Description
The Spectroscopy and Coherent Scattering (SCS) instrument of the European X-ray free-electron laser (EuXFEL) provides time-resolved tools to investigate electronic, spin and lattice structure of complex materials and reveal the material dynamics on the nanometer length scale and the femtosecond time scale using optical lasers as pump and soft X-rays as probe.
Femtosecond lasers are important tools to modify and control the properties of quantum materials on ultrashort time scales. Resonant inelastic X-ray scattering (RIXS) spectroscopy has emerged over the last decades as a powerful method to explore low-energy orbital, spin, lattice and charge excitations in quantum materials. The main limiting factor to attain RIXS at high energy- and time-resolution is the low repetition rate and photon flux of the first-generation XFEL. Taking advantage of the Megahertz repetition rate of the EuXFEL, the high-resolution spectrometer of the Heisenberg User consortium (hRIXS) enables users to perform RIXS spectroscopy of material dynamics in nonequilibrium with unprecedented time- and energy-resolution [1]. We report from the user-assisted commissioning program addressing charge transfer (CT) excitations in NiO transition metal oxide. The paradigmatic charge transfer insulator and antiferromagnet NiO was excited across the CT gap by a 50 fs laser pulse at 266 nm. We observe the initial creation of localized charge transfer excitons, and their subsequent decay with a time constant of about 2 ps into a metastable state that persists over several tens of ps. These first results demonstrate the potential of time-resolved RIXS at MHz-repetition rate XFELs to explore nonequilibrium dynamics of quantum materials.
Furthermore, in the context of topological states of matter, a key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions [2]. The nucleation of this phase, followed in real time via single-shot small angle X-ray scattering (SAXS) after infrared laser excitation, is mediated by a transient fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. These observations provide fundamental insights into the nature of topological phase transitions, and together with atomistic spin dynamics simulations, lead to a detailed microscopic understanding of all-optical topological switching.
[1] Schlappa et al., arXiv:2403.08461v1
[2] Büttner et al., Nature Materials 20, 30 (2021)
