Origin of charge density wave in the kagome metal CsV3 Sb5 as revealed by optical spectroscopy

We report on a study of the optical properties of CsV3Sb5 for a large number of temperatures above and below the charge-density-wave (CDW) transition. Above the CDW transition, the low-frequency optical conductivity reveals two Drude components with distinct widths. An examination of the band structure allows us to ascribe the narrow Drude component to a light-electron-like and multiple Dirac bands, and the broad Drude component to heavy-hole bands near the M points which form saddle points near the Fermi level. Upon entering the CDW state, the opening of the CDW gap is clearly observed. A large portion of the broad Drude component is removed by the gap, whereas the narrow Drude component is not affected. Meanwhile, an absorption peak associated with interband transitions involving the saddle points shifts to higher energy and grows in weight. These observations attest to the importance of saddle point nesting in driving the CDW instability in CsV3Sb5.