Bulk evidence for a time-reversal symmetry broken superconducting state in URu2Si2

URu2Si2 is claimed to be a chiral d-wave superconductor with a k(z) (k(x) +/- ik(y)) time-reversal symmetry broken orbital component for the Cooper pair wave function, which contains both nodal points and lines of nodes. To study the magnetic response of such an unconventional state through a bulk, thermodynamic probe, we measured the magnetic torque t in very high-quality, well-characterized URu2Si2 single crystals at high magnetic fields H and at very low temperatures T. The magnetization M(H) proportional to tau(H)/H of URu2Si2, in its superconducting state and for angles within 15. from the ab plane, reveals a change in its sign for H approaching H-c2: from a clear diamagnetic response dominated by the pinning of vortices to a state with a smaller but paramagneticlike hysteretic response which disappears at H-c2, thus implying that it is intrinsically related to the superconducting state. We argue that this anomalous, angular-dependent behavior is evidence for a time-reversal symmetry broken superconducting state in URu2Si2, although not necessarily for the k(z) (k(x) +/- ik(y)) state.