Absence of time reversal symmetry breaking in La_{2-x}Sr_{x}CuO_{4}
  We have performed zero-field muon spin rotation measurements on single crystals of La_{2-x}Sr_{x}CuO_{4} to search for spontaneous currents in the pseudo-gap state. By comparing measurements on materials across the phase diagram, we put strict upper limits on any possible time-reversal symmetry breaking fields that could be associated with the pseudo-gap. Comparison between experimental limits and proposed circulating current states effectively eliminates the possibility that such states exist in this family of materials. 
G. J. MacDougall, A. A. Aczel, J.P. Carlo, T. Ito, J. Rodriguez, P. L.   Russo, Y. J. Uemura, S. Wakimoto, and G. M. Luke