  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.
