$^{181}$Ta Nuclear quadrupole resonance study of non-centrosymmetric superconductor PbTaSe$_2$

Abstract

We report on a pure 181 Ta-nuclear quadrupole resonance (NQR) measurement of PbTaSe 2 at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the TaSe 2 layer. We observed the 181 Ta-NQR spectrum of the intrinsic structure with space group P 6m2, which agrees well with density functional theory (DFT) 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 Bardeen-Cooper-Schrieffer (BCS) theory.The gap size obtained by 181 Ta-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 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 superconducting gap size inherent to multiple Fermi surfaces theoretically proposed in PbTaSe 2 .