Thermal Diffusivity Above Mott-Ioffe-Regel Limit

TL;DR

This study measures thermal diffusivity in cuprates above the Mott-Ioffe-Regel limit, revealing linear temperature dependence and Planckian relaxation, indicating both phonons and electrons contribute to thermal transport.

Contribution

It provides high-resolution measurements showing the inverse thermal diffusivity's linearity in temperature and introduces a model linking it to Planckian relaxation and a crossover diffusion constant.

Findings

Inverse thermal diffusivity is linear in temperature.

The slope relates to Planckian relaxation time and a diffusion velocity.

The intercept indicates a crossover between coherent and incoherent quasiparticles.

Abstract

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 $\tau\approx\hbar/k_BT$ and a thermal diffusion velocity $v_B$, which is close, but larger than the sound velocity. The intercept $b$ represent 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.