Temperature Evolution of the Pseudogap State in the Infra-Red Response of Underdoped La_(2-x)Sr_xCuO_4

TL;DR

This study investigates the temperature-dependent evolution of the pseudogap state in underdoped La_{2-x}Sr_xCuO_4 using infrared spectroscopy, revealing suppression in scattering rates indicative of pseudogap formation up to 400 K.

Contribution

It provides detailed infrared spectral analysis of underdoped LSCO, demonstrating the temperature evolution of the pseudogap state in both in-plane and out-of-plane responses.

Findings

Suppression of scattering rate below high-frequency extrapolation indicates pseudogap presence.

Pseudogap effects persist up to 400 K in underdoped LSCO.

Distinct temperature dependence observed in in-plane and out-of-plane spectra.

Abstract

The ab-plane optical spectra of two single crystals of underdoped La_(2-x)Sr_xCuO_4 were investigated. The reflectivity of La_(1.87)Sr_(0.13)CuO_4 has been measured in the frequency range 30- 9,000 cm-1 (0.004 -- 1 eV) both parallel and perpendicular to the CuO_2 planes, whereas La_(1.86)Sr_(0.14)CuO_4 was studied only in the ab-plane. The extended Drude model shows that the frequency-dependent effective scattering rate 1/tau(omega)is strongly suppressed below the high-frequency straight-line extrapolation, a signature of the pseudogap state. This suppression can be seen from temperatures below the superconducting transition up to 400 K. In the case of underdoped LSCO the straight-line extrapolation is temperature independent below 200 K, whereas above 200 K there is a strong temperature dependence of the high-frequency 1/tau(omega,T). The out-of-plane direction is also examined for evidence of the pseudogap state.