Evidence for broken time-reversal symmetry in the superconducting phase   of URu$_2$Si$_2$
  Recent experimental and theoretical interest in the superconducting phase of the heavy fermion material URu$_2$Si$_2$ has led to a number of proposals in which the superconducting order parameter breaks time-reversal symmetry (TRS). In this study we measured polar Kerr effect (PKE) as a function of temperature for several high-quality single crystals of URu$_2$Si$_2$. We find an onset of PKE below the superconducting transition that is consistent with a TRS-breaking order parameter. This effect appears to be independent of an additional, possibly extrinsic, PKE generated above the hidden order transition at $T_{HO}=17.5$ K, and contains structure below $T_c$ suggestive of additional physics within the superconducting state. 
E. R. Schemm, R. E. Baumbach, P. H. Tobash, F. Ronning, E. D. Bauer,   A. Kapitulnik
cond-mat.supr-con