Thermal Diffusivity Above The Mott-Ioffe-Regel Limit

We present high-resolution thermal diffusivity measurements on several near optimally doped electron- and hole-doped cuprate systems in a temperature range that passes through the Mott-Ioffe-Regel limit, above which the quasiparticle picture fails. Our primary observations are that the inverse thermal diffusivity is linear in temperature and can be fitted to D-Q(-1) = aT + b. The slope a is interpreted through the Planckian relaxation time tau approximate to h/k(B)T and a thermal diffusion velocity v(B), which is close, but larger than the sound velocity. The intercept b represents a crossover diffusion constant that separates coherent from incoherent quasiparticles. These observations suggest that both phonons and electrons participate in the thermal transport, while reaching the Planckian limit for relaxation time.