Glasslike cross-plane thermal conductivity of the kagome metals RbV3Sb5 and CsV3Sb5

In this paper, we report on the thermal conductivity of ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$ and ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ with three-dimensional charge density wave phase transitions from 40 to 500 K measured by pump-probe thermoreflectance techniques. At room temperature, the in-plane (basal plane) thermal conductivities are found to be moderate, with $12\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ of ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$ and $8.8\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, and ultralow cross-plane (stacking direction) thermal conductivities are observed, with $0.72\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ of ${\mathrm{RbV}}_{3}{\mathrm{Sb}}_{5}$ and $0.49\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. A unique glasslike temperature dependence in the cross-plane thermal conductivity is observed, which decreases monotonically even lower than the Cahill-Pohl limit as the temperature decreases below the phase transition point ${T}_{\mathrm{CDW}}$. This temperature dependence is found to obey the hopping transport picture. In addition, a peak in cross-plane thermal conductivity is observed at ${T}_{\mathrm{CDW}}$ as a fingerprint of the modulated structural distortion along the stacking direction.