Thursday, September 27, 2012

More recent hot topics

Still working on proposals, so this will be brief.  However, as mentioned in my previous post, I did want to add in some topics/open questions that are fairly hot right now:
  • Topological insulators - These materials are nominally band insulators, in that they have a filled band of electronic states, an energy gap, and an empty conduction band.  However, unlike ordinary band insulators, these have an odd number of states that live at their surface that exist in the band gap.  Because of strong spin-orbit coupling, the spin of a carrier in one of these surface states is locked in a particular orientation relative to the carrier's momentum.  That tends to suppress large angle scattering by ordinary disorder, since ordinary disorder scattering doesn't flip spin.  One big question is, can these materials be grown in such a way that the bulk really is insulating?  During crystal growth, it is energetically cheap to form point defects in many of these materials that act as dopants, leading to serious bulk conduction.  Recent work has found materials (e.g., Bi2Te2Se) that are less problematic, but no one has (to my knowledge) figured out a way to grow really insulating thin films that retain the cool surface state properties.  A second question is, can one actually employ these surface states for anything useful?
  • Gating in strongly correlated materials - pioneered by Iwasa's group in Japan, there has been a boon in using ionic liquids (essentially molten organic salts) as electrolytes in gating experiments.  These liquids allow one to obtain surface charge densities comparable to those possible in chemical doping, on the order of one charge per unit cell on the surface.  That's enough to do interesting physics in strongly correlated materials (e.g., gating an insulating copper oxide layer into superconductivity).  How far can one push this?  Can this technique be used to develop a transistor based on the Mott metal-insulator transition?  
  • Quantum computing - this isn't new, of course, but there seems to be more and more work going on toward making some form of solid state, scalable quantum computer.  Which of the competing approaches will win out?  Spins, with their amazingly long coherence times in isotopically pure Si and diamond?  Superconducting flux or charge qubits?  It does not look like there is any fundamental reason why you couldn't have quantum computers, but it's an enormous technical challenge.
  • Optomechanics - there are a number of groups out there having lots of fun looking at micro- or nanoelectromechanical systems and coupling them to optics.  This lets you do optical cooling methods to put the mechanical systems into low-occupation quantum states; this lets you entangle the light with with mechanical system; etc.  What are the ultimate limits here?  Could this usher in a new style of precision measurement, or lead to new quantum information manipulations?
  • Plasmonics - we're firmly out of the stage now where every weird metal nanostructure with a plasmon resonance was netting a high profile paper.  Instead, people are looking at using plasmons to confine light to deep subwavelength scales, for super-tiny optical emitters, detectors, etc.  How small a laser can one make using plasmonics?  What other quantum optics tricks can one play with these tools?  Can plasmonic effects be engineered to improve photovoltaics significantly, or photocatalysis?
There are many more, of course.  Suggestions and discussion, as always, are appreciated.

Wednesday, September 19, 2012

Controversies/hot topics in condensed matter, revisited

A little over six years ago (!), I wrote this post, where I listed a whole series of topics that were hot/exciting/controversial at the time.  While I'm traveling for a NSF site visit and working on a couple of proposals, I thought that now might be a good time to bring up this list again.  Here is a too-brief update/scorecard, with current statements in blue.  In a followup post I will try to add some new topics, and I invite suggestions/submissions in the comments.  The previous list was:
  • 2d metal-insulator transition - What is the mechanism for the apparent metal-insulator transition in 2d electron and hole systems at low densities? Is it profound or not?  I admit, I haven't followed this as closely as I should have.  The old argument had been that the scaling theory of localization (a theory that essentially neglects electron-electron interactions) says that any 2d electronic system becomes an insulator at zero temperature in the presence of arbitrarily weak (but nonzero) disorder. 
  • High-Tc - what is the mechanism of high temperature superconductivity? What is the ultimate limit of Tc? What is the "bad metal", and what is the pseudogap, really? How important are stripes and checkerboards? Is the phrase "doped Mott insulator" really a generic description of these systems?  We still don't have a definitive, broadly agreed answer to these questions, though progress has been made, in large part due to continually improving sample material quality.  It sure looks like there can be other phases (including ones involving spatial patterns of charge like stripes and checkerboards) that compete with superconductivity.  It also looks like high temperature superconductivity often "dies" as T is increased due to loss of global phase coherence.  That is, there are still paired electrons above the (bulk) critical temperature, but the pairs lack the coordinated quantum mechanical evolution that gives what we think of as the hallmarks of the superconducting state (the perfect expulsion of magnetic flux at low magnetic fields; zero electrical resistance).  Since I wrote the above, a whole new class of superconducting materials, the iron pnictides, has been discovered.  While in their normal state they are generally not Mott insulators, electronic correlations and the competition between different correlated ground states do seem to be important, something broadly similar to what is seen in the cuprates.  
  • Quantum criticality and heavy fermions - Do we really understand these systems? What are the excitations in the "local moment" phase? What is the connection to high-Tc, if any?  My faculty colleague who is an expert on quantum criticality would give a definite, though qualified "yes" to that last question.  I think (and please correct me in the comments if you disagree) that how to properly describe low energy electronic excitations of systems when the quasiparticle picture breaks down (that is, when the carriers don't act roughly like ordinary electrons, but instead are "incoherent") is still up in the air. 
  • Manganites - What sets the length scale for inhomogeneities in these materials?   I believe this is still up for discussion, though it's known that the effects of disorder and strain can make it very challenging to pull out truly intrinsic physics.
  • Quantum coherence and mesoscopics - Do we really have a complete understanding of mesoscopic physics and decoherence at this point? What about in correlated materials?  In normal ("boring") metals at low temperatures and in ordinary semiconductors, it looks like we do have a pretty good handle on what's going on, though there are still some systems where the details can get very complicated.  As for strongly correlated materials (when electron-electron interactions are very important), I still have not seen a lot of work directly looking at the issue.  This is related to the point above about quantum criticality - if you can't readily describe the low energy excitations of the system as particle-like, then it can be tricky to think about their quantum coherent properties.
  • Quantum Hall systems - Are there really non-Abelian states at certain filling factors? In bilayers, is there excitonic condensation?  Cautious answers on both these counts appear to be "yes".  There has been quite a bit of lovely work looking at the 5/2 fractional quantum Hall state (including very cute stuff by my postdoctoral mentor) that seems entirely consistent with non-Abelian physics.  Likewise, the recent work making the case for Majorana fermions in semiconductor/superconductor hybrid systems shows that there is hope of really studying systems with non-Abelian excitations.  In the case of the quantum Hall bilayers, work by Jim Eisenstein's group at Cal Tech looks very exciting (if I'm leaving out someone, my apologies - I haven't followed the area that closely).
  • 1d systems - Is there conclusive evidence of spin-charge separation and Luttinger liquid behavior in semiconductor nanowires? Nanotubes?  I think the case for spin-charge separation is better now than it was six years ago, due to very nice work by multiple groups (Yacoby now at Harvard; the gang at Cavendish in Cambridge, for example).
  • Mixed valence compounds - Is there or is there not charge ordering at low temperatures in Fe3O4, something that's been argued about for literally 60 years now?  This seems to have been settled in the Fe3O4 case:  the system has some amount of charge disproportionation, orbital ordering, and the electron-phonon coupling is not negligible in looking at the physics here.
  • Two-channel Kondo physics - Is there firm evidence for the two-channel Kondo effect and non-Fermi liquid behavior in some physical system?  A qualified "yes", in that the Goldhaber-Gordon group at Stanford made a tunable quantum dot system that can sit at a point that looks like 2-channel Kondo physics is relevant.  However, I haven't seen anyone else following up on this, probably in part b/c it's very hard.
  • Molecular electronics - Is there really improving agreement between experiment and theory? Can novel correlation physics be studied in molecular systems? Can molecules exhibit intrinsic (to the molecule) electronic functionality?  In order, "yes", "yes" (interesting underscreened Kondo physics and other quantum impurity problems, for example), and "yes" (e.g., optically driven switching between isomers with different conductances), though as I've said for years, we're not going to be building computers out of these things - they're tools for looking at physics and chemistry at the nanometer scale.
  • Organic semiconductors - What is the ultimate limit of charge mobility in these materials? Are there novel electronic correlation effects to be seen? Can one see a metal-insulator transition in these systems?  In order, "it still remains to be seen" (though mobilities on the order of 10-100 cm^2/Vs have been shown); "maybe" (if one counts experiments looking at charge transfer salts, Mott transitions, etc.), and "yes" (if one uses, e.g., ionic liquids to obtain exceedingly high carrier densities).
  • Nanomechanical systems - Can we demonstrate true "quantum mechanics", in the sense of a mechanical system that acts quantum mechanically?  Yes - see Science's breakthrough of the year in 2010, for example.
  • Micro/nano systems to address "fundamental physics" - Can we measure gravity on the 100 nm length scale? Are there experiments with Josephson junctions that can probe "dark energy"?  "Not yet" and "Not yet", though like atomic physics I think it would not be surprising if condensed matter produced some systems that could be used in precision measurement tests looking at these kinds of issues.

Thursday, September 13, 2012

Room temperature superconductivity? Probably not yet.

A paper appeared on the arxiv recently (published in Advanced Materials) from a group in Leipzig reporting magnetic measurements that the authors argue are suggestive of some kind of room temperature superconductivity in highly oriented pyrolitic graphite (that's enhanced after a particular treatment involving water).  Unsurprisingly, this got a bit of attention.  So, what is the deal?

Well, it's been known for a long time that clean graphite has a very large diamagnetic response.  That means that when a magnetic field is applied to the material from the outside, the field inside the material is less than the external field.  This can happen in a couple of ways.  In a type-I superconductor, at low applied magnetic fields, persistent "screening" currents are set up that completely cancel the externally applied field.  This perfect diamagnetism is called the Meissner effect, and is a signature of superconductivity.  (Why this happens is actually a pretty deep question - just accept for now that it does.)

The investigators have spent a long time staring at the magnetization of their graphite as a function of applied magnetic field and temperature, and they argue that what they see could be a signature of some kind of "granular" superconductivity.  This means that the bulk of the material is not superconducting.  In fact, you can compare the measured diamagnetism with what one would expect for a perfect diamagnet, and if this is superconductivity, only about 0.01% of the sample is superconducting.  Still, the systematics that they find are interesting and definitely worth further investigation.   It's important to know, though, that there have been similar discussions for over a decade.  We're not there yet.

Friday, September 07, 2012

TED talks

My local public radio station has been repeatedly promoting the TED Radio Hour, which involves (to paraphrase the promo) people having 18 minutes to give the talk of their lives.  The TED folks have certainly gone very far in promotion - they do a great job in making all of their talks look like things worthy of listening.  Looking on the TED site, it's interesting to see what there is that may be relevant to readers of this blog.  Searching on "condensed matter" (without the quotes) returns only a single talk, by the extraordinarily creative George Whitesides.  Searching on "nanoscale" returns eight talks, including one by Paul Rothemund on DNA origami and one by Angela Belcher on her work on nano-enabled batteries.  A search on "solid state" returns nothing relevant at all.  This has made me think about what I'd say if I had the chance to give a talk like this - one where it's supposed to be accessible to a really general audience.  Two topics come to mind. 

First, someone at some point should give a TED talk that really spells out how enormous the impact of solid state physics really is on our daily lives.  This would require a couple of minutes talking about what we mean by "solid state physics", and what it tells us.  This would also require some discussion about the divide between science and engineering,  the nature of basic science, and the eventual usefulness of abstract knowledge.  In the end, you can tie together the ideal gas law (the need to use statistics to understand large numbers of particles), the Pauli principle (which explains the periodic table and how electrons arrange themselves), the need for better telephone amplifiers (Bell Labs and the transistor), all eventually resulting in the cell phone in your pocket, computers, the internet, etc.

Second, I'd love to jump into some of our work that looks at how heating and dissipation happen at the molecular scale.  When you push current through a wire, the wire gets hot.  How does that happen?  What does "hot" mean?  How does energy get from the battery into the microscopic degrees of freedom in the wire?  What happens if the wire is really small, like atomic-scale?  What does it mean for something to be "irreversible"?   This could be a lot of fun.  Of course, the total number of scientists that give these talks is tiny and they are august (e.g., Rothemund and Belcher are both MacArthur Fellows; Whitesides has won just about everything except the Nobel, and that wouldn't be a surprise).  Still, it never hurts to fantasize a bit.

Sunday, September 02, 2012

Cheating, plagiarism, and honor codes

The internet has been buzzing about the rumored cheating scandal at Harvard, where more than half of the students in an introductory political science class are implicated in plagiarism and/or improper collusion on a take-home exam.  It sounds like there were several coincident issues here.  First, the take-home final was "open book, open notes, open internet".  That might be fine under some circumstances and certainly corresponds to real-life conditions - I've always hated the contrived nature of closed-book, timed exams.  However, if students are not properly trained in how to cite material (a big "if" at Harvard), this approach could easily lead to short-answer responses that sound very similar to each other, a case of textual "convergent evolution" rather than actual collusion.  However, an article at Salon makes this whole mess sound even worse than that.  It sounds like real collusion and cheating were common and had been for years, and the professor basically implied that the course was an easy A.

I was surprised to learn that Harvard had no honor code system.  I'd been an undergrad at Princeton, which has a very seriously run honor system; I'd been a grad student at Stanford, where the situation was similar (though they did ask faculty not to put students in a position where they'd be "tempted", like a take-home closed book exam - I always thought this to be hypocritical.  Either you trust students or you don't.), and I'm a faculty member at Rice, where there is a very seriously run honor system.  While no system is perfect, and those truly determined to cheat will still try to cheat, based on my experience I think that having a broadly known, student-run (or at least run with heavy student participation) academic justice system is better than the alternative.  It does, however, rely critically on faculty buy-in.  If the faculty think that the honor system is broken (either too lenient or too cumbersome), or the faculty are so detached from teaching that they don't care or realize the impact that teaching has on other students, then an honor system won't work. 

On the one hand, cheating is in many ways easier than ever before, because of the free flow of information enabled by the internet.  Students can download solution manuals for nearly every science textbook, for example.  However, technology also makes it possible to compare assignments and spot plagiarism more readily than ever before.  What I find very distressing is the continual erosion of the meaning of academic and intellectual honesty.  This ranges from the death-of-a-thousand-cuts little stuff like grabbing images off the internet without attribution, all the way to the vilifying of science as just as subjective as opinion.  There is something very insidious about deciding to marginalize fact-checking.  If places like Harvard don't take the truth and intellectual honesty seriously, how can we be surprised when the average person is deeply cynical about everything that is claimed to be true?