Friday, September 15, 2006

This week in cond-mat

Two papers from the past week, the first of which gives us a chance to discuss one of the on-going controversies in condensed matter physics.
cond-mat/0609301 - Lai et al., Linear temperature dependence of conductivity in Si two-dimensional electrons near the apparent metal-to-insulator transition
For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder. So, the question is whether the real (interacting) case, with an apparent transition between metallic and insulating states, is profound (that is, a real quantum phase transition) or not (e.g., a percolative transition caused by the system breaking up into disconnected puddles of carriers as the concentration is lowered). There are some interesting pieces of evidence pointing in each direction. This paper weighs in using very nice Si quantum wells in SiGe, showing evidence consistent with a percolative crossover in the conductivity. Anyone out there care to comment on the state of this debate in general? Has there been a really slam dunk experiment out there that I've missed by avoiding this problem?

cond-mat/0609297 - Naik et al., Cooling a nanomechanical resonator with quantum back-action (also available in Nature)
This paper is one I need to read more carefully. These folks have constructed a nanomechanical resonator (operates at about 20 MHz), and are using a superconducting single-electron transistor (SSET) measured at high frequency to detect the resonator's motion. This is a great system for testing ideas about quantum measurement and back-action of the detector on the system being measured. In this case, they find that for the right settings of the SSET detector, they can actually cool the resonator (as determined by the noise temperature of the resonator, inferred from the readout of the detector) using the detector. The claim is that this is analogous to laser cooling in some sense, bit without a closer reading, I don't see how this really works. This shows that I need to think more and read more about this detector back-action business.


Peter Armitage said...

Hi Doug. I have some thoughts on this 2D MIT stuff, but it'll take more time than I have now...

I will say that I've always loved that the original Kravchenko paper was published in PRB. Not that PRB isn't a perfectly lovely and reputable journal with many important results, but I can't think of a similarly seminal paper that was published there. Anyone?

Did they have problems with getting published somewhere higher up the hierachy? Or was it just a matter of when you have a result of that magnitude it doesn't where you send it.

If the former, then it reflects poorly on some forever anonymous referee or editor. And although I have a hard time wrapping my Science/Nature raised mind around such a move ;), if it is the latter, then how refreshing is that in this era of "...and still MORE hype...."?

Anyone know the story?

Alison Chaiken said...

The claim is that this is analogous to laser cooling in some sense, bit without a closer reading, I don't see how this really works.

I think that both cooling mechanisms are essentially evaporative in that only certain modes couple energy out. In the laser case, a red-detuned photon beam only interacts with atoms or molecules that are coming towards the light source. In the SSET case the analogous feature seems to be the frequency asymmetry of the quantum noise. Why exactly the quantum noise is asymmetric is beyond me.

Doug Natelson said...

Hey Peter - I agree that it's cool how the original Kravchenko paper is a PRB. I suspect that this may have something to do with the refereeing process, though I don't know for sure. I do have one example where something like this happened because of author choice. My faculty colleague, Rui-Rui Du, is a big 2d electron person. Remember all the stuff about stripes and anisotropies at high Landau levels? He published his first paper on that stuff in Solid State Communications in January '99, essentially at the same time that Mike Lilly and Jim Eisenstein published a PRL on the same topic. In the end, both papers have been very highly cited, even though SSC is generally a lower impact journal.