Electronic Inhomogeneity and Breakdown of the Universal Thermal Conductivity of Cuprate Superconductors

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity κ of YBa₂Cu₃Oy, La_2-xSrₓCuO₄, and Bi₂Sr₂CaCu₂O_8+δ. Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity κ₀/T, as well as the in-plane anisotropy of κ₀/T in Bi₂Sr₂CaCu₂O_8+δ, indicate a breakdown of the universal thermal conductivity, a notable theoretical prediction for d-wave superconductors. Our results point to an important role of electronic inhomogeneities, which are not considered in the standard perturbation theory for thermal conductivity, in the underdoped to optimally doped regime.