Infrared probe of the charge density wave gap in ScV₆Sn₆

The V-based kagome metals AV(3)Sb(5) (A = K, Rb, Cs) exhibit a cascade of exotic quantum phenomena including charge density wave (CDW) order and superconductivity. Considerable effort has been made to understand the nature of the CDW phase of AV(3)Sb(5), but the origin remains elusive. A new family of the V-based kagome metals RV6Sn6 (R = Y, Sc, or rare-earth ions) has attracted recent interest. Among RV6Sn6, only ScV6Sn6 shows a CDW order. Thus, RV6Sn6 can be a new platform for investigating the nature of the CDW phase of the V-based kagome metals. Here, combining infrared spectroscopy with density-functional theory (DFT) calculations, we investigate the electronic response of RV6Sn6 (R = Y, Sc). While the optical conductivity sigma(1)(omega) spectra of YV6Sn6 show no anomaly from 10 to 300 K, those of ScV6Sn6 exhibit drastic changes below the CDW transition temperature T-CDW approximate to 92K: the suppression of the Drude responses and the appearance of the absorption peaks at about 34 and 270 meV. A distinct multipeak structure in the energy region between 270 and 800 meV due to the interband transitions associated with the van Hove singularities (vHSs) at the M point is hardly affected by the CDWtransition, implying the robustness of the vHSs at the M point against the CDW transition. Our DFT calculations demonstrate that the vHSs at the M point remain intact in the CDW phase of ScV6Sn6 and the CDW gaps corresponding to the absorption peak at 270 meV open most clearly on the k(z) = 1/3 and 1/2 planes. The calculated phonon dispersions of the pristine phase of ScV6Sn6 reveal that the structural instability with the imaginary phonon frequencies on the A-H-L plane (k(z) = 1/2) and along the (M) over bar-(K) over bar line (k(z) = 1/3) induces the out-of-plane charge modulation, indicating that the CDW transition of ScV6Sn6 is associated with its structural phase transition.