Layer specific observation of slow thermal equilibration in ultrathin metallic nanostructures by femtosecond x-ray diffraction

TL;DR

This study uses femtosecond x-ray diffraction to reveal layer-specific heat transfer dynamics in ultrathin gold-nickel nanostructures, showing unexpected slow heat transfer from nickel to gold despite rapid initial heating of nickel.

Contribution

It provides the first layer-specific observation of ultrafast thermal equilibration in nanoscale metallic multilayers, highlighting the role of electron-specific heat and electron-phonon coupling.

Findings

Nickel heats rapidly while gold remains cold initially.

Heat transfer from Ni to Au is much slower than predicted by classical models.

Electron-specific heat differences influence energy flow in nanoscale layers.

Abstract

Ultrafast heat transport in nanoscale metal multilayers is of great interest in the context of optically-induced demagnetization, remagnetization and switching. We investigate the structural response and the energy flow in the ultrathin double-layer system Gold (Au) on ferromagnetic Nickel (Ni) by ultrafast x-ray diffraction (UXRD). The penetration depth of light exceeds the bilayer thickness, preventing unambiguous layer-specific information from optical probes. Even though the excitation pulse is incident from the Au side, we observe a very rapid heating of the Ni lattice, whereas the Au lattice initially remains cold; the subsequent heat transfer from Ni to the Au lattice is found to be two orders of magnitude slower than predicted by the conventional heat equation and much slower than electron-phonon coupling times in Au. Both observations are independent of the excitation wavelength, although for the same fluence 400nm light excites electrons in Au ten times more than 800nm light. Simple model calculations show that the different specific heat of electrons in Ni and Au as well as the different electron-phonon coupling rapidly force the majority of thermal energy into the Ni lattice. Our results show that femtosecond UXRD provides an experimental account of heat transport over single digit nanometer distances as the thermal framework for ultrafast spin dynamics.