Origin of charge density wave in the kagome metal CsV 3 Sb 5 as revealed by optical spectroscopy

The recently discovered kagome metals $A$V${}_{3}$Sb${}_{5}$ ($A$=K, Rb or Cs) have generated great interest as they exhibit an unconventional charge density wave (CDW), superconductivity, and nontrivial band topology. Here, the authors combine infrared spectroscopy and first-principles calculations to shed light on the origin of the CDW. They find that below the CDW transition the optical conductivity component associated with the saddle points is significantly suppressed, while that arising from the light-electron and Dirac bands is not affected. These observations attest to the importance of saddle point nesting in driving the CDW instability in CsV${}_{3}$Sb${}_{5}$.