s-wave superconductivity in kagome metal CsV${}_3$Sb${}_5$ revealed by ${}^{121/123}$Sb NQR and ${}^{51}$V NMR measurements

Abstract

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 $T_{\mathrm{c}}=2.5$ K. Both ${}^{51}$V NMR spectra and ${}^{121/123}$Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between $91$ K and $94$ K manifests that it is a first order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and $1/T_{1}T$ shows a Hebel--Slichter coherence peak just below $T_{\mathrm{c}}$, indicating that CsV${}_3$Sb${}_5$ is an s-wave superconductor.