Supercurrents and spontaneous time-reversal symmetry breaking by nonmagnetic disorder in unconventional superconductors

We explore the emergence of orbital currents and associated time-reversal symmetry breaking (TRSB) arising solely from nonmagnetic disorder in unconventional superconductors. This study is relevant to unconventional superconductors reporting signs of TRSB below Tc. Specifically, we solve a model of a disordered d-wave superconductor and trace the origin of current loops for sufficiently high disorder levels. Since the pure d-wave superconducting state does not exhibit TRSB by itself, it is surprising that such persistent currents arise purely from nonmagnetic disorder. In this talk, a detailed theoretical investigation of this effect will be presented. We show that time-reversal symmetry breaking occurs in a broad range of disorder levels in a d-wave superconductor and that the generation of the currents can be traced to local extended s-wave pairing. The locally induced s-wave pairing leads to energetically favored regions of s+id order which is known to support spontaneous currents in the presence of inhomogeneous density modulations.