Thermal diffusivity above the Mott-Ioffe-Regel limit

Abstract

Understanding transport of charge excitations in strongly correlated electron systems exhibiting the ``strange metal state without long-lived quasiparticle excitations has been a long-standing challenge in condensed matter physics. Here, the authors demonstrate that thermal diffusivity, which tracks energy-thermalization processes, exhibits a limiting Planckian relaxation time and a unique thermal diffusion velocity. These reflect both the contribution of phonons and electrons. This result is bound by the thermal diffusivity of the electrons at the Mott-Ioffe-Regel limit, where the mean-free-path reaches atomic distances.