Hydrodynamic heat transport in low thermal conductivity materials

Phonon hydrodynamics was mostly identified for high thermal conductivity materials due to their dominant phonon-phonon scattering. However, we show that even a low-thermal conductivity material, such as chalcogenide GeTe can exhibit phonon hydrodynamics depending on the mutual influence of phonon-phonon and extrinsic scattering processes. Our study involves the first-principles density functional method along with the direct solution of linearized Boltzmann transport equations to analyze the thermal transport in crystalline GeTe for a wide range (4-300 K) of temperatures. A phonon hydrodynamic regime is found to appear at low temperature analyzing the average scattering rates for normal, umklapp, and other resistive processes. Further, using the kinetic-collective model, the Knudsen number is determined which suggests the consistent hydrodynamic behavior of phonon thermal transport for GeTe. Finally, grain size and vacancies are found strongly to modify the hydrodynamic window.


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