Decoherence in Disordered Conductors at Low Temperatures, the effect of Soft Local Excitations

TL;DR

This study proves that electron dephasing rates must vanish at zero temperature unless there's ground state degeneracy, and experimentally shows that apparent saturation is due to non-linear effects and local defects, not fundamental zero-point fluctuations.

Contribution

The paper provides a thermodynamic proof that dephasing rates vanish at zero temperature and experimentally clarifies the causes of apparent saturation in low-temperature dephasing.

Findings

Dephasing rate must vanish as temperature approaches zero in most systems.

Apparent saturation of dephasing rate is due to non-linear transport effects.

Heavy impurities cause a low-temperature shoulder in dephasing rate.

Abstract

The conduction electrons' dephasing rate, $\tau_{\phi}^{-1}$, is expected to vanish with the temperature. A very intriguing apparent saturation of this dephasing rate in several systems was recently reported at very low temperatures. The suggestion that this represents dephasing by zero-point fluctuations has generated both theoretical and experimental controversies. We start by proving that the dephasing rate must vanish at the $T\to 0$ limit, unless a large ground state degeneracy exists. This thermodynamic proof includes most systems of relevance and it is valid for any determination of $\tau_{\phi}$ from {\em linear} transport measurements. In fact, our experiments demonstrate unequivocally that indeed when strictly linear transport is used, the apparent low-temperature saturation of $\tau_{\phi}$ is eliminated. However, the conditions to be in the linear transport regime are more strict than hitherto expected. Another novel result of the experiments is that introducing heavy nonmagnetic impurities (gold) in our samples produces, even in linear transport, a shoulder in the dephasing rate at very low temperatures. We then show theoretically that low-lying local defects may produce a relatively large dephasing rate at low temperatures. However, as expected, this rate in fact vanishes when $T \to 0$, in agreement with our experimental observations.