  This work reports the thermal conductivity of RbV3Sb5 and CsV3Sb5 with
three-dimensional charge density wave phase transitions from 80 K to 400 K
measured by pump-probe thermoreflectance techniques. The in-plane (basal plane)
thermal conductivities are found moderate, i.e., 12 W/mK of RbV3Sb5 and 8.8
W/mK of CsV3Sb5 at 300 K. Low cross-plane (stacking direction) thermal
conductivities are observed, with 0.72 W/mK of RbV3Sb5 and 0.49 W/mK of CsV3Sb5
at 300 K. A unique glass-like 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.
