Nodeless Bulk Superconductivity in the Time-Reversal Symmetry Breaking Bi/Ni Bilayer System

Epitaxial bilayer films of Bi(110) and Ni host a time-reversal symmetry breaking superconducting order with an unexpectedly high transition temperature Tc=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 to 2 THz as a function of temperature and magnetic field. We analyze the data in the context of a Bardeen-Cooper-Schrieffer-like superconductor with a finite normal-state scattering rate. In a zero magnetic field, all states in the film become fully gapped, providing important constraints into possible pairing symmetries. Our data appear 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.