Introduction
Understanding the electronic structure of complex materials exhibiting strong electron correlation effects remains one of the key topics in solid state research. The microscopic origin of their macroscopic phenomena of significant technological relevance are often linked to their spectrum of low-energy excitations and their relaxation dynamics. Developing a fundamental understanding is scientifically intriguing, since these phenomena involve the interplay of different fundamental interactions and the most relevant mechanism are often still not identified. As experimental scientist, we aim at providing unprecedented insight into these phenomena, conceiving novel approaches to probe materials with unparalleled accuracy, for example, in terms of spectral or temporal resolution. Our experimental techniques fully exploit the unique properties of advanced X-ray spectroscopy and scattering techniques, which allow selective studies of specific aspects within complex compound materials and explore their phase diagram. The development of novel experimental techniques and their associated instrumentation is therefore an integral, important part of our research activity and we have developed a worldwide recognized expertise in this field.
Scientific strategy
Our general guideline is to concentrate within the research area of correlated materials on topics which take full advantage of the unique properties of X-ray based probe techniques and our expertise in advanced X-ray spectroscopy and scattering techniques. Over the past years this has led to the development of two main research directions:
The investigation of electronic properties by high resolution resonant inelastic X-ray scattering (RIXS), which we realize primarily at our two state-of-the-art instruments installed as permanent end stations at Synchrotron SOLEIL. To further broaden our experimental capabilities we have developed in collaboration with the LCPMR group of Marc Simon a new instrument for hard X-ray photoemission spectroscopy (HAXPES) on solid samples and in gas phase.
The aim of our second research direction is to understand femtosecond laser pulse induced magnetization dynamics. With the recent advent of novel femtosecond pulsed X-ray sources, advanced X-ray based techniques can now be applied to complement all-optical laser based experiments and we employ in our experiments a variety of probe techniques based on resonant elastic X-ray scattering at magnetically dichroic absorption resonances.
A goal of the coming years is to merge these two research directions by employing femtosecond time resolved X-ray spectroscopy techniques like RIXS to investigate electron dynamics in strongly correlated materials. To further strengthen our role in these rapidly developing scientific areas, we continue to develop our local, national and international collaborations. Our capability to fabricate and characterize complex sample structures, which we have developed at the LCPMR over the past years, plays thereby an important role. We will for this reason continue to dedicate a significant amount of time and resources to technique and instrumentation development for both, sample fabrication and characterization at the LCPMR and our research activities at external X-ray facilities.
Overview on scientific activities
The two RIXS spectrometer developed by our group are now fully operational and we are now exploiting the unique capabilities offered by them. The AEHRA instrument, installed as a permanent end station on the SEXTANTS beamline of SOLEIL, is one of the few operational soft X-ray RIXS spectrometers with a resolving power higher than 5,000. Our own research focused initially on the investigation of low-energy excitations that cannot be accessed otherwise, e.g., by UV-vis spectroscopy. Over the past years, we broadened the spectrum of scientific topics and formed collaborations with colleagues from local, European and other foreign research institutions. As a photon-based technique RIXS is well suited to probe the electronic structure of matter under extreme conditions, e.g., in the presence of strong electromagnetic fields or under high pressure conditions. For the latter, hard X-ray RIXS is in particular well-suited due to the X-rays' long penetration length. This has enabled us to employ the exceptional sensitivity of RIXS to investigate the local electronic and magnetic properties of materials with unconventional behaviors like heavy fermion compounds and transition metal based superconductors. Both RIXS instruments, AEHRA and the hard X-ray instrument at the GALAXIES beamline, are now fully operational and exhibit excellent performance. Both instruments are open to outside users and the demand exceeds largely the available beamtime, which underlines the recognition the instrument receives worldwide.
The discovery of ultrafast demagnetization dynamics by Eric Beaurepaire and his colleagues in 1996 has given rise to the today worldwide very active research area of femtomagnetism. With the advent of femtosecond pulsed X-ray sources, it has become possible to employ advanced X-ray techniques to the investigation of the mechanism underlying this intriguing phenomenon. We have initially concentrated on resonant elastic X-ray scattering techniques to probe magnetization dynamics, which add to femtosecond time resolution element selectivity and nanometer spatial resolution. Our first results, obtained in parallel at the HHG source of the Laboratoire d’Optique Appliquée and the XUV-FEL FLASH in Hamburg, revealed clear evidence for the occurrence of spin transport by the IR laser excited, hot valence band electrons, a mechanism which had been proposed to govern the ultrafast demagnetization only two years before. This led to a shift of our research activities and we realized a series of experiments further confirming our initial observations. We note that these experiments realized at XFEL sources take full advantage of their unique properties and we are today one of the principal investigators of magnetization dynamics at XFEL sources worldwide. To broaden our experimental capabilities at the XUV-FEL FLASH we have developed a versatile instrument, which has already enabled us to realize different experiments like single X-ray pulse based X-ray streaking, THz-pump - X-ray probe studies and X-ray pump – X-ray probe measurements using a split-and-delay unit, which we integrated into our setup.
As part of our collaboration with the Laboratoire d'Optique Appliquée (LOA), we developed together a novel high harmonic generation (HHG) beamline and end station dedicated to and optimized for the investigation of ultrafast charge and spin dynamics in solids. With this instrument we have recently demonstrated efficient generation of high harmonics with a high degree of circular polarization. Many well-established and powerful magnetically dichroic X-ray spectroscopy and imaging techniques can now be applied for femtosecond time resolved studies at HHG sources.