Nonequilibrium phonon distribution in current-driven nanostructures*

Electric energy of current flowing in materials or devices is dissipated mostly as Joule heat. However, in nanoscale systems, the generated phonons can escape before they thermalize, which can result in the breakdown of the Joule heating approximation.
We demonstrate a strongly non-equilibrium phonon distribution in current-driven metallic microwires, which cannot be described in terms of heat [1]. Our main observation is a linear dependence of resistance on current at cryogenic temperatures, qualitatively inconsistent with the effects of Joule heating, as confirmed by the simulations of heat flow. As the temperature is increased, the zero-current singularity becomes smoothed out, but the linear dependence remains apparent at sufficiently large currents even near ambient temperatures. A kinetic model based on the quasi-ballistic escape approximation for the nonequilibrium phonons generated by electron scattering reproduces our main observations. The demonstrated effects are important for optimizing thermal management in electronic and thermoelectric nanodevices, and for the analysis of current-induced thermal effects at nanoscale.
[1] G.X. Chen, R. Freeman, A. Zholud, S. Urazhdin, Phys. Rev. X 10, 011064 (2020)