Wednesday, July 01, 2009

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....

4 comments:

Dave Bacon said...

Cue proposals to build a quantum computer out of the system in 3...2...1

Anonymous said...

The results are very surprizing. How can the states be coherent over 100 microns or so at nitrogen temperature? The AB oscillations appear when applying a voltage larger than 200 mV. This is really too large.

Maybe I do not know enough about CDW and the measurements make sense. However, the quality of the oscillations is not spectacular and not convincing.

Doug Natelson said...

Dave - I'm sure someone has already called up DARPA....

Anon. - The peak in the Fourier transform of resistance vs. 1/B (the insets to their Fig. 2) look pretty well defined, comparable to what is seen in other mesoscopic experiments in normal metal rings (at much lower T and much smaller circumference). The apparent need for the high bias would be to un-pin the CDW. Clearly if the CDW is pinned and there's basically no CDW current, one would not expect oscillations. Note that the required bias is quite a bit lower at 79 K, consistent with this idea.

It's a very interesting question, how to think about what that applied bias voltage is doing. In ordinary mesoscopics, an applied bias that big would make me worry about both heating and smearing due to energy averaging. Here, it's less obvious.

I'd love to see the oscillations over a much larger range of magnetic fields. If the oscillations continue over tens of Gauss, that would be more persuasive, at least to me.

Schlupp said...

Hehe, reading "Fröhlich superconducting quantum device" I can't help thinking (at least at first, before I switch my physics mind on) "Cheerful superconducting quantum device".