Superconducting Gap Structure of the Noncentrosymmetric Topological Superconductor Candidate HfRuP

We investigate the gap symmetry of the topological superconductor candidate HfRuP, which crystallizes in a noncentrosymmetric hexagonal crystal structure, using muon spin rotation/relaxation (mu SR) measurements in transverse-field (TF) geometry. The temperature and magnetic field dependencies of the superconducting relaxation rate derived from the TF-mu SR spectra can be well described by an isotropic s-wave gap. The superconducting carrier density n(s) = 1.41(1) x 10(26) m(-3) and the magnetic penetration depth, lambda(0) = 603(2) nm, were calculated from the TF- mu SR data. Interestingly, the ratio between the superconducting transition temperature and the superfluid density, T-c/lambda(-2)(0) similar to 3.3, is very close to those of unconventional superconductors. Further, our zero-field (ZF) mu SR results do not show any considerable change in the muon spin relaxation above and below the superconducting transition temperature, suggesting that time-reversal symmetry is preserved in the superconducting state of this superconductor.