181Ta nuclear quadrupole resonance study of the noncentrosymmetric superconductor PbTaSe2

We report on a pure Ta-181 nuclear quadrupole resonance (NQR) measurement of PbTaSe2 at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the TaSe2 layer. We observed the Ta-181 NQR spectrum of the intrinsic structure with space group P6m2, which agrees well with density functional theory calculations. The nuclear spin relaxation rate 1/T-1 shows an exponential decrease well below T-c, indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by Ta-181 NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of Ta 5d orbitals, where the average pairing interactions are expected to be weaker than in the BCS model, are primarily probed. The temperature dependence of 1/T-1 below T-c can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of the superconducting gap size inherent in multiple Fermi surfaces theoretically proposed in PbTaSe2.