Evidence of fully gapped superconductivity in NbReSi: A combined ?SR and NMR study

We report a comprehensive study of the noncentrosymmetric NbReSi superconductor by means of muon-spin rotation and relaxation (??SR) and nuclear magnetic resonance (NMR) techniques. NbReSi is a bulk superconductor with Tc = 6.5 K, characterized by a large upper critical field, which exceeds the Pauli limit. Both the superfluid density psc(T ) (determined via transverse-field ??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 Tc, as evinced from zero-field ??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.