This week in cond-mat

Here's a couple of preprints that I've found interesting in the last week. Note that I'm not going to be surveying the published literature as much, since there are other resources such as the Virtual Journal of Nanoscale Science that do an excellent job of that (though they miss papers in ACS journals, which increasingly contain results at the border between chemistry and condensed matter physics).

cond-mat/0603173 - Manfra et al., Reentrant anisotropic phases in a two-dimensional hole system
I'm not writing about this one just because Mike Manfra and I used to share an office at Bell Labs. Two-dimensional electron gases (2degs) have been a workhorse physical system over the last 25 years, showing a number of fascinating many-body pieces of physics, including the integer quantum Hall effect (which has led to the definition of the standard Ohm!), the fractional quantum Hall effect (a demonstration of a correlated electronic state that has excitations with fractional quantum numbers, including fractional charge), apparent zero-resistance states under microwave illumination, and interlayer quantum coherence in bilayer electron-hole systems. Another weird effect observed recently is the onset of big anisotropies in the electrical resistance of such 2degs in very clean material at very low temperatures. The explanation for this spontaneous anisotropy is generally thought to involve the electronic system breaking up into some kind of stripes. With the recent development of new high quality two-dimensional hole systems, now one can test this idea. In the new cond-mat paper, Manfra et al. find that the anisotropies are very different in the hole system than the electronic analog, and discuss how details of the single-electron states (like the presence of strong spin-orbit scattering in the hole case that is absent in the electron case) can matter greatly.

cond-mat/0603108 - Badzey and Mohanty, Coherent signal amplification in bistable nanomechanical oscillators by stochastic resonance (also Nature 437, 995 (2005)).
Stochastic resonance is a neat phenomenon, when nonlinear systems can sometimes exhibit improved signal to noise when additional noise is introduced deliberately(!). This paper is a cute implementation of this idea, using bistable nanomechanical resonators as the nonlinear element. When you think about it, bistability (the resonators seem to have two competing, well-defined oscillatory states, one with high amplitude and one with low amplitude) is about as nonlinear a response as you can get. While some of this group's earlier work with these resonators has engendered some controversy, this paper is very pretty.