Nodeless superconductivity in the centro- and noncentrosymmetric rhenium-boron superconductors

Abstract

We report a comprehensive study of the centrosymmetric Re${}_3$B and noncentrosymmetric Re${}_7$B${}_3$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with $T_c$ = 5.1 K (Re${}_3$B) and 3.3 K (Re${}_7$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/relaxation ($\hat{I}¼$SR). In both Re${}_3$B and Re${}_7$B${}_3$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via tranverse-field $\hat{I}¼$SR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by temperature-dependent upper critical fields $H_{c2}(T)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $\hat{I}¼$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of both Re${}_3$B and Re${}_7$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.

9 pages, 6 figures, accepted by Phys. Rev. B