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

In this paper, we report on the thermal conductivity of RbV3Sb5 and CsV3Sb5 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 12Wm-1K-1 of RbV3Sb5 and 8.8Wm-1K-1 of CsV3Sb5, and ultralow cross-plane (stacking direction) thermal conductivities are observed, with 0.72Wm-1K-1 of RbV3Sb5 and 0.49Wm-1K-1 of CsV3Sb5. 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 TCDW. This temperature dependence is found to obey the hopping transport picture. In addition, a peak in cross-plane thermal conductivity is observed at TCDW as a fingerprint of the modulated structural distortion along the stacking direction.