Rotational Symmetry Breaking in the Hidden-Order Phase of URu2Si2
  A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called "hidden order" phase transition in the heavy fermion compound URu2Si2, at transition temperature T_h=17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T_h, which breaks the four-fold rotational symmetry of the tetragonal URu2Si2. Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic "nematic" phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry. 
R. Okazaki, T. Shibauchi, H. J. Shi, Y. Haga, T. D. Matsuda, E.   Yamamoto, Y. Onuki, H. Ikeda, Y. Matsuda
cond-mat.str-el cond-mat.supr-con