Fully gapped spin-singlet superconductivity in noncentrosymmetricPbTaSe2:Pb207nuclear magnetic resonance study

We report the $²⁰⁷\mathrm{Pb}$ nuclear magnetic resonance (NMR) measurements on polycrystalline sample of ${\mathrm{PbTaSe}}₂$ 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 ${T}_{\mathrm{c}}=4.05\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ 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 $2\mathrm{\ensuremath{\Delta}}=3.5\phantom{\rule{4pt}{0ex}}{k}_{\mathrm{B}}{T}_{\mathrm{c}}$ in ${\mathrm{PbTaSe}}₂$.