Coherent and Incoherent Time-Resolved Diffuse Scattering Intensity from First Principles
Abstract
Time-resolved diffuse scattering from X-ray free-electron lasers and ultrafast electron diffraction gives access to the nonequilibrium lattice dynamics of photoexcited solids in reciprocal space. Existing interpretations have often relied on simplified models. Here we present a first-principles formulation of the time-dependent first-order diffuse intensity in which coherent and incoherent contributions emerge on equal footing. By assuming the probe-pulse coherence time to be longer than the characteristic electronic timescales, but shorter than those of the ionic motion, we work in the density-matrix formalism and derive equations of motion for the displacement-displacement correlators in a basis of instantaneous, out-of-equilibrium Born-Oppenheimer normal modes. We show how the incoherent contribution, governed by the phonon occupations, and a coherent contribution, governed by the anomalous phonon correlators, emerge. The non-trivial coherent dynamics is shown to be induced both by the photoinduced phonon-frequency shift and by electron-phonon scattering off the nonequilibrium carrier distribution. We implement the resulting equations using Wannier interpolation in the \textsc{EPI}q code and apply them to photoexcited germanium. The calculated diffuse intensity reproduces both the oscillation frequencies and the rising incoherent background observed experimentally, opening the way to a systematic interpretation of time-resolved diffuse scattering in ultrafast experiments.
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BibTeX
@misc{furci2026coherent,
title = {{Coherent and Incoherent Time-Resolved Diffuse Scattering Intensity from First Principles}},
author = {Matteo Furci and Giovanni Marini and Stefano Mocatti and Matteo Calandra},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22006},
eprint = {2610.22006},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22006},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Coherent and Incoherent Time-Resolved Diffuse Scattering Intensity from First Principles
AU - Matteo Furci
AU - Giovanni Marini
AU - Stefano Mocatti
AU - Matteo Calandra
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22006
UR - https://arxiv.org/abs/2610.22006
AB - Time-resolved diffuse scattering from X-ray free-electron lasers and ultrafast electron diffraction gives access to the nonequilibrium lattice dynamics of photoexcited solids in reciprocal space. Existing interpretations have often relied on simplified models. Here we present a first-principles formulation of the time-dependent first-order diffuse intensity in which coherent and incoherent contributions emerge on equal footing. By assuming the probe-pulse coherence time to be longer than the characteristic electronic timescales, but shorter than those of the ionic motion, we work in the density-matrix formalism and derive equations of motion for the displacement-displacement correlators in a basis of instantaneous, out-of-equilibrium Born-Oppenheimer normal modes. We show how the incoherent contribution, governed by the phonon occupations, and a coherent contribution, governed by the anomalous phonon correlators, emerge. The non-trivial coherent dynamics is shown to be induced both by the photoinduced phonon-frequency shift and by electron-phonon scattering off the nonequilibrium carrier distribution. We implement the resulting equations using Wannier interpolation in the \textsc{EPI}q code and apply them to photoexcited germanium. The calculated diffuse intensity reproduces both the oscillation frequencies and the rising incoherent background observed experimentally, opening the way to a systematic interpretation of time-resolved diffuse scattering in ultrafast experiments.
ER -