Monday, January 30, 2012

This is damned peculiar....

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

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

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

Tuesday, January 24, 2012

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.

Monday, January 16, 2012

"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.

Wednesday, January 11, 2012

From around the web


Wednesday, January 04, 2012

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.

Monday, January 02, 2012

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?