$s$-wave superconductivity in kagome metal CsV$_{3}$Sb$_{5}$ revealed by $^{121/123}$Sb NQR and $^{51}$V NMR measurements

We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{\rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra splits after charge density wave (CDW) transition, which demonstrates a commensurate CDW order. Coexistence of high temperature phase and CDW order phase between $91$ K and $94$ K manifest it is a first order phase transition. At low temperature, EFG fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decrease below $T_{\rm c}$ and $1/T_{1}T$ shows a Hebel-Slichter coherence peak just below $T_{\rm c}$, indicating that CsV$_3$Sb$_5$ is an $s$-wave superconductor.