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.