Infrared probe of charge density wave gap in ScV₆Sn₆

The V-based kagome metals AV₃Sb₅ (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 AV₃Sb₅, but the origin remains elusive. A new family of the V-based kagome metals RV₆Sn₆ (R = 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. In this talk, we will discuss the electronic response of RV₆Sn₆ (R = Y, Sc) investigated by the infrared spectroscopy and DFT calculations. While the optical conductivity spectra of YV₆Sn₆ show no anomaly from 10 K to 300 K, those of ScV₆Sn₆ exhibit drastic changes below the CDW transition temperature T_CDW ≈ 92 K: the suppression of the Drude responses and the appearance of the absorption peaks at about 34 and 270 meV. Our DFT calculations demonstrate that 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(bar) - K(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.