Fully gapped superconductivity in centrosymmetric and noncentrosymmetric Re-B compounds probed with μSR

We present a comprehensive study on the superconducting properties of ${\mathrm{Re}}₇{\mathrm{B}}₃$ and ${\mathrm{Re}}₃\mathrm{B}$ through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation ($\ensuremath{\mu}\mathrm{SR}$) experiments on polycrystalline samples. ${\mathrm{Re}}₇{\mathrm{B}}₃$ (${T}_C$ = 3.2 K) is a noncentrosymmetric type-II ($\ensuremath{\kappa}\phantom{\rule{4pt}{0ex}}\ensuremath{\simeq}$ 9.27) superconductor in the weak-coupling (${\ensuremath{\lambda}}_{e\ensuremath{-}ph}$ = 0.54) regime. On the other hand, ${\mathrm{Re}}₃\mathrm{B}$ (${T}_C$ = 5.19 K) is a centrosymmetric type-II ($\ensuremath{\kappa}\phantom{\rule{4pt}{0ex}}\ensuremath{\simeq}$ 34.55) superconductor in the moderate-coupling (${\ensuremath{\lambda}}_{e\ensuremath{-}ph}$ = 0.64) regime. Our transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements show evidence for isotropically gapped BCS-type superconductivity with normalized gap (${\mathrm{\ensuremath{\Delta}}}₀/{k}_B{T}_C$) values of 1.69 (${\mathrm{Re}}₇{\mathrm{B}}₃$) and 1.75 (${\mathrm{Re}}₃\mathrm{B}$).