Time-reversal symmetry breaking in superconductors through loop pair-current order*

We propose a superconducting instability where microscopic loop pair-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 an upper bound for the resulting magnetisation, 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.