Microstructure effects on the phase transition behavior of a prototypical quantum material

Materials with insulator metal transitions promise advanced functionalities for future information technology. Patterning on the microscale is key for miniaturized functional devices, but material properties may vary spatially across microstructures. Characterization of these miniaturized devices requires electronic structure probes with sufficient spatial resolution to understand the influence of structure size and shape on functional properties. The present study demonstrates the use of imaging soft X ray absorption spectroscopy with a spatial resolution better than 2 amp; 956;m to study the insulator metal transition in vanadium dioxide thin film microstructures. This novel technique reveals that the transition temperature for the conversion from insulating to metallic vanadium dioxide is lowered by 1.2 K 0.4 K close to the structure edges compared to the center. Facilitated strain release during the phase transition is discussed as origin of the observed behavior. The experimental approach enables a detailed understanding of how the electronic properties of quantum materials depend on their patterning at the micrometer scal

Ultrafast manipulation of the NiO antiferromagnetic order via sub gap optical excitation

Wide band gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub gap excitation in charge transfer insulators has been shown to couple to low energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge transfer insulator NiO to demonstrate that 1.5 eV sub gap optical excitation leads to a renormalised NiO band gap in combination with a significant reduction of the antiferromagnetic order. We employ element specific X ray reflectivity at the FLASH free electron laser to demonstrate the reduction of the upper band edge at the O 1s 2p core valence resonance K edge whereas the antiferromagnetic order is probed via X ray magnetic linear dichroism XMLD at the Ni 2p 3d resonance L2 edge . Comparing the transient XMLD spectral line shape to ground state measurements allows us to extract a spin temperature rise of 65 5 K for time delays longer than 400 fs while at earlier times a non equilibrium spin state is formed. We identify transient mid gap states being formed during the first 200 fs accompanied by a band gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band ga

The TRIXS end station for femtosecond timeresolved resonant inelastic x ray scattering experiments at the soft x ray free electron laser FLASH

We present the experimental end station TRIXS dedicated to time resolved soft x ray resonant inelastic x ray scattering RIXS experiments on solid samples at the free electron laser FLASH. Using monochromatized ultrashort femtosecond XUV soft x ray photon pulses in combination with a synchronized optical laser in a pump probe scheme, the TRIXS setup allows measuring sub picosecond time resolved highresolution RIXS spectra in the energy range from 35 eV to 210 eV, thus spanning the M edge M1 and M2,3 absorption resonances of 3d transition metals and N4,5 edges of rare earth elements. A Kirkpatrick Baez refocusing mirror system at the first branch of the plane grating monochromator beamline PG1 provides a focus of 6 6 lm2 FWHM at the sample. The RIXS spectrometer reaches an energy resolution of 35 160 meV over the entire spectral range. The optical laser system based on a chirped pulse optical parametric amplifier provides approximately 100 fs FWHM long photon pulses at the fundamental wavelength of 800 nm and a fluence of 120 mJ cm2 at a sample for optical pump XUV probe measurements. Furthermore, optical frequency conversion enables experiments at 400 nm or 267 nm with a fluence of 80 and 30 mJ cm2, respectively. Some of the first pump probe RIXS spectra measured with this setup are shown. The measured time resolution for time resolved RIXS measurements has been characterized as 287 fs FWHM for the used energy resolutio

Electron population dynamics in resonant non linear x ray absorption in nickel at a free electron laser

Free electron lasers provide bright, ultrashort, and monochromatic x ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution The high fluence of their x ray pulses can also easily enter the regime of the non linear x ray matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non linear processes for future experiment designs. Here, we show non linear changes in the edge absorption of metallic nickel thin films, measured with fluences up to 60 amp; 8201;J cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non linear changes. Our findings provide important insights for the design and evaluation of future high fluence free electron laser experiments and contribute to the understanding of non linear electron dynamics in x ray absorption processes in solids at the femtosecond timescal