Phonon focusing at room temperature

Abstract

Wavelike heat transport in solids, known as phonon focusing, has so far been observed only at cryogenic temperatures, limiting both its investigation and potential applications. Here we demonstrate phonon focusing at room temperature in boron arsenide, a material with high thermal conductivity. The measured ray-like temperature patterns and heat propagation dynamics match with first-principles Boltzmann transport simulations. We show that the observed heat dynamics originate from the spatial redistribution of non-equilibrium phonon waves with long propagation lengths. Moreover, our first-principles theory identifies distinct transport symmetries governed by crystallographic orientation, which we experimentally validate across multiple samples. These findings offer opportunities for directional and nanoscale control of phonon waves, non-equilibrium phonon distributions, and phonon–carrier interactions for next-generation thermal and quantum technologies at room temperature.