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

TL;DR

This study reveals that Re$_3$B and Re$_7$B$_3$ superconductors exhibit nodeless, multigap superconductivity with preserved time-reversal symmetry, regardless of their centrosymmetric or noncentrosymmetric crystal structures.

Contribution

It demonstrates that inversion symmetry breaking and spin-orbit coupling are not necessary for multigap superconductivity in rhenium-boron compounds.

Findings

Both compounds are bulk superconductors with $T_c$ of 5.1 K and 3.3 K.

Evidence of nodeless, multigap superconductivity from $ ext{μ}$SR and heat capacity.

No spontaneous magnetic fields indicate preserved time-reversal symmetry.

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 ($\mu$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 $\mu$SR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by temperature-dependent upper critical fields $H_\mathrm{c2}(T)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $\mu$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.