Optical Thermal Diffusivity Measurement in Bad Metals

Local measurements of thermal diffusivity are used to analyze the transport of heat in the bad metallic regime of several strongly correlated materials. Thermal diffusivity D_Q was measured for several cuprate systems in their so-called bad metal regime, including both electron and hole doped cuprates. The temperature dependence of normal state resistivity of these systems are either linear or close-to-quadratic, while the overall magnitudes exceed the Mott-Ioffe-Regel limit at approximately 250K, indicating failure of the conventional quasiparticle transport picture. We found that the various doping of (Nd/Pr/Sm)_1-xCe_xCuO₄ all have T-linear D_Q^-1. We interpret our results through a strong electron-phonon scattering picture where both electron and lattice system saturates a quantum scattering time bound of ~h/kBT, and the slope of the T-linear D_Q^-1 is associated with a characteristic intermediate speed between the speed of sound and the Fermi velocity. Our results suggest that neither well-defined electron nor phonon quasiparticles might be present in these systems, and that thermal transport is carried out by a collective ”soup” of strongly coupled electrons and phonons.