Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu

TL;DR

This study demonstrates that in nanoscale copper layers, hot electron energy transport is predominantly diffusive rather than ballistic when the layer thickness exceeds twice the inelastic mean free path, using ultrafast x-ray diffraction.

Contribution

The paper provides experimental evidence distinguishing diffusive from ballistic energy transport in nanoscale copper layers based on fluence-dependent measurements.

Findings

Energy transport in Cu becomes more efficient with increased fluence.

Diffusive transport dominates for Cu layers larger than twice the inelastic mean free path.

The approach can distinguish diffusion from ballistic transport in nanoscale structures.

Abstract

When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer into a buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. The non-linear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence. This fluence-dependent transport efficiency is reproduced by a diffusive energy transport model and serves as a generally applicable experimental approach to distinguish diffusion from ballistic transport. Following this approach, we identify diffusive electronic energy transport to govern the spatial energy distribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path.