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

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity \kappa of YBa_{2}Cu_{3}O_{y}, La_{2-x}Sr_{x}CuO_{4} and Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}. Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity \kappa_{0}/T, as well as the in-plane anisotropy of \kappa_{0}/T in Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}, 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, that are not considered in the standard perturbation theory for thermal conductivity, in the under- to optimally-doped regime.