I mean in a second) electronic ground states that are "gapped", meaning that the lowest energy excited states for the many-electron system are separated from the ground state by an energy range where there are no allowed states. When I say that a ground state is special, I mean that it has some particular order parameter (or broken symmetry) that is distinct from that of the excited states. In this sense, a band insulator or semiconductor is not special - the many-body filled valence band states really don't have any different symmetries than the empty conduction band states. However, the superconducting ground state is special, with broken gauge symmetry (when compared to the normal metallic state) and a minimum energy (the gap energy) required to make any excitations (in this case, by breaking apart a Cooper pair). Fractional quantum Hall states are similarly gapped. The consequence of that energy gap is that the ground state can be very robust. In particular, the gap means that low energy (compared to the gap) inelastic processes cannot perturb the system, since there are no allowed final states around. This is one reason why it is possible to see macroscopic quantum effects in superconductors, as long as T is small compared to the gap.

The authors of this paper have decided to see whether such macroscopic quantum effects (detectable via quantum interference measurements analogous to the two-slit experiment) can survive in another gapped system. The distinction here is that the special state is something called a charge density wave (CDW), where the electronic density in a material (in this case tantalum trisulfide) spontaneously takes on a spatially periodic modulation. This gapped state kicks in at much higher temperatures than typical superconducting transitions. The authors have been able to measure quantum interference robustly in their device at liquid nitrogen temperatures, which is pretty impressive, and there is reason to believe that this could be extended to room temperature. The sample fabrication is very impressive, by the way. You can't just take a sheet of this stuff and punch a hole in it to make your ring-shaped interferometer. Instead, you have to actually curl a sheet up into a tube. Neat stuff, and quite surprising to me. I need to read up more about CDWs....

A cool result

There's a new asap paper in Nano Letters that is very slick. There has been a lot of interest in the last few years in plasmonics - the controlled manipulation of plasmons, collective oscillations of the electronic fluid in metals. Plasmons are pretty remarkable excitations. Because they involve displacement of the electron density, they necessarily result in local electric fields near metal surfaces (useful for optical antenna sorts of effects), and they can (under the right circumstances) couple efficiently to electromagnetic radiation. Plasmon response to light can be very pronounced, ranging from resonant scattering or absorption (for example, why certain types of glass are colored) to more complex dispersive effects, including negative (effective) indices of refraction. Plasmons are also responsible for helping light to transmit through sub-wavelength apertures. However, as far as I know, until now none of these effects have depended in any significant way on the angular momentum of light. In this new result, researchers from the Technion in Israel have designed aperture structures that can couple selectively to left- or right-circularly polarized light. The trick is in finding a situation such that the angular momentum of the light (essentially the spin of the photons) couples selectively to plasmon modes in the apertures that have matching orbital angular momentum. I don't fully understand how the two experiments described in the paper work, but it's a neat, clever result.

Four items

Four items, and a physics post later in the week.

• Is "just-in-time" supply chain management truly the work of the devil, or merely incredibly annoying? We've had a problem with a gate valve on a piece of cleanroom equipment at my institution, and the vendor (a) has no spare valves; (b) has no spare parts for the valves; and (c) says it'll take around 4 weeks to fab a replacement valve. Now, I understand why a business wouldn't want a huge inventory sitting on shelves, and that there are real fixed costs associated with inventories. Still, how hard would it be to have some spare parts, particularly when these things don't go bad when stored? I can tell you that it doesn't make me predisposed to ever buy anything from this supplier again. So, while it may be penny-wise, it sure feels pound-foolish for companies to alienate customers by having no backup supplies at all.
• Ahh, scientific publishing. Two folks from Cornell used an amusing computer program to generate a grammatically correct but completely nonsensical fake paper (pdf). They then got that paper accepted to an open-access journal, without the knowledge of the editor (!), with the strong implication being that this publisher was willing to publish literally anything as long as the authors are willing to pay the fees. Wonderful. I've suspected for a while (basically when a couple of publishers spammed me about being a contributing editor on journals I'd never heard of, back when I was a brand new assistant prof) that there are some shady practices out there.
• Also regarding scientific publishing, I was shocked and appalled (ok, not really, but certainly surprised) when I got the proofs of an article that we have coming out in Phys Rev B. Why? Because it was clear from the marked-up "author query" version of the manuscript that the AIP production office had converted our beautiful LaTeX manuscript into Microsoft Word format for editing. What is the world coming to?!
• Lastly, I was fortunate enough to receive a new iPod Touch as a gift. Anyone out there have suggestions for must-have apps?
  

The revolution will be twittered.

Not a physics post, but an observation. There is a major event going on in Iran right now - protests involving many thousands of people; rioting; the most political upheaval since the 1979 revolution. I hope that everything works out for the best - any country with a Supreme Leader needs a new governance structure, IMO. Anyway, twitter is being used as a major tool by the Iranian protesters. So much for my general perception that twitter was only for people more self-indulgent than bloggers (ahem.). It's fascinating and alarming to watch events unfold from halfway around the world, while CNN reports on things like Sarah Palin/David Letterman feuds. It's as though the "news" network has forgotten what real news is....

Nanoscale, the book

No, I have not compiled my blog postings into dead-tree format. Nor have I finished my textbook based on my graduate nanoscale physics course sequence. Instead, I wanted to point out this book, which is a cute volume with lots of computer-rendered pictures of crystal structures and the like. It's an admirable attempt to give the reader a sense of the atomic-scale composition of materials, along with brief, informative, often fun descriptions. While there are a few minor typos that seem to be caused by autocorrection run amok, the book remains entertaining and educational, with very well crafted illustrations. The book has its own website, too.

This week in cond-mat

Two papers appeared on the arxiv in the last couple of days concerning the very hot topic of quantum-limited measurement. I'm no expert in the area, but here's a quick summary of the idea.... Anyone who's read anything about quantum mechanics is familiar with the popular "gamma-ray microscope" thought experiment meant to highlight the Heisenberg uncertainty relation. In lay terms, trying to use light to determine the location of a particle with arbitrarily high precision requires, in a simple thought experiment, light of a correspondingly short wavelength. Shorter wavelength = higher energy photons = higher momentum photons = big momentum transfer to the particle. Thus, the more precisely you localize the particle, the less you know about its momentum. This is an adequate handwave for the popular press, but the real situation can be more subtle. Still, in the general problem of quantum measurement, one is often concerned about "back action" - the fact that coupling your system to a detector (thus enabling you to make some kind of measurement of an observable) generally perturbs the equations of motion of the system itself. It turns out, under certain very special circumstances, it is possible to design a measurement and pick observables such that the effect of back action is essentially confined to some variable that you don't care about. The net result in that case is that you can measure your particular observable to higher precision than a simplified uncertainty argument would suggest is possible.

Two groups, those of Keith Schwab at Cal Tech (paper here) and Konrad Lehnert at Boulder/JILA (paper here), have managed to do this type of measurement, looking at the position of a nanoscale mechanical resonator. In both cases, they are able to couple the resonator to a microwave LC resonator in such a way that they can measure the mechanical displacement better than the standard quantum limit. These measurements are very technologically impressive, and they open up the path toward really exciting possibilities, including entanglement of different nanomechanical systems, clever cooling schemes, and true quantum mechanics measurements.

Random tidbits

Several minor things....

• I've got an article on single-molecule electronics coming out in the June issue of Physics World. It's reasonably accessible, and I'm pretty happy with how it turned out, though I wish there had been more space to discuss the theoretical challenges.
• This is damned cool. I had an undergrad course that was like the baby version of this - building up transistors into logic gates; then using logic gates to build a shift register; then building and programming a little 6502-based computer to run a model train network. This guy's work puts all that to shame by comparison.
• The pseudonymous Kyle Finchsigmate, always entertaining and clever (often profane), has started a wiki site devoted to chemistry experimental techniques. In comments about that I came across this site from Rochester. I think it would be great to have a site like this about experimental physics, though clearly it would take a lot of work from many people to have it be any good....
• I've been asked by a reader to solicit discussion and opinions about the various journal online manuscript submission/review systems. Which ones are good, and which ones are lousy? From what I can tell, the APS system is decent (though it always seems to complain erroneously about mistakes in my references and article lengths), and the Paragon system from ACS is quite good. The Nature publishing group one also seems to be put together well. I'm not a fan of "Manuscript Central" or whatever it is that Elsevier and IEEE use. What do you all think?
• Thank goodness McLeroy was not confirmed as head of the TX board of education.
  
• This'll be the last update for about the next 9 days or so, since I'll be traveling with very limited 'net access.
  

Plastic Fantastic thoughts

Reading Eugenie Reich's Plastic Fantastic brought me right back to the heady days of my postdoc, job search, and nearly a year spent with a student chasing what turned out to be fabricated results. In hindsight I learned an awful lot about human nature and the sociology of science, and some of that is conveyed to readers of this book, though not all.

First, the book review. I think Reich writes well, and I think she did a good job simplifying the science where appropriate for a more general audience. Criticizing the details (e.g., I wasn't a big fan of her definition of "polaron") misses the larger point (you don't need to know what a polaron is to appreciate the fact that Schon didn't fully get what polarons are either). Personally I think it would have been useful to spend more time on standard scientific practice at Bell Labs - Schon's claims (going back to his doctoral work in Germany) that he didn't keep notebooks or save primary data aren't just damning - they're completely outside what I saw essentially everyone else do, both at Bell and in grad school. How on earth did this happen? How did no one immediately supervising Schon never notice that he had no notebooks?! The idea that researchers at Bell were so independent that no one would ever notice this is crazy. I also think it would have been good to spend a bit more time on the denoument, at least discussing further the major issues raised by this whole affair: what are the responsibilities of co-authors? What are the responsibilities of managers? There were also some nuances of what happened as the scandal broke that I didn't see (though I could've missed them on a quick read), including some choice remarks by Batlogg that were rather remarkable at the time. [One other point: Reich points out that the Departments of Defense and Energy don't have central offices of research integrity. Strictly speaking, that's right, but the way it's written makes it sound like DOD and DOE never even consider the matter, which is not true. Since 2000, anyway, DOE has used the following (pdf) policy regarding research misconduct, which is basically the blanket federal policy applied at DOD as well.] In the end, the book is very effective at what it does, though it raises many more questions than it answers.

Regarding specific comments of others.... I don't think management was dealt with unfairly here in general. I didn't feel like Cherry was particularly singled out. Also, the book doesn't convey well one factor that I think is important to remember: most of the immediate managers (e.g., Rogers, Capasso) were running large, active research programs of their own. There's no question that between that and the corporate turmoil from the collapse of telecom, these people had other things on their minds than trying to manage Schon. Now, that being said, how in the name of all that is holy did these people not realize that Schon's publication rate was simply unphysical? NO ONE can write a paper every two weeks for two years. Didn't this raise questions at the journals, too? One other comment about management that was raised only indirectly.... There were a number of people who were thrilled to claim (effectively) some share of the credit for this stuff when things looked good, but were quick to disavow all responsibility when things went bad. You can't have it both ways.

(One final point that has nothing to do with the author: the choice to put a silhouette of Icarus on the cover is deeply flawed. Icarus actually flew.)

Anyone read this yet?

I was in Barnes & Noble yesterday evening and saw a copy of Plastic Fantastic in their science section. This is Eugenie Reich's telling of the Schön saga. Anyone out there had a chance to read this yet? Steve? Don? I'll have to pick up a copy at some point.

Wolfram|Alpha: not too impressive.

By now many of you have run across Wolfram|Alpha, billed by its creator as a "computational knowledge engine". I've been goofing around with it a little over the past two days, and I'm not too impressed, though there are some cute things in there. The demonstration video, narrated by Wolfram himself, is very slick, and gives you the impression that Wolfram|Alpha can take even minimalistic requests (e.g., "Germany US GDP") and provide lots of computed output (US and German GDPs side by side as a function of time, in various different currency units and normalizations, for example). That is sort of true, for a very limited subset of queries. As one might expect from the people who developed Mathematica, Wolfram|Alpha can also do some symbolic math, including graphing of functions.

Unfortunately, it would appear that their model is to have these kinds of limited queries templated by hand on their side. Trying to ask well-defined questions about comparatively simple things ("What is the resistance of a wire?"), which you might expect from the demo to call up a pretty set of dialog boxes, etc., instead gives you "Wolfram|Alpha isn't sure what to do with your input." In this particular example, just "resistance of a wire" calls up dialog boxes about US and UK wire gauges and is at least somewhat useful. For a parser to do fine with "resistance of a wire" and gag on "what is the resistance of a wire" is pretty sad these days.

Bottom line: the idea of Wolfram|Alpha is cute, but right now it's entirely too much like playing an old text adventure game:
----
You are facing a brown, wooden door set in a dark green frame. There is a doorbell button here.

>Ring the doorbell.

I do not know how to do that.

>ring doorbell

I do not know how to do that.

>push button

You push the button, and from within the house you hear a distant chime.

Faking APS email not a good way to be taken seriously

Many of us know the joy of getting email from, err, enthusiastic amateurs claiming to have solved all of the great problems of modern physics (often involving the invalidation of quantum mechanics, relativity, or both). This morning's allotment was particularly amusing, though. Subject line: Giant Revolution in the Physics Science. From: [allegedly] aps@aps.org. (Really from someone in Hungary.) It explicitly claims to be a message on behalf of about a dozen physicists (presumably not with their actual permission), including last year's Nobel Laureates. Even better, it asks us all to contact the Royal Swedish Academy (complete with contact information) and pressure them to award the Nobel in physics to a Hungarian physicist who "reinterprets the total known experimental results and uses solely the mathematical apparatus of dynamics and electrodynamics". Amateurishly spoofing email from people is no way to promote yourself....

This week in cond-mat

Two recent arxiv papers caught my eye. I'm not working on graphene, but these are both pretty interesting results.

arxiv:0905.0923 - Mak et al., Observation of an Electric-Field Induced Band Gap in Bilayer Graphene by Infrared Spectroscopy
The authors, from Tony Heinz's group at Columbia, make a field-effect device out of bilayer graphene (identified optically thanks to its particular Raman spectrum) and an electrolyte. As I'd mentioned once before, by using electrolytes it is possible to achieve very large gated charge densities in transistor-style devices. In this case, the authors find that they can turn bilayer graphene from a semimetal-like system (with touching valence and conduction bands at the charge neutrality point) to a semiconductor (as determined via optical measurements), with a band gap induced and controlled by the gate. I need to read more carefully just how this works, but it shows how these kinds of experiments (moving a good fraction of a charge per unit cell around) can alter band structure profoundly.

arxiv:0905.1712 - Li et al., Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
This paper, published this week online in Science, may end up being quite important. The authors show that they can grow mostly single-layer graphene on copper supports. Copper can be annealed to produce large (several mm) crystallites, so significant areas of graphene can be made this way, templated with comparatively few defects. The big step here compared to earlier work on growing graphene using Ru or Ni substrates is that the resulting material seems to be self-limiting in thickness because of the mutual solubility limits of C in Cu and Cu in C. The authors can also transfer the graphene to other substrates, including Si chips, a necessary step for any would-be electronics applications.

Star Trek

I saw the new Star Trek movie last night, and it was extremely fun. Great special effects, with nods to all the appropriate elements of the original series. I also loved the score - Michael Giacchino rocks. (Sorry for the wiki link - his website seems to be down.) Sure, the science can be goofy, and the plot has some flaws, but somehow it really felt like old-school Trek in all the right ways.

Talks in 90 seconds

At the workshop I'd mentioned, because of time constraints only 11 of the participants had the chance to give full talks. To try to get a better sense of what everyone else was working on, the organizers let anyone else who wanted to speak give a 90 second talk after dinner on the first night. This was a fascinating exercise, taken in good fun by all involved. The time limit was enforced by a big analog timer with a loud buzz.... As you might imagine, conveying what your group is doing given just 90 seconds is a challenge. About 19 people tried this, using anywhere from one to ten (!) powerpoint slides. It worked surprisingly well, actually. A neat idea.

The workshop experience

I just finished attending a three-day workshop near Washington, DC sponsored by the National Nanotechnology Initiative (and other "participating agencies", since technically the US Department of Defense is not strictly part of the NNI, for reasons which escape me), focused on "nanotechnology-enabled sensing". Since I've only backed into anything sensing related in the last couple of years, this was very educational for me. The workshop was put together, in part, by Roger van Zee from NIST and Gernot Pomrenke from AFOSR, and was tasked with producing a report on the topic. The report is actually supposed to be useful to diverse a diverse audience, from technical contacts at the various agencies to Congressional staffers and the White House Office of Science and Technology Policy. I learned a lot, and met many interesting people from other universities, national labs, industry, and federal agencies.

As you might imagine, producing the report is quite a task. You have to corral 30-40 PhD researchers (who all have their own areas of expertise and writing styles), and build up a consensus document that is comprehensive, readable, brief but with some technical depth, and covers an extremely broad topic. Sensing is particularly tricky, since there are many many transduction mechanisms, many many things that people want to detect (including small molecule chemicals, biomolecules, cells, physical variables, EM radiation), and lots of ancillary requirements (biocompatibility? specificity? portability? low power?). It's like herding cats, but in the end I think this report should do a good job of highlighting what nano can do for sensing. (Here's the five-word executive summary: Sensors good, nanosensors much better. See? Who says that it's tough to write for non-scientists....)

Small really is different

Shameless self-promotion part II. The actual scientific result that just came out in Nature is rather surprising. There are two "ordinary" settings for Kondo physics: a magnetic impurity in an otherwise nonmagnetic host (e.g., dilute Mn atoms in Cu), or a quantum dot containing an unpaired electron. In the former case, the conduction electrons of the host metal can lower their kinetic energy by trying to occupy a singly occupied d orbital of the magnetic impurity. However, because of the Coulomb repulsion of the other electrons on the impurity atom, really doing this is classically forbidden by energy conservation. Still, quantum mechanics lets that forbidden state exist as a virtual intermediate state in a scattering process that takes a conduction electron from the host, flips the spin of the impurity atom, and spits out an electron into a different conduction band state. In the quantum dot case, an analogous magnetic dance takes place, in which the spin of the unpaired electron on the dot is flipped, and an electron is transferred across the dot. This Kondo scattering process affects the electronic conduction through the dot in a particular, identifiable way.

The surprising result in our case is that we see indications of this Kondo process in atomic-scale junctions between chemically homogeneous (e.g., all the atoms are Ni) ferromagnetic metals. The data are pretty clear, and indicate that this spin-related process competes with ordinary ferromagnetic exchange in these nanostructures. It would appear, from accompanying theory calculations by our coauthors, that the very act of whittling the ferromagnetic metal down to the atomic-scale junction is enough to mess with the electronic properties of the metal that we'd ordinarily consider to be intrinsic. The bottom line is, when worrying about the magnetic properties of truly nanoscale structures (with many surface atoms), one may need to keep track of relatively exotic ("strong correlation") physics like the Kondo effect.

An ideal collaboration

I just had about the best possible experience with a collaboration that one can expect to have. Indeed, I worry that I've now used up my "collaboration karma". Here's how these things are supposed to work....

Back at the APS March Meeting in 2006, my student Zach was presenting his masters work on electronic conduction through atomic-scale Ni junctions. Specifically, he had been doing some experiments to try and examine whether atomic-scale contacts between ferromagnetic metals had unusually large changes in electrical resistance when placed in a changing magnetic field, as had been reported in the literature. (We found that the answer is "No", but the magnetoresistance does depend in detail on the precise atomic configuration of the device. This work was independently confirmed simultaneously by Dan Ralph's group at Cornell.) Anyway, at the end of the session, I met Carlos Untiedt, who was just getting going as a faculty member at the University of Alicante in Spain. I'd read some of Carlos' earlier work on metal junctions made using mechanical means, and he'd read our work, too. He mentioned to me that the Spanish government has a program that allows Spanish graduate students to spend time abroad working in other labs, and suggested that we try this at some point. I said that this sounded like a good idea, and we should do it.

Fast forward to the beginning of 2008, when Carlos and I got back in touch. He had a very good student eager and interested to come and visit, and, even better, they had some exciting data that they'd been taking in mechanically-controlled (STM-style, middle of this page) atomic-scale metal junctions. The main advantage of mechanical junctions is that you can break and re-form them many times, giving you serious statistical information about junction properties. Now, in my lab we often use an alternative technique for making atomic-scale junctions that doesn't involve mechanical motion. While our method (electromigration) is more time-consuming and therefore not well suited to really large statistical samples, it has one main advantage: the junctions we make have enough geometric stability that we can look at a single junction over many temperatures. This can't really be done in STM-style junctions. This was a relatively rare situation: there was an ideal point of scientific collaboration, and we had the person and the resources to make things happen.

So, we did it. Carlos' student, M. Reyes Calvo, came and spent a little under four months working in my lab with my group. She was able to make junctions with our approach that were analogous to the ones that she'd been studying in Spain, and measured them as a function of temperature in our system. The results were very nicely consistent with her data from Spain, and the whole scientific story hung together well. After her visit and a number of fun conversations with theorist colleagues at Alicante, a paper was written that came out today in Nature. It just doesn't work any better than that. I'll write about the science in a separate post....

Nice speech.

President Obama addressed the US National Academy of Sciences this morning (video link here). Students, take note. In addition to boosting funding for basic research and making the R&D tax credit permanent, he's talking about tripling (!) the number of NSF graduate fellowships.

Just stop.

Attention TX state and federal officials with R next to their names. Let me clue you in on a couple of points.
1) Secession is not an option. See the US Civil War.
2) TX does not have the authority to break up into smaller states autonomously. That went out the window when TX was re-admitted to the Union after the Civil War.

Bloviating about this pointless drivel makes the entire state look bad. Don't you realize that this garbage makes it difficult to convince smart people to move here, because it looks like the state is governed by idiots?

(This is my last Texas post for a long while - I promise.)

Random favor....

I use the free version of google analytics to do some simple tracking of page views, etc. on both this blog and on my group webpage, just for fun. For some strange reason, the little javascript doodad that allows google to track hits works just fine on all of my group-related pages (like this one and this one), but fails on my publications page. If someone out there with greater expertise or sharper eyes than me could take a look at the html source and explain to me what's wrong with my publications page, I'd greatly appreciate it. Thanks. UPDATE: Thanks - all fixed, I think. Behold the power of teh intarwebs.

Cold fusion, the longer story.

I fully expect angry comments about this....

Here's how a cold fusion experiment is supposed to work, broadly. One takes an electrochemical cell containing either regular water or D₂O, and as one electrode uses palladium (prepared in some meticulous way, to be discussed further below). Then one sets the electrochemical conditions such that hydrogen (or deuterium) ions are electrochemically favored to go into the palladium lattice, up to some very high loading. It's been known for decades that Pd likes to take up hydrogen, so the fact that one can do this is of no surprise. Now, while all this is going on, one carefully monitors the temperatures of the electrodes, the water, etc. The experimental claim, coarsely described, is that after some time, cells containing heavy water under these conditions begin to get hot (but not cells containing ordinary water!). Ideally one does good calorimetry and can measure the amount of energy that comes out of the cell in the form of heat, vs. the amount of energy put in in the form of integrated electrochemical current times voltage. The claim is that in some such experiments, the inferred amount of energy out is much larger than the electrical energy in. This is "excess heat".

So, what's the problem? Well, there are several issues.
1) Calorimetry can be a tricky business. This was the main criticism of the original Pons and Fleischmann work. From what I can tell, people have been much more careful about this than twenty years ago.
2) The experiments just aren't reproducible, in many senses of the term. For example, the temperature-vs-time evolutions of nominally identical cells are completely different, and all over the map. There are big fluctuations on many timescales all over the place. Sometimes the thermal output is big, sometimes it's small. This is generally swept aside by those doing the experiments, who take a wildly fluctuating response, integrate it, and claim reproducibility because the net integral ends up having the desired sign. Not the desired magnitude, just the desired sign. What would I expect to see in a well-controlled experiment? Take one large piece of palladium, cut it into thirds, and set up three identical cells. The temperature-time histories of these things should really reproduce. If you can't do that, then you don't have a controlled experiment. This isn't a small thing.
3) The cells stop working after a while. Unsurprisingly the time period varies from cell to cell. Now, why should this happen unless the underlying process is chemical in some way? By the way, some cells (but not all) "revive" when the electrochemical conditions are changed. Again, all of this is massively variable, even between nominally identical cells in the same labs.
4) The claim of excess heat assumes that there's no chemistry taking place. For example, what if I made that assumption and looked at my car engine? The amount of electrical power input by each spark plug is miniscule compared to the total power out. If I neglected chemical reactions, I'd come to the conclusion that something amazing was going on. Furthermore, if I normalized the output power by, say, the number of platinum atoms at the tips of the spark plugs, I might then conclude that the only way of achieving such power out was something like nuclear. That's the hazard of ignoring possible chemical channels. The issue here is that palladium is known to be highly catalytic, and there are certainly diffusion processes within solids that can be strongly influenced by isotopic differences. Moreover, the claim is also that surface prep of the Pd is of absolutely critical importance. Again, this sounds to me like catalysis, not a bulk effect. Now, you'd think this could all be resolved by analytical chemistry - look at the cell materials before and after running. Look at the water before and after running. However, remember that the different folks doing this disagree on basic analytical chemistry issues like the possible production of helium, tritium, etc. That has to make you wonder about how trustworty their collective analyses are.

Now, I'm not saying that there's nothing worth examining here. The DOD clearly thinks its worth looking into, and it would be nice to get this straightened out once and for all. However, 60 Minutes notwithstanding, the work is just not reproducible in the sense that most experimental physicists would use.

Cold fusion.

Tonight 60 Minutes is airing a report on cold fusion research. I haven't seen the report, but this whole business is what I was referring to obliquely back when I wrote this. I'll write more about this soon, since it provides a nice vehicle for talking about good experiments and what we mean when we say that something is "reproducible" and "controlled". The short version: (1) Don't go out and buy palladium futures just yet. (2) There is something weird going on in the experiments, but it's not at all clear that it has anything to do with nuclear processes, since that would require at least two "miracles" (= totally unexpected deviations from established physics). (3) A careful look at this by careful people is probably worthwhile, but massive hype is a bad idea.

This just in....

The governor of Texas doesn't always think before he talks. sigh.

What is temperature?

Another in my off-and-on series about condensed matter physics concepts.

Everyone has an intuitive grasp of what they mean by "temperature", but for the most part only physicists know the rigorous definition. Temperature, colloquially, is some measure of the energy stored in a system. If two systems having different temperatures are placed "in contact" (so that energy can flow between them via microscopic interactions like atoms vibrating into each other), there is a net flow of energy from the high temperature system to the low temperature system, until the temperatures equilibrate. Fun fact: the nerves in your skin don't actually sense temperature; rather, they sense the flow of thermal energy. Metals below body temperature generally feel cold to the touch because they are very effective at conducting away thermal energy. Plastics at the same temperature feel warmer because they are much worse thermal conductors.

Anyway, temperature is defined more rigorously than this touchy-feely business. Consider a system (e.g., a gas) that has a certain amount of total energy. That system can have many configurations (e.g., positions and momenta of the gas molecules) that all have the same total energy. Often there are so many configurations in play that we keep track of the log of the number of available configurations, which we call (to within a constant that gives us nice units) S. Now, what happens if we give the system a little more total energy? Well, (almost always) this changes the number of available configurations. (In the gas example, now more of the molecules have access to higher momenta, for example.) How many more configurations? A simple assumption here is that the change in E is linearly proportional to the change in S. The proportionality factor is exactly T, the temperature. At low temperatures, a given change in E implies a comparatively large change in the number of available configurations; conversely, at high temperatures, a given change in E really doesn't increase the number of available configurations very much. (I know that someone is going to object to my gas example, since even for a single molecule in a box there are, classically, an infinite number of possible configurations for the molecule even if we only keep track of positions. Don't worry about that too much - we have mathematically solid ways to deal with this.)

Those in the know will already be aware that S is the entropy of the system. The requirement that the total S always increase or stay the same for a closed system ends up implying just the familiar properties of temperature and average energy flow that we know from everyday experience.

Writing and word limits

I'm working on an article for nonspecialists about a nano topic, and all I can say is, writing concisely for nonexperts is much more difficult than writing concisely for experts.