Fig. 2.

Anisotropy of the $ab$-plane thermal diffusivity as a function of temperature of YBCO_6.60 (green circles) and YBCO_6.75 (orange squares). Charge density order occurs at around $140-150$ K in both materials (25) and is marked by the gray region. Note that anisotropies decrease significantly below the transition, signifying the nontrivial role that the electronic system plays in the thermal transport. The solid line is the electrical anisotropy in the $ab$ plane on similar crystals adopted from ref. 12.