The GALAXIES beamline at the SOLEIL synchrotron: inelastic X-ray scattering and photoelectron spectroscopy in the hard X-ray range

Subfemtosecond Control of Molecular Fragmentation by Hard X-Ray Photons

Tuning hard x-ray excitation energy along Cl 1s→σ^{*} resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO

Contributions to the spectroscopic response of defects and dopants in Al-doped ZnO films are determined combining X-ray spectroscopies and DFT.

Double-Core-Hole States in Neon: Lifetime, Post-Collision Interaction, and Spectral Assignment

Using synchrotron radiation and high-resolution electron spectroscopy, we have directly observed and identified specific photoelectrons from K^{-2}V states in neon corresponding to simultaneous 1s ionization and 1s→valence excitation. The natural lifetime broadening of the K^{-2}V states and the relative intensities of different types of shakeup channels have been determined experimentally and compared to ab initio calculations. Moreover, the high-energy Auger spectrum resulting from the decay of Ne^{2+}K^{-2} and Ne^{+}K^{-2}V states as well as from participator Auger decay from Ne^{+}K^{-1}L^{-1}V states, has been measured and assigned in detail utilizing the characteristic differences in lifetime broadenings of these core hole states. Furthermore, post collision interaction broadening of Auger peaks is clearly observed only in the hypersatellite spectrum from K^{-2} states, due to the energy sharing between the two 1s photoelectrons which favors the emission of one slow and one fast electron.

A computationally efficient method to solve the Takagi–Taupin equations for a large deformed crystal

A treatise is presented on solving the Takagi–Taupin equations in the case of a strain field with an additional, spatially slowly varying component (owing to, for example, heat expansion or angular compression). It is shown that such a component typically has a negligible effect on the shape of the reflectivity curve when considering the reflectivity of a microscopic surface area of the crystal. However, it makes the centroid of that curve shift in terms of the wavelength (or the incidence angle) as a function of the position of the mentioned area, which alters the shape of the overall reflectivity curve integrated over the crystal's macroscopic surface. The validity of the method is demonstrated by comparing computed reflectivity curves with experimental ones for bent silicon wafers. A good agreement is observed.

Hard-X-Ray-Induced Multistep Ultrafast Dissociation

Creation of deep core holes with very short (τ≤1  fs) lifetimes triggers a chain of relaxation events leading to extensive nuclear dynamics on a few-femtosecond time scale. Here we demonstrate a general multistep ultrafast dissociation on an example of HCl following Cl 1s→σ^{*} excitation. Intermediate states with one or multiple holes in the shallower core electron shells are generated in the course of the decay cascades. The repulsive character and large gradients of the potential energy surfaces of these intermediates enable ultrafast fragmentation after the absorption of a hard x-ray photon.

Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3 and SrTiO3. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from strong Coulomb interactions between transition metal and oxygen ions. Such electronic correlations offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. Here we show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metal-oxygen bond. Using the perovskite nickelate (RNiO3) family as a template, we probe charge reconstruction at interfaces with gadolinium titanate GdTiO3. X-ray absorption spectroscopy shows that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate, exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence.

Electronic state-lifetime interference in resonant Auger spectra: a tool to disentangle overlapping core-excited states

Thanks to a new fit approach, electronic state-lifetime interference terms are extracted and used to disentangle overlapping states.

CeRu4Sn6: a strongly correlated material with nontrivial topology

Topological insulators form a novel state of matter that provides new opportunities to create unique quantum phenomena. While the materials used so far are based on semiconductors, recent theoretical studies predict that also strongly correlated systems can show non-trivial topological properties, thereby allowing even the emergence of surface phenomena that are not possible with topological band insulators. From a practical point of view, it is also expected that strong correlations will reduce the disturbing impact of defects or impurities, and at the same increase the Fermi velocities of the topological surface states. The challenge is now to discover such correlated materials. Here, using advanced x-ray spectroscopies in combination with band structure calculations, we infer that CeRu₄Sn₆ is a strongly correlated material with non-trivial topology.

Electronic Properties of BaFe2As2 upon Doping and Pressure: The Prominent Role of the As p Orbitals

Using high-resolution, lifetime removed, x-ray absorption spectroscopy at the As K edge, we evidence the strong sensitivity of the As electronic structure upon electron doping with Co or pressure change in BaFe2As2, at room temperature. Our results unravel the prominent role played by As-4p orbitals in the electronic properties of the Fe pnictide superconductors. We propose a unique picture to describe the overall effect of both external parameter doping and pressure, resolving the apparent contradiction between angle-resolved photoemission spectroscopy, transport, and absorption results, with the As-p states as a key ingredient.