Unusual nernst effect suggesting time-reversal violation in the striped cuprate superconductor La _2-xBa _xCuO ₄

The striped cuprate La _2-xBa _xCuO ₄ (x=18) undergoes several transitions below the charge-ordering temperature T _co=54K. From Nernst experiments, we find that, below T _co, there exists a large, anomalous Nernst signal e _N,even(H,T) that is symmetric in field H, and remains finite as H→0. The time-reversal violating signal suggests that, below T _co, vortices of one sign are spontaneously created to relieve interlayer phase frustration.