Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh2B2

Abstract

NbRh2B2 crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below K. Here we show that the temperature dependence of the upper critical field deviates from the behaviour expected for both Werthamer–Helfand–Hohenberg and the Ginzburg–Landau models and that T exceeds the Pauli paramagnetic limit, T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either s-wave or -wave, and the pressure dependence of reveals the superconducting gap is primarily s-wave in character. The magnetic penetration depth nm. Heat capacity measurements reveal the presence of a multigap -wave superconducting order parameter and moderate electron–phonon coupling.