Broken time-reversal symmetry detected by tunneling spectroscopy of superconducting Pd-doped CaAgP

TL;DR

This study uses tunneling spectroscopy to detect broken time-reversal symmetry in superconducting Pd-doped CaAgP, revealing novel properties of topological nodal-line semimetals with surface superconductivity.

Contribution

It demonstrates the sensitivity of tunneling spectroscopy to broken TRS in a topological superconductor, providing new insights into superconductivity in nodal-line semimetals.

Findings

Broken TRS confirmed by magnetic field response of conductance spectra

Zero-bias peaks with asymmetric structures reversed by magnetic field direction

Superconductivity strongly coupled to external magnetic field

Abstract

The appearance of broken time-reversal symmetry (TRS) in superconducting states is an intriguing issue in solid-state physics because of the incompatibility of the spontaneous magnetic field and the Meissner effect. We identify broken TRS in Pd-doped CaAgP (CaAg$_{0.9}$Pd$_{0.1}$P) by tunneling spectroscopy through the magnetic field response of conductance spectra. CaAg$_{0.9}$Pd$_{0.1}$P is a nodal-line semimetal with exotic electronic states such as drumhead surface states and surface superconductivity. Tunneling conductance spectra acquired at the side surfaces of CaAg$_{0.9}$Pd$_{0.1}$P under an applied magnetic field exhibit broad zero-bias peaks with small asymmetric structures. Surprisingly, the asymmetric structures are reversed exactly by flipping the field direction. On the basis of an analysis which stands on the formula of tunneling junctions for unconventional superconductors, these results are consistent with the pair potential of the superconductivity breaks the TRS and is strongly coupled to an external magnetic field. We reveal the novel character of superconducting nodal-line semimetals by developing the TRS sensitivity of tunneling spectroscopy. Our results serve as an exploration of broken TRS in superconducting states realized in topological materials.