Multigap superconductivity in centrosymmetric and noncentrosymmetric rhenium-boron superconductors

We report a comprehensive study of the centrosymmetric Re3B and noncentrosymmetric Re7B3 superconductors. At a macroscopic level, their bulk superconductivity (SC), with Tc=5.1K (Re3B) and 3.3 K (Re7B3), 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 (μSR). In both Re3B and Re7B3 the low-T zero-field electronic specific heat and the superfluid density (determined via transverse-field μSR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by the temperature-dependent upper critical fields Hc2(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 μSR measurements, indicates a preserved time-reversal symmetry in the superconducting state of both Re3B and Re7B3. 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.