Fully gapped spin-singlet superconductivity in noncentrosymmetric PbTaSe$_2$ : $^{207}$Pb-NMR

We report the $^{207}$Pb nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe$_2$ with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate $1/T_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\rm c}$ = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below $T$ = 0.7 $T_{\rm c}$. The Knight shift $K$ decreases below $T_{\rm c}$. These results suggest spin-singlet superconductivity with a fully-opened gap $2\Delta = 3.5$ $k_{\rm B}T_{\rm c}$ in PbTaSe$_2$.