Optical phonons along the c axis of YBa_2Cu_3O_{6+x}, for x=0.5 -> 0.95

TL;DR

This study measures the c-axis phonon spectra of YBCO_{6+x} single crystals across various doping levels, revealing doping-dependent phonon broadening, splitting, and anomalies that evolve with temperature and doping, indicating complex lattice dynamics.

Contribution

It provides detailed doping and temperature-dependent phonon spectra of YBCO_{6+x}, highlighting new phonon splitting and broadening phenomena linked to oxygen content and electronic properties.

Findings

Phonon modes broaden and split with decreased doping.

Anomalies near T_c occur well above the transition temperature.

A broad phonon band develops at 400 cm^{-1} in the normal state.

Abstract

The c-axis polarized phonon spectra of single crystals of YBCO_{6+x} have been measured for the doping range x=0.5 -> 0.95, between 10 K and 300 K. The low background electronic conductivity, determined by Kramers-Kronig analysis of the reflectance, leads to a rich phonon structure. With decreased doping the five normally-active B_{1u} modes broaden and the high-frequency apical oxygen mode splits into two components. We associate the higher of these with the two-fold coordinated copper-oxygen sticks. The 155 cm^{-1} low-frequency mode, which involves the apical and chain-oxygens, splits into at least three components with decreasing doping. Some phonon anomalies which occur near T_c in the highly-doped material occur well above T_c in the oxygen-reduced systems. An unusual broad phonon band develops in the normal state at approx 400 cm^{-1}, which becomes more intense at low doping and low temperatures, borrowing oscillator strength from apical and plane oxygen modes resulting in a major transformation of the phonon spectrum below approx 150 K.