Anomalous Thermal Diffusivity in Underdoped YBa$_2$Cu$_3$O$_{6+x}$

Understand

We present local optical measurements of thermal diffusivity in the $ab$ plane of underdoped YBCO crystals.

• We find that the diffusivity anisotropy is comparable to reported values of the electrical resistivity anisotropy, suggesting that the anisotropies have the same origin.
• The anisotropy drops sharply below the charge order transition.
• We interpret our results through a strong electron-phonon scattering picture and find that both electronic and phononic contributions to the diffusivity saturate a proposed bound.