Time-Reversal-Symmetry-Breaking Superconductivity in Epitaxial Bismuth/Nickel Bilayers

TL;DR

This paper reports the discovery of time-reversal symmetry-breaking superconductivity in epitaxial Bi/Ni bilayer films, a 2D system lacking inversion symmetry, characterized by the polar Kerr effect and supported by a theoretical model involving magnetic fluctuations.

Contribution

It provides the first evidence of TRS-breaking superconductivity in a 2D non-centrosymmetric system and introduces a theoretical framework for its pairing mechanism.

Findings

Observation of polar Kerr effect at superconducting transition

TRS-breaking superconductivity in a 2D Bi/Ni bilayer

Proposal of a magnetic fluctuation-induced pairing model

Abstract

Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of 3D crystals with bulk inversion symmetry. Here we report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce superconducting pairing of the dxy = +- idx^2y^2 orbital symmetry between the electrons in Bi. In this model the order parameter spontaneously breaks the TRS and has a non-zero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.