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

TL;DR

This study measures thermal diffusivity in underdoped YBCO crystals, revealing anisotropy linked to charge order and suggesting the absence of well-defined quasiparticles due to strong scattering effects.

Contribution

First local optical measurements of thermal diffusivity in underdoped YBCO, connecting anisotropy to charge order and quasiparticle behavior.

Findings

Diffusivity anisotropy matches electrical resistivity anisotropy.

Anisotropy sharply decreases below charge order transition.

Both electronic and phononic diffusivities saturate a proposed bound.

Abstract

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. Our results suggest that neither well-defined electron nor phonon quasiparticles are present in this material.