Infrared properties of La$_{2-x}$(Ca,Sr)$_x$CaCu$_2$O$_{6+\delta}$ single crystals

TL;DR

This study investigates the infrared optical properties of La$_{2-x}$(Ca,Sr)$_x$CaCu$_2$O$_{6+\\delta}$ single crystals, revealing charge-transfer gaps, spectral weight shifts with doping, and pseudogap features, contributing to understanding high-temperature superconductivity.

Contribution

It provides detailed optical spectra and analysis of charge-transfer and pseudogap phenomena in bilayer cuprates with different doping levels.

Findings

Charge-transfer gap observed at ~15000 cm$^{-1}$.

Spectral weight shifts to low frequencies with increased doping.

Pseudogap features similar to other bilayer cuprates.

Abstract

The in-plane optical properties of two crystals of the bilayer cuprate \LSxCCO, one with excess Ca and $x=0.10$ and the other with Sr and $x=0.15$, were investigated over the frequency range of 45--25000 cm$^{-1}$. The optical conductivity has been derived from Kramers-Kronig transformation. Each crystal exhibits a peak at around 15000 cm$^{-1}$ which corresponds to the charge-transfer gap of the parent insulator. With increasing carrier density, spectral weight shifts from the CT excitation to the low-$\omega$ region. For the superconducting sample ($x=0.15$), the optical conductivity displays a peak in the far-infrared region, which shifts toward zero frequency with decreasing temperature. The temperature-dependent behavior favors a dynamical localization picture. A ``pseudogap'' feature is observed in the low-frequency reflectance and the scattering rate spectra. Both the energy scale and the temperature dependence of the ``pseudogap'' are similar to other bilayer cuprates.