S-Wave Superconductivity in Kagome Metal CsV3Sb5 Revealed by 121/123Sb NQR and 51V NMR Measurements

We report 121/123Sb nuclear quadrupole resonance (NQR) and 51V nuclear magnetic resonance (NMR) measure-ments on kagome metal CsV3Sb5 with Tc =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/T1T shows a Hebel Slichter coherence peak just below Tc,indicating that CsV3Sb5 is an s-wave superconductor.