Magnetoresistance and dephasing in a two-dimensional electron gas at intermediate conductances

TL;DR

This study combines theory and experiments to analyze negative magnetoresistance in a 2D electron gas at intermediate conductances, revealing the influence of higher-order effects and localization on dephasing and magnetoresistance behavior.

Contribution

It provides a comprehensive analysis of negative MR in 2D electron gases at intermediate conductances, incorporating higher-order corrections and clarifying the interpretation of dephasing rates.

Findings

Negative MR at intermediate conductance is described by standard WL formulas with a reduced prefactor.

Second-loop corrections dominate over Coulomb interaction effects at moderate conductance levels.

In the weak insulator regime, the fitted dephasing rate reflects localization effects, not true dephasing.

Abstract

We study, both theoretically and experimentally, the negative magnetoresistance (MR) of a two-dimensional (2D) electron gas in a weak transverse magnetic field $B$. The analysis is carried out in a wide range of zero-$B$ conductances $g$ (measured in units of $e^2/h$), including the range of intermediate conductances, $g\sim 1$. Interpretation of the experimental results obtained for a 2D electron gas in GaAs/In$_x$Ga$_{1-x}$As/GaAs single quantum well structures is based on the theory which takes into account terms of higher orders in $1/g$, stemming from both the interference contribution and the mutual effect of weak localization (WL) and Coulomb interaction. We demonstrate that at intermediate conductances the negative MR is described by the standard WL "digamma-functions" expression, but with a reduced prefactor $\alpha$. We also show that at not very high $g$ the second-loop corrections dominate over the contribution of the interaction in the Cooper channel, and therefore appear to be the main source of the lowering of the prefactor, $\alpha\simeq 1-2/\pi g$. We further analyze the regime of a "weak insulator", when the zero-$B$ conductance is low $g(B=0)<1$ due to the localization at low $T$, whereas the Drude conductance is high, $g_0>>1.$ In this regime, while the MR still can be fitted by the digamma-functions formula, the experimentally obtained value of the dephasing rate has nothing to do with the true one. The corresponding fitting parameter in the low-$T$ limit is determined by the localization length and may therefore saturate at $T\to 0$, even though the true dephasing rate vanishes.