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 τ in very high-quality, well-characterized URu₂Si₂ single crystals at high magnetic fields H and at very low temperatures T. The magnetization M(H)â̂τ(H)/H of URu₂Si₂, in its superconducting state and for angles within 15^â̂̃ 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.