Evidence of fully-gapped superconductivity in NbReSi: A combined $\hat{I}¼$SR and NMR study

Abstract

We report a comprehensive study of the noncentrosymmetric NbReSi superconductor by means of muon-spin rotation and relaxation ($\hat{I}¼$SR) and nuclear magnetic resonance (NMR) techniques. NbReSi is a bulk superconductor with $T_c=6.5$ K, characterized by a large upper critical field, which exceeds the Pauli limit. Both the superfluid density $\ddot{I}{}_{\mathrm{sc}}(T)$ (determined via transverse-field $\hat{I}¼$SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity (SC) in NbReSi. We also find signatures of multigap SC, here evidenced by the field-dependent muon-spin relaxation rate and the electronic specific-heat coefficient. The absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field $\hat{I}¼$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of NbReSi. Finally, we discuss possible reasons for the unusually large upper critical field of NbReSi, most likely arising from its anisotropic crystal structure.

9 pages, 7 figures; accepted by Phys. Rev. B