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

Jarecki, J. ; Mehner, L. ; Mattern, M. ; Jurgilaitis, A. LU ; Zeuschner, S. P. ; Ahn, B. LU ; Baltrusch, F. ; Ekström, J. C. LU ; Kroon, D. LU and Herzog, M. , et al. (2026) In Applied Physics Letters 128(17).
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 leverage the fluence-dependent transport efficiency to distinguish ballistic and diffusive electron transport in an approach not relying on the transport velocity. We follow the energy that is rapidly transferred across Cu layers of different thicknesses via hot electrons from a photo-excited Pt layer into a buried Ni detection layer. In the Ni layer, the transported energy linearly relates to a rapid lattice expansion, which we probe via ultrafast x-ray diffraction. A... (More)

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 leverage the fluence-dependent transport efficiency to distinguish ballistic and diffusive electron transport in an approach not relying on the transport velocity. We follow the energy that is rapidly transferred across Cu layers of different thicknesses via hot electrons from a photo-excited Pt layer into a buried Ni detection layer. In the Ni layer, the transported energy linearly relates to a rapid lattice expansion, which we probe via ultrafast x-ray diffraction. A nonlinear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence, which is inconsistent with a ballistic scenario but reproduced by a diffusive energy transport model. We already identify that for a Cu thickness of about 50 nm, i.e., about twice the electronic inelastic mean free path, diffusive electronic energy transport dominates the spatial energy distribution. Our experimental approach is generally applicable to distinguish diffusion from ballistic energy transport.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Applied Physics Letters
volume
128
issue
17
article number
172204
publisher
American Institute of Physics (AIP)
external identifiers
  • scopus:105037837754
ISSN
0003-6951
DOI
10.1063/5.0318923
language
English
LU publication?
yes
id
5ac696c4-8621-416b-b125-05973abecdbd
date added to LUP
2026-08-27 11:26:45
date last changed
2026-08-27 11:27:27
@article{5ac696c4-8621-416b-b125-05973abecdbd,
  abstract     = {{<p>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 leverage the fluence-dependent transport efficiency to distinguish ballistic and diffusive electron transport in an approach not relying on the transport velocity. We follow the energy that is rapidly transferred across Cu layers of different thicknesses via hot electrons from a photo-excited Pt layer into a buried Ni detection layer. In the Ni layer, the transported energy linearly relates to a rapid lattice expansion, which we probe via ultrafast x-ray diffraction. A nonlinear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence, which is inconsistent with a ballistic scenario but reproduced by a diffusive energy transport model. We already identify that for a Cu thickness of about 50 nm, i.e., about twice the electronic inelastic mean free path, diffusive electronic energy transport dominates the spatial energy distribution. Our experimental approach is generally applicable to distinguish diffusion from ballistic energy transport.</p>}},
  author       = {{Jarecki, J. and Mehner, L. and Mattern, M. and Jurgilaitis, A. and Zeuschner, S. P. and Ahn, B. and Baltrusch, F. and Ekström, J. C. and Kroon, D. and Herzog, M. and Walz, C. and Weber, F. C. and Larsson, J. and Hehn, M. and Pudell, J. E. and Schick, D. and Von Reppert, A. and Bargheer, M.}},
  issn         = {{0003-6951}},
  language     = {{eng}},
  number       = {{17}},
  publisher    = {{American Institute of Physics (AIP)}},
  series       = {{Applied Physics Letters}},
  title        = {{Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu}},
  url          = {{http://dx.doi.org/10.1063/5.0318923}},
  doi          = {{10.1063/5.0318923}},
  volume       = {{128}},
  year         = {{2026}},
}