Optical conductivity of charge carriers interacting with a two-level systems reservoir

TL;DR

This paper studies how charge carriers interacting with a reservoir of two-level systems affect optical conductivity and resistivity, revealing regimes with non-Drude behavior and contrasting with harmonic oscillator baths.

Contribution

It introduces a functional-integral approach to analyze charge dynamics with two-level systems, highlighting novel regimes and differences from harmonic bath models.

Findings

Identification of three regimes based on the spectral function.

Observation of non-Drude optical conductivity in certain parameter ranges.

Contrast with harmonic oscillator bath behavior at low temperatures.

Abstract

Using the functional-integral method we investigate the effective dynamics of a charged particle coupled to a set of two-level systems as a function of temperature and external electric field. The optical conductivity and the direct current (dc) resistivity induced by the reservoir are computed. Three different regimes are found depending on the two-level system spectral function, which may lead to a non-Drude optical conductivity in a certain range of parameters. Our results contrast to the behavior found when considering the usual bath of harmonic oscillators which we are able to recover in the limit of very low temperatures.