Sunday, February 25, 2007

This week in cond-mat

One theory paper, and two experimental papers this time.
cond-mat/0702446 - Poggio et al., Feedback cooling of a cantilever's fundamental mode below 5 mK
Suppose you had a mechanical resonator (mass on a spring). At moderate temperatures you know from the good, old equipartition theorem that the average kinetic energy and average potential energy in the resonator would each equal 1/2 k_{B}T. (Note to self: get LaTeX working in blogger....) At low enough temperatures (k_{B}T < \hbar \omega), you should instead think about the number of vibrational quanta in the resonator. Suppose you could actively damp the resonator - if it's moving toward you, you push back to slow it down. It is possible to effectively cool the resonator this way (though in a Maxwell's demon sense, there's no such thing as a free lunch). How far you can go depends on the noise in your measurement system used for the feedback. In this paper by Dan Rugar's group, they demonstrate that they can cool a Si cantilever from a base temperature of around 4.2 K all the way down to 5 mK, limited by the noise in their feedback system. This is impressive, and of obvious interest to those who want to examine the fundamental quantum properties of mechanical systems (including detector back-action).

cond-mat/0702472 - Kalb et al., Organic small-molecule field-effect transistors with Cytop(tm) gate dielectric: eliminating gate bias stress effects
A persistent problem with organic FETs is that their performance degrades if the gate is biased for long periods. There can be many reasons for this, but one major issue involves the interaction between the semiconductor and the gate dielectric. It is widely believed that in many OFETs charge leaking through the gate dielectric introduces defects and trap states right at the channel interface in the organic semiconductor. Here, Batlogg's group at ETH seems to have found, with collaborators, a fluoropolymer dielectric that doesn't seem to have these problems, and has impressive breakdown strength as well. I'll have to look into getting some.

cond-mat/0702505 - Khodas et al., One-dimensional Fermi-Luttinger liquid
Fermi liquid theory is the standard model of electrons in metals (as well as normal-state liquid 3He). The upshot of FLT is that the quasiparticles of the interacting electron gas look very much like weakly interacting electrons, and have well defined quantum numbers (spin 1/2, charge -e, k-vectors and band indices). In 1d, though, FLT doesn't do well. Luttinger, by assuming that the dispersion E(k) of the carriers around the Fermi points is linear, came up with an exact solution to the 1d problem now called the Luttinger liquid (LL). The LL has some very interesting properties, including separate spin and charge excitations. In this paper, Glazman, Pustilnik, Khamanev, and Khodas consider what happens when the dispersion a the Fermi points is more realistic: linear with a little bit of quadratic correction. This breaks particle-hole symmetry around the Fermi points, and has some profound effects on the structure of the density of states. This is a long paper, and while I think I get the main point, I haven't had a chance to look at it thoroughly. It seems important, though, since the slight nonlinear correction considered here seems very physically reasonable for many systems.

1 comment:

Alison Chaiken said...

Thanks for the link to the paper about the near-quantum-limit cantilever. It's a beautiful example of a system that can be understood by undergraduates and yet is at the leading edge of experimental science. Lots of new tests of quantum mechanics appear to be at hand. What will happen when the Einstein-Podolsky-Rosen experiment is performed with mechanical resonators? Can quantum-limit mechanical systems be used to explore the physics of gravity at small length scales? At the moment Bose-Einstein condensates are at the forefront in probing the limits of QM but mechanical systems will provide a useful alternative.