Time-reversal symmetry breaking in superconductors through loop Josephson-current order

We propose a superconducting instability where loop Josephson-currents form spontaneously within a unit cell at the critical temperature, Tc. Such currents break time-reversal symmetry (TRS) without needing an unconventional pairing mechanism. Using Ginzburg-Landau theory we show how they emerge in a toy model and estimate the size of the resulting magnetization, which is consistent with recent muon-spin relaxation experiments. We discuss the crystal symmetry requirements and show that they are met by the Re6X (X=Zr, Hf, Ti) family of TRS-breaking, but otherwise seemingly conventional, superconductors.