Magneto-optical signatures of a cascade of transitions in   La$_2-x$Ba$_x$CuO$_4$
  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]. 
Hovnatan Karapetyan, M. Hucker, G. D. Gu, J. M. Tranquada, M. M.   Fejer, Jing Xia, and A. Kapitulnik
cond-mat.supr-con