Optical properties of the pseudogap state in underdoped cuprates

TL;DR

This paper explains the optical properties of underdoped cuprates in the pseudogap state using the resonating valence bond spin-liquid model, emphasizing the role of correlations and Fermi surface changes over effective mass increases.

Contribution

It demonstrates that the pseudogap's effects on optical conductivity are due to Gutzwiller factors and Fermi surface shrinking, not effective mass enhancement, aligning with experimental observations.

Findings

Persistent Drude response at low doping levels.

No significant increase in optical effective mass observed.

Optical conductivity reduction explained by correlations and Fermi surface changes.

Abstract

Recent optical measurements of deeply underdoped cuprates have revealed that a coherent Drude response persists well below the end of the superconducting dome. In addition, no large increase in optical effective mass has been observed, even at dopings as low as 1%. We show that this behavior is consistent with the resonating valence bond spin-liquid model proposed by Yang, Rice, and Zhang. In this model, the overall reduction in optical conductivity in the approach to the Mott insulating state is caused not by an increase in effective mass, but by a Gutzwiller factor, which describes decreased coherence due to correlations, and by a shrinking of the Fermi surface, which decreases the number of available charge carriers. We also show that in this model, the pseudogap does not modify the low-temperature, low-frequency behavior, though the magnitude of the conductivity is greatly reduced by the Gutzwiller factor. Similarly, the profile of the temperature dependence of the microwave conductivity is largely unchanged in shape, but the Gutzwiller factor is essential in understanding the observed difference in magnitude between ortho-I and -II YBa$_2$Cu$_3$O$_y$.