Low-energy gap associated with the anomalous symmetry-breaking states in CsV₃Sb₅

Several anomalous symmetry-breaking states have been reported at temperatures well below the charge-density wave (CDW) transition in CsV₃Sb₅. Whether these states are concomitant with an electronic structure change remains elusive. Here, we present a study of the optical properties of CsV₃Sb₅ in the CDW state. In addition to the CDW gap, which opens below T_CDW = 91 K, a conspicuous suppression of the optical conductivity develops around 200 cm⁻¹ below T^∗ ≃ 50 K, indicating the opening of another low-energy gap well below the CDW transition. A detailed Drude-Lorentz analysis manifests that, similar to the CDW gap, this low-energy gap also opens in the saddle-point bands with V-3d character near the M point. Our experimental results not only reveal a Fermi surface reconstruction coinciding with the emergence of the anomalous symmetry-breaking states, but also underline the importance of the V-3d orbitals in driving these anomalous states in CsV₃Sb₅.