Probing dynamics in quantum materials with femtosecond X-rays

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. In quantum materials, ultrashort light pulses can induce transitions between electronic phases, switch ferroic orders and unveil non-equilibrium emergent behaviours. Here, we review the use of femtosecond X-ray pulses in tracking the underlying dynamics of the structural, electronic and magnetic order in these systems.