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

We present a comprehensive study on the superconducting properties of Re7B3 and Re3B through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation (μSR) experiments on polycrystalline samples. Re7B3 (TC = 3.2 K) is a noncentrosymmetric type-II (κ≃ 9.27) superconductor in the weak-coupling (λe−ph = 0.54) regime. On the other hand, Re3B (TC = 5.19 K) is a centrosymmetric type-II (κ≃ 34.55) superconductor in the moderate-coupling (λe−ph = 0.64) regime. Our transverse-field μSR measurements show evidence for isotropically gapped BCS-type superconductivity with normalized gap (Δ0/kBTC) values of 1.69 (Re7B3) and 1.75 (Re3B).