Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh${}_2$B${}_2$

Abstract

NbRh${}_2$B${}_2$ crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below $7.46(5)$~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 ${\mu }_0{H}_{c2}\left(0\right)\approx 18$~T exceeds the Pauli paramagnetic limit, ${\mu }_0{H}_{\mathrm{P}}=13.9$~T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either $s$-wave or $\left(s+s\right)$-wave, and the pressure dependence of $T_{\mathrm{c}}$ reveals the superconducting gap is primarily $s$-wave in character. The magnetic penetration depth $\lambda (0)=595(5)$~nm. Heat capacity measurements reveal the presence of a multigap $\left(s+s\right)$-wave superconducting order parameter and moderate electron-phonon coupling.

18 pages, 7 figures, 1 table