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?

Tidbits.

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

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

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

Shifting gears

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

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

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