Infrared probe of the charge density wave gap in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Sc</mml:mi><mml:msub><mml:mi mathvariant="normal">V</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mi>Sn</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

The V-based kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A$ = 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 $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, but the origin remains elusive. A new family of the V-based kagome metals $R{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ ($R$ = Y, Sc, or rare-earth ions) has attracted recent interest. Among $R{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$, only $\mathrm{Sc}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ shows a CDW order. Thus, $R{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ 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 $R{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ ($R$ = Y, Sc). While the optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ spectra of $\mathrm{Y}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ show no anomaly from 10 to 300 K, those of $\mathrm{Sc}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ exhibit drastic changes below the CDW transition temperature ${T}_{\mathrm{CDW}}\ensuremath{\approx}92\phantom{\rule{0.16em}{0ex}}\mathrm{K}$: 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 CDW transition, 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 $\mathrm{Sc}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ and the CDW gaps corresponding to the absorption peak at 270 meV open most clearly on the ${k}_{\mathrm{z}}=1/3$ and 1/2 planes. The calculated phonon dispersions of the pristine phase of $\mathrm{Sc}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ reveal that the structural instability with the imaginary phonon frequencies on the A-H-L plane (${k}_{\mathrm{z}}=1/2$) and along the $\overline{\mathrm{M}}\text{\ensuremath{-}}\overline{\mathrm{K}}$ line (${k}_{\mathrm{z}}=1/3$) induces the out-of-plane charge modulation, indicating that the CDW transition of $\mathrm{Sc}{\mathrm{V}}_{6}{\mathrm{Sn}}_{6}$ is associated with its structural phase transition.

Topics & Concepts