Temperature Dependence of the Conductivity Sum Rule in the Normal State due to Inelastic Scattering

TL;DR

This paper investigates how inelastic scattering affects the temperature dependence of the optical sum rule in the normal state, emphasizing the role of spectral function changes over Sommerfeld expansion contributions.

Contribution

It demonstrates that the primary source of temperature dependence is from the spectral function's temperature variation, not the Sommerfeld expansion, providing analytical insights into this behavior.

Findings

Most temperature dependence arises from spectral function changes.

Low boson frequencies lead to linear temperature dependence.

Analytical expressions clarify the spectral function's role.

Abstract

We examine the temperature dependence of the optical sum rule in the normal state due to interactions. To be concrete we adopt a weak coupling approach which uses an electron-boson exchange model to describe inelastic scattering of the electrons with a boson, in the Migdal approximation. While a number of recent works attribute the temperature dependence in the normal state to that which arises in a Sommerfeld expansion, we show that in a wide parameter regime this contribution can be quite small. Instead, most of the temperature dependence arises from the zeroth order term in the `expansion', through the temperature dependence of the spectral function, and the interaction parameters contained therein. For low boson frequencies this circumstance causes a linear T-dependence in the sum rule. We develop some analytical expressions and understanding of the temperature dependence.