Nodeless bulk superconductivity in the time-reversal symmetry breaking Bi/Ni bilayer system

TL;DR

This study reports on the discovery of nodeless, time-reversal symmetry breaking superconductivity in Bi/Ni bilayers, with evidence suggesting unconventional p-wave pairing and a high transition temperature of 4.1 K.

Contribution

It provides the first detailed electrodynamic measurements indicating nodeless, TRS-breaking superconductivity in Bi/Ni bilayers, challenging existing pairing symmetry models.

Findings

All states become fully gapped at zero magnetic field.

Data rules out odd-frequency pairing.

Superconductivity develops across the entire bilayer, indicating possible p-wave nature.

Abstract

Epitaxial bilayer films of Bi(110) and Ni host a time-reversal symmetry (TRS) breaking superconducting order with an unexpectedly high transition temperature $T_c = 4.1$ K. Using time-domain THz spectroscopy, we measure the low energy electrodynamic response of a Bi/Ni bilayer thin film from $0.2$ THz to $2$ THz as a function of temperature and magnetic field. We analyze the data in the context of a BCS-like superconductor with a finite normal-state scattering rate. In zero magnetic field, all states in the film become fully gapped, providing important constraints into possible pairing symmetries. Our data appears to rule out the odd-frequency pairing that is natural for many ferromagnetic-superconductor interfaces. By analyzing the magnetic field-dependent response in terms of a pair-breaking parameter, we determine that superconductivity develops over the entire bilayer sample which may point to the $p$-wave like nature of unconventional superconductivity.