Magneto-optical signatures of a cascade of transitions in La$_2-x$Ba$_x$CuO$_4$

TL;DR

This study investigates the sequence of phase transitions in La$_2-x$Ba$_x$CuO$_4$ using Kerr effect and birefringence measurements, revealing time-reversal-symmetry breaking occurring at high temperatures.

Contribution

It provides the first detailed Kerr effect analysis of La$_2-x$Ba$_x$CuO$_4$, linking structural transitions to TRSB onset and suggesting TRSB occurs well above room temperature.

Findings

Kerr signal onset coincides with lower temperature tetragonal phase

Hysteresis indicates TRSB occurs above room temperature

Structural phase transitions are precisely located by birefringence

Abstract

Recent experiments in the original cuprate high temperature superconductor, La$_2-x$Ba$_x$CuO$_4$, have revealed a remarkable sequence of transitions [1]. Here we investigate such crystals with Kerr effect which is sensitive to time-reversal-symmetry breaking (TRSB). Concurrent birefringence measurements accurately locate the structural phase transitions from high-temperature tetragonal to low temperature orthorhombic, and then to lower temperature tetragonal, at which temperature a strong Kerr signal onsets. Hysteretic behavior of the Kerr signal suggests that TRSB occurs well above room temperature, an effect that was previously observed in high quality YBa$_2$Cu$_3$O$_{6+x} crystals [2].