Phonon-dominated thermal transport in an ultrathin Au-Ni bilayer

Thermal conductivity in metals is normally predominantly electronic. However, at small distances, the phonon contribution to thermal transport becomes increasingly important, especially in metals with weak electron-phonon coupling such as Au. In this report, we show that thermal transport in an Au-Ni bilayer becomes predominantly phononic when the Au thickness is ~6 nm or less. We analyze recent ultrafast x-ray diffraction measurements of heat transport in an ultrathin metal bilayer consisting of 5.6 nm Au and 12 nm Ni [1] together with earlier measurements on a thicker bilayer [2] and show that in the experiment [1] more than 50% of the heat flow from the Ni lattice to Au lattice was carried by phonons, despite a high thermal boundary resistance for phonon heat transport due to a large mismatch of the phonon spectra. The measurements reported in Ref. [1] offer a way to study heat transport by phonons in multilayer structures on the single digit nanometer scale and could be used for direct testing of non-equilibrium molecular dynamics simulations.

[1] J. Pudell et al., Nature Commun. 9, 3335 (2018).
[2] G. M. Choi et al., Phys. Rev. B 89, 064307 (2014).