X-ray free-electron lasers – a new probe for quantum matter

The advent of x-ray free-electron lasers (FEL) provides us with a tool to investigate the lattice and electronic dynamics of solids at the inherent time (femtoseconds) and length (Ångströms) scale. This new generation of accelerator-based photon sources delivers extremely brilliant, ultrashort and coherent x-ray pulses that are primarily used to map out ultrafast dynamics, but also for taking snapshots with single x-ray pulses and/or employing their coherence. First, I will introduce the unique properties of x-ray FELs and sketch the current state-of-the-art in the field of condensed matter, as well as the progress of the SwissFEL project. Then, I will illustrate the capabilities of x-ray FELs at the example of a study from the Linac Coherent Light Source (LCLS). 
Identifying the mechanism that leads to the emergence of superconductivity in iron-based materials remains the subject of active research. Amongst spin-driven scenarios, it has also been suggested that electron correlations enhance the electron-phonon coupling in these materials, but direct experimental verification has been lacking. We show that the electron-phonon coupling strength in FeSe can be directly quantified by combining two ultrafast experiments into a “coherent lock-in” measurement in the terahertz regime: x-ray diffraction tracks the light-induced coherent lattice motion, while photoemission monitors the coherent, orbital-resolved changes in the electronic band structure. Comparison with theory reveals an enhancement of the coupling strength owing to correlation effects, highlighting the importance of a cooperative interplay between electron-electron and electron-phonon interactions. By relying only on a linear, coherent response, this approach denotes a purely experimental and model-free technique, which opens the way for unbiased tests of emergent phenomena in correlated materials.