Ta 181 nuclear quadrupole resonance study of the noncentrosymmetric superconductor PbTaSe 2

We report on a pure $^{181}\mathrm{Ta}$ nuclear quadrupole resonance (NQR) measurement of ${\mathrm{PbTaSe}}_{2}$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the ${\mathrm{TaSe}}_{2}$ layer. We observed the $^{181}\mathrm{Ta}$ NQR spectrum of the intrinsic structure with space group $P\overline{6}m2$, which agrees well with density functional theory calculations. The nuclear spin relaxation rate $1/{T}_{1}$ shows an exponential decrease well below ${T}_{{}_{\mathrm{c}}}$, indicating that the superconducting state is fully gapped in the framework of BCS theory. The gap size obtained by $^{181}\mathrm{Ta}$ NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of $\mathrm{Ta}\phantom{\rule{4pt}{0ex}}5d$ 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 ${T}_{{}_{\mathrm{c}}}$ 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 ${\mathrm{PbTaSe}}_{2}$.