To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation ($\mu$SR) studies of superconducting noncentrosymmetric Re$_{ 0.82}$Nb$_{ 0.18}$ ($T_c = 8.8$ K) and centrosymmetric Re ($T_c = 2.7$ K). In Re$_{ 0.82}$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 Re$_{ 0.82}$Nb$_{ 0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $\mu$SR below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric Re$T$ ($T$ = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg$_{ 10}$Ir$_{ 19}$B$_{ 16}$ and Nb$_{ 0.5}$Os$_{ 0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and Re$T$. We discuss the superconducting order parameter symmetries that are compatible with the observations.
Categories: cond-mat.supr-con