Systematics of c-axis Phonons in the Thallium and Bismuth Based Cuprate Superconductors

TL;DR

This study investigates the c-axis phonons in various thallium and bismuth-based cuprate superconductors using infrared reflectivity, providing detailed phonon mode analysis and comparing different measurement techniques.

Contribution

It offers a comprehensive analysis of c-axis phonons in Tl- and Bi-based cuprates, including revised mode assignments and comparison of LO frequencies from different experimental methods.

Findings

Accurate determination of TO and LO phonon frequencies.

Revised assignment of oxygen phonon modes.

Explanation of discrepancies between different measurement techniques.

Abstract

We present grazing incidence reflectivity measurements in the far infrared region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6, Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly obtained. The reflectivity curves are well fitted by a series of Lorentz oscillators. In this way the transverse (TO) phonon frequencies were accurately determined. On the basis of the comparative study of the Bi and Tl based cuprates with different number of CuO2 layers per unit cell, we suggest modifications of the assignment of the main oxygen modes. We compare the LO frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic Josephson junction characteristics with our measurements, and explain the discrepancy in LO frequencies obtained by the two different methods.