Interband effects in the c-axis optical conductivity in YBaCuO

TL;DR

This paper calculates the c-axis optical conductivity in layered metals like YBaCuO, highlighting the role of interband effects and their temperature dependence, which influence the optical response over a wide frequency range.

Contribution

It introduces a detailed theoretical model including interband effects in the optical conductivity of layered superconductors, emphasizing their significance in the c-axis response.

Findings

Interband term contributes significantly to c-axis optical conductivity.

Interband response is weakly temperature dependent.

Low-energy response can develop a gap or pseudogap with decreasing temperature.

Abstract

The normal state optical conductivity is calculated for a layered metal with two layers per unit cell coupled through a transverse hopping matrix element $t_\perp$. The optical response involves an interband term in addition to the more familiar intraband term which leads to the usual Drude form. The interband term is only weakly temperature dependent, even for an inelastic scattering rate which is linear in T. It gives a $c$-axis response which extends in frequency over the entire band width although there can be structure on this energy scale which reflects details of the electronic structure. In particular, at low energy, the $c$-axis response can develop a gap or pseudogap as the temperature is lowered. At high temperature, a Drude response will be seen only if the intraband transitions, which are of order $t_\perp^4$, become important compared with the interband transitions which are of order $t_\perp^2$.