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 $1\mathrm{/}{T}_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\mathrm{c}}$ = 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 $T_{\mathrm{c}}$. The Knight shift $K$ decreases below $T_{\mathrm{c}}$. These results suggest spin-singlet superconductivity with a fully-opened gap $2\mathrm{\Delta }=3.5$ $k_{\mathrm{B}}{T}_{\mathrm{c}}$ in PbTaSe${}_2$.