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

TL;DR

This paper introduces a novel superconducting state where spontaneous supercurrent loops break time-reversal symmetry, potentially explaining experimental observations in Re6X superconductors.

Contribution

It proposes a new superconducting instability involving loop super-currents within a unit cell, supported by Ginzburg-Landau theory and symmetry analysis.

Findings

Supercurrent loops spontaneously form at the transition temperature.

Time-reversal symmetry is broken in the superconducting state.

Estimated internal fields align with experimental data.

Abstract

We propose a superconducting instability where microscopic supercurrent loops form spontaneously within a unit cell at the superconducting transition temperature with only uniform, onsite and intra-orbital singlet pairing. As a result of the circulating currents time-reversal symmetry is spontaneously broken in the superconducting state. Using Ginzburg-Landau theory, we describe in detail how these currents emerge in a toy model. We discuss the crystallographic symmetry requirements to realize such a state and show that they are met by the Re6X (X=Zr, Hf, Ti) family of time-reversal symmetry breaking, but otherwise seemingly conventional, superconductors. We estimate an upper bound for the resulting internal fields and find it to be consistent with recent muon-spin relaxation experiments.