Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors

TL;DR

This paper models the impact of a pseudogap on the optical conductivity in underdoped cuprate superconductors, revealing a gap formation below a characteristic temperature and differing behaviors between underdoped and overdoped regimes.

Contribution

It introduces a phenomenological d-wave pseudogap into the calculation of c-axis optical conductivity, explaining experimental observations in underdoped cuprates.

Findings

A pseudogap causes a gap in optical conductivity below T*

The pseudogap effect is more pronounced in in-plane than interplane conductivity

Incoherent transmission best describes underdoped c-axis conductivity

Abstract

We calculate the c-axis infrared conductivity $\sigma_c(\omega)$ in underdoped cuprate superconductors for spinfluctuation exchange scattering within the CuO$_2$-planes including a phenomenological d-wave pseudogap of amplitude $E_g$. For temperatures decreasing below a temperature $T^* \sim E_g/2$, a gap for $\omega < 2E_g$ develops in $\sigma_c(\omega)$ in the incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting' behavior, i.e. it increases for low temperatures above the constant behavior for $E_g=0$. We find that the pseudogap structure in the in-plane optical conductivity is about twice as big as in the interplane conductivity $\sigma_c(\omega)$, in qualitative agreement with experiment. This is a consequence of the fact that the spinfluctuation exchange interaction is suppressed at low frequencies as a result of the opening of the pseudogap. While the c-axis conductivity in the underdoped regime is described best by incoherent transmission, in the overdoped regime coherent conductance gives a better description.