Understanding the photo-physics of driven materials requires detailed measurements of multiple microscopic degrees of freedom at ultrafast time resolution. Femtosecond X-rays are key to this endeavor, as they can probe the dynamics of structural, electronic and magnetic degrees of freedom. To this end, we perform hard and soft x-ray diffraction experiments aimed at clarifying the mechanisms behind photo-induced phases. While traditional diffraction experiments reveal insights on long range orders, time-resolved resonant inelastic x-ray scattering gives access to the dynamics of the momentum-dependent excitation spectrum of a material. This is key in revealing precursors of long-range orders, such as magnetic correlations. In addition, femtosecond x-ray techniques also allow for probing excitations directly in the time domain. The diffuse scattering between Bragg peaks yields momentum-resolved information on phonon anharmonicities and was key in determining possible pathways to light-induced ferroelectricity. We perform these experiments at several free-electron laser facilities around the world such as PAL-XFEL in Korea, LCLS in the United States, European XFEL in Germany, and SwissFEL in Switzerland.
Measuring non-equilibrium dynamics in complex solids with ultrashort x-ray pulses
M. Buzzi, M. Först, A. Cavalleri
Philosophical Transactions A, 377, 20170478 (2019)
Strong interactions between electrons give rise to the complexity of quantum materials, which exhibit exotic functional properties and extreme susceptibility to external perturbations. A growing research trend involves the study of these materials away from equilibrium, especially in cases in which the stimulation with optical pulses can coherently enhance cooperative orders. Time-resolved X-ray probes are integral to this type of research, as they can be used to track atomic and electronic structures as they evolve on ultrafast timescales. Here, we review a series of recent experiments where femtosecond X-ray diffraction was used to measure dynamics of complex solids.
Probing dynamics in quantum materials with femtosecond X-rays
M. Buzzi, M. Först, R. Mankowsky, and A. Cavalleri
Nature Reviews Materials, 3, 299–311 (2018)
Optical pulses are routinely used to drive dynamic changes in the properties of solids. In quantum materials, many new phenomena have been discovered, including ultrafast transitions between electronic phases, switching of ferroic orders and non- equilibrium emergent behaviours, such as photoinduced superconductivity. Understanding the underlying non- equilibrium physics requires detailed measurements of multiple microscopic degrees of freedom at ultrafast time resolution. Femtosecond X- rays are key to this endeavour, as they can probe the dynamics of structural, electronic and magnetic degrees of freedom. Here, we review a series of representative experimental studies in which ultrashort X- ray pulses from free- electron lasers have been used, opening up new horizons for materials research.