In collaboration with Jan Vorberger, MPIKS Dresden, we investigate fundamental aspects of electron-phonon coupling in a simple metal: arxiv.org/abs/1507.03743.
#### Synopsis of the manuscript

The coupling between electrons and phonons in solids determines fundamental effects ranging from transport and dissipation phenomena to many-body effects like superconductivity, and consequently is one of the central topics in condensed matter physics.

The strength of the electron-lattice interaction is not an observable, however, and cannot be directly accessed by experiments. One experimental route for determining an effective electron-phonon coupling constant is based on ultrafast techniques, where the coupling between electrons and phonons is extracted from the dynamics of thermal equilibration after impulsive excitation of the electrons. This requires some modelling, with the canonical approach being a two-temperature model (TTM), which has been employed in thousands of publications.

We re-examine this approach by investigating the electron-phonon coupling in aluminium as the prototype of a simple free-electron metal. Employing femtosecond electron diffraction, with the highest time resolution currently available, as well as first principle calculations, we provide theoretical and experimental evidence that the basic assumption of the TTM, i.e. the notion that Bose-Einstein statistics describe the phonon population at all times, is improper.

We introduce a refined model named non-thermal lattice model (NLM). By considering the coupling between the electrons and the three phonon branches individually, we are able to describe the key mechanism transiently leading to a non-thermal lattice. We find perfect agreement between the NLM with all electron-phonon coupling parameters taken from *ab initio* calculations and the experimental results, resolving a long-standing ambiguity in the strength of electron-phonon coupling between theory and time-resolved experiments.

While we introduce this approach for the case of aluminium, we consider the NLM to provide a significantly improved representation of electron-phonon interaction in a range of metals. In addition, the concept of dividing the phononic degrees of freedom into subsets is generally applicable to describe ultrafast structural dynamics in a broad range of materials.

**Authors**: Lutz Waldecker, Roman Bertoni, Ralph Ernstorfer, Jan Vorberger

**Preprint** available at arxiv.org/abs/1507.03743.

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