Multigap superconductivity in centrosymmetric and noncentrosymmetric rhenium-boron superconductors
Tian Shang, Weiwei Xie, Jianzhou Zhao, Yuxuan Chen, Dariusz Jakub Gawryluk, M. Medarde, M. Shi, Huiqiu Yuan, E. Pomjakushina, T. Shiroka
Abstract
We report a comprehensive study of the centrosymmetric ${\mathrm{Re}}_{3}\mathrm{B}$ and noncentrosymmetric ${\mathrm{Re}}_{7}{\mathrm{B}}_{3}$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with ${T}_{c}=5.1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (${\mathrm{Re}}_{3}\mathrm{B}$) and 3.3 K (${\mathrm{Re}}_{7}{\mathrm{B}}_{3}$), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$). In both ${\mathrm{Re}}_{3}\mathrm{B}$ and ${\mathrm{Re}}_{7}{\mathrm{B}}_{3}$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via transverse-field $\ensuremath{\mu}\mathrm{SR}$) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by the temperature-dependent upper critical fields ${H}_{\mathrm{c}2}(T)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements, indicates a preserved time-reversal symmetry in the superconducting state of both ${\mathrm{Re}}_{3}\mathrm{B}$ and ${\mathrm{Re}}_{7}{\mathrm{B}}_{3}$. Our results suggest that a lack of inversion symmetry and the accompanying antisymmetric spin-orbit coupling effects are not essential for the occurrence of multigap SC in these rhenium-boron compounds.