Time-Reversal Symmetry Breaking in Re-Based Superconductors

Abstract

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) studies of superconducting noncentrosymmetric ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ (${T}_{c}=8.8\text{ }\text{ }\mathrm{K}$) and centrosymmetric Re (${T}_{c}=2.7\text{ }\text{ }\mathrm{K}$). In ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$, the low--temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $\ensuremath{\mu}\mathrm{SR}$ below ${T}_{c}$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric $\mathrm{Re}T$ ($T=\text{transition}$ metal), yet its preservation in the isostructural noncentrosymmetric superconductors ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ and ${\mathrm{Nb}}_{0.5}{\mathrm{Os}}_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and $\mathrm{Re}T$. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.