Saturday, November 17, 2007

This week in cond-mat

Two papers this week. I'll write about our own at a later date. These two are both connected to on-going long-term controversies in condensed matter/mesoscopic/nanoscale physics.

arxiv:0711.1810 - Capron et al., Low temperature dephasing in irradiated metallic wires
In the orthodox picture of metals (thought to be valid for relatively weak disorder), the quantum coherence time of electrons is expected to diverge toward infinity as the temperature approaches zero. Think about a two-slit experiment for electrons. If the electrons are well isolated from their environment, they can diffract off the slits and land on the screen, producing an interference pattern. If the electrons are coupled to environmental degrees of freedom that can change their state when the electron goes by, the relative phase of the electron wavefunctions going through each of the slits gets scrambled by that interaction, washing out the interference. In the usual 2-slit experiment, the degrees of freedom are those of detectors at the slits. Within a disordered metal, those environmental degrees of freedom can be lattice vibrations, other electrons, or magnetic impurities. For a decade now there has been an ongoing controversy about whether the coherence time (as inferred from some quantum correction to the classical electronic conductance) really does diverge, or whether it saturates as T -> 0. Intrinsic saturation would be a big deal - it would imply that the quasiparticle picture of electrons (Fermi liquid theory) fails at the low T limit. In this paper, the authors perform a very careful control experiment, looking at whether structural damage to silver nanowires can, by itself, introduce extra degrees of freedom that cause decoherence. They get this damage by ion-implanting Ag ions into Ag nanowires. The results show no sign of extra decoherence due to this irradiation.

arxiv:0711.1464 - Baenninger et al., Low-temperature collapse of electron localisation (sic) in two dimensions
Another ongoing brouhaha has been about whether electrons confined to two dimensions have an insulating or metallic ground state in the presence of any disorder. Without interactions, the "Gang of Four" (Anderson, Abrahams, Ramakrishnan, and Licciardello) showed that even infinitesimal disorder leads to localization and an insulating ground state for an infinite 2d system. Of course, real electrons do interact with each other, and real systems are of finite size. One big complication in this whole discussion is in trying to tell the difference between a real, uniform, insulating state and the breakup of your system into inhomogeneous "puddles" of electrons due to the disorder potential. The Cambridge group has done some careful experiments in mesoscopic samples of rather clean 2d electron gas, and they've found that small regions with higher temperature resistances far exceeding the quantum of resistance (~ h/e^2 ~ 26 kOhms) can show a crossover at low temperatures to what looks like a metallic state. I haven't been following this controversy in detail, but these data look very interesting, and I will have to read this closely.

1 comment:

Schlupp said...

Doug, when will the later date for your paper be?

I really like your paper discussion posts.