Phonon-dominated energy transport in purely metallic heterostructures

TL;DR

This study demonstrates that in ultrathin Au/Ni bilayers, energy transport after laser excitation is primarily driven by phonons rather than electrons, due to weak electron-phonon coupling in gold, as shown by ultrafast x-ray diffraction and modeling.

Contribution

It provides direct experimental evidence and modeling that phonons dominate thermal transport in metallic heterostructures with ultrathin gold layers.

Findings

Energy transfer from electrons to phonons occurs within a few picoseconds.

Phonons dominate thermal transport in ultrathin gold films.

Weak electron-phonon coupling in gold suppresses electron-mediated heat conduction.

Abstract

We use ultrafast x-ray diffraction to quantify the transport of energy in laser-excited nanoscale Au/Ni bilayers. Electron transport and efficient electron-phonon coupling in Ni convert the laser-deposited energy in the conduction electrons within a few picoseconds into a strong non-equilibrium between hot Ni and cold Au phonons at the bilayer interface. Modeling of the subsequent equilibration dynamics within various two-temperature models confirms that for ultrathin Au films the thermal transport is dominated by phonons instead of conduction electrons because of the weak electron-phonon coupling in Au.