Magnetoresistance and Dephasing in a Two-dimensional Electron Gas at Intermediate Conductances

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 e2/h), including the range of intermediate conductances g∼1. Interpretation of the experimental results obtained for a 2D electron gas in GaAs/InxGa 1-xAs/GaAs single quantum well structures is based on a theory that takes into account terms of higher orders in 1/g. We show that the standard weak localization (WL) theory is adequate for g ≳ 5. Calculating the corrections of second order in 1/g to the MR, stemming from both the interference contribution and the mutual effect of WL and Coulomb interaction, we expand the range of a quantitative agreement between the theory and experiment down to significantly lower conductances g∼1. We demonstrate that at intermediate conductances the negative MR is described by the standard WL "digamma-functions" expression, but with a reduced prefactor α. 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 α≃1-2/πg. The fitting of the MR allows us to measure the true value of the phase breaking time within a wide conductance range g≳1. 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 temperature, whereas the Drude conductance is high g0≫1, so that a weak magnetic field delocalizes electronic states. 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→0, even though the true dephasing rate vanishes. © 2004 The American Physical Society.