S-Wave Superconductivity in Kagome Metal CsV_3Sb_5 Revealed by ^121/123Sb NQR and ^51V NMR Measurements

We report ^121/123Sb nuclear quadrupole resonance (NQR) and ^51V nuclear magnetic resonance (NMR) measurements on kagome metal CsV_3Sb_5 with T_\rm c=2.5 K. Both ^51V NMR spectra and ^121/123Sb 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_1T shows a Hebel–Slichter coherence peak just below T_\rm c, indicating that CsV_3Sb_5 is an s-wave superconductor.