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$_2$.
Categories: cond-mat.supr-con