X ray matter interactions are intrinsically weak, and the high energy and momentum of X rays pose significant challenges to applying strong light matter coupling techniques that are highly effective at longer wavelengths for controlling and manipulating radiation. Techniques such as enhanced coupling between light and electrons at a metal dielectric interface or within nanostructures, as well as the Purcell effect where spontaneous emission is amplified near a metallic surface are not applicable to X rays due to their fundamentally different energy and momentum scales. Here we present a novel approach for coupling X rays to surface plasmon polaritons by entangling X ray photons with SPPs in the ultraviolet range through X ray to UV spontaneous parametric down conversion in aluminum. The distinct characteristics of the SPPs are imprinted onto the angular and energy dependence of the detected X ray photons, as demonstrated in this work. Our results highlight the potential to control X rays using SPPs, unlocking exciting opportunities to enhance X ray matter interactions and explore plasmonic phenomena with atomic scale resolution a capability uniquely enabled by X rays.

Synchrotron radiation facilities provide highly polarized x-ray beams across a wide energy range. However, the exact type and degree of polarization vary according to the beamline and experimental setup. To accurately determine the angle and degree of linear polarization, a portable x-ray polarimeter has been developed. This setup consists of a silicon drift detector that rotates around a target made of high-density polyethylene. The imprint generated in the angular distribution of scattered photons from the target at a 90-degree angle between the incident x-rays and detector has been exploited to determine the beam polarization. Measurements were conducted at the GALAXIES beamline of the SOLEIL synchrotron. The expected angular distribution of the scattered photons for a given beam polarization was obtained through simulations using the Geant4 simulation toolkit. An excellent agreement between simulations and the collected data has been obtained, validating the setup and enabling a precise determination of the beam polarization.

We report on the pressure evolution of the electronic and crystal structures of the noncentrosymmetric antiferromagnet EuCoGe3. Using a diamond anvil cell, we performed high pressure fluorescence detected near-edge x-ray absorption spectroscopy at the Eu L3, Co K, and Ge K edges and synchrotron powder x-ray diffraction. In the Eu L3 spectrum, both divalent and trivalent Eu peaks are observed from the lowest pressure measurement (~2 GPa). By increasing pressure, the relative intensity of the trivalent Eu peak increases, and an average Eu valence continuously increases from 2.2 at 2 GPa to 2.31 at~50 GPa. On the other hand, no discernible changes are observed in the Co K and Ge K spectra as a function of pressure. With the increase in pressure, lattice parameters continuously decrease without changing I4mm symmetry. Our study revealed a robust divalent Eu state and an unchanged crystal symmetry of EuCoGe3 against pressure.

We report on the pressure evolution of the electronic and crystal structures of the noncentrosymmetric antiferromagnet EuCoGe3. Using a diamond anvil cell, we performed high pressure fluorescence detected near-edge x-ray absorption spectroscopy at the Eu L3, Co K, and Ge K edges and synchrotron powder x-ray diffraction. In the Eu L3 spectrum, both divalent and trivalent Eu peaks are observed from the lowest pressure measurement (similar to 2 GPa). By increasing pressure, the relative intensity of the trivalent Eu peak increases, and an average Eu valence continuously increases from 2.2 at 2 GPa to 2.31 at similar to 50 GPa. On the other hand, no discernible changes are observed in the Co K and Ge K spectra as a function of pressure. With the increase in pressure, lattice parameters continuously decrease without changing I4mm symmetry. Our study revealed a robust divalent Eu state and an unchanged crystal symmetry of EuCoGe3 against pressure.

We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred mu m distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface.

Texture-induced magnetic anisotropy is realized in iron oxide thin films deposited at room temperature onto Si(1 0 0) substrates using reactive magnetron sputtering of a Fe2O3 target. Diversities in morphology and magnetic properties are evidenced by electron microscopy and vibrating sample magnetometry. Additional insight provided by X-ray absorption spectroscopy and X-ray magnetic circular dichroism reveals fundamental differences in chemical composition and element selective magnetic properties between the films deposited in the presence of Ar and N2 reactive gas. Taking advantage of hard X-ray magnetometry, substrate-free magnetization profiles of the films were derived and deconvoluted into major magnetic components. In addition to ferromagnetic components with anisotropy defined by texture a significant contribution of (super)paramagnetic components due to rich interface structure of these nanocomposite films is observed.

We characterize experimentally and theoretically the high-energy dielectric screening properties of the prototypical correlated metal SrVO$_3$. The dynamical structure factor measured by inelastic x-ray scattering spectroscopy as a function of momentum transfer is in very good agreement with first-principles calculations in the adiabatic local density approximation to time-dependent density-functional theory. Our results reveal the crucial importance of crystal local fields in the charge response function of correlated materials: They lead to depolarization effects for localised excitations and couple spectra from different Brillouin zones.

High-energy-resolution fluorescence-detected (HERFD) near-edge x-ray-absorption fine-structure measurements performed at the Pb and W ${L}_{3}$ absorption edges have been used to study the pressure dependence of the local atomic structure of $\mathrm{PbW}{\mathrm{O}}_{4}$ from 1 bar up to 22 GPa. Comparison of measured spectra with density-functional theory structural calculations and simulations based on solution of the Bethe-Salpeter equation establish an unambiguous $\mathrm{PbW}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{I}$ to $\mathrm{PbW}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{III}$ phase transition around 7 GPa. No evidence of either the Raspite or Fergusonite structures is found. The smaller core-hole lifetime broadening afforded by HERFD allows unique experimental determination of crystal-field splitting and its sensitivity to the oxygen cage geometry surrounding the Pb and W atoms thus giving definitive identification of the high-pressure phase of $\mathrm{PbW}{\mathrm{O}}_{4}$.

Among europium compounds, pressure induced valence transitions and/or intermediate valence states are often observed. In such systems, applying pressure of several GPa can drive a Eu valence from divalent to almost trivalent. Non-centrosymmetric EuRhGe3 possesses magnetic Eu2+ ions and exhibits antiferromagnetic ordering at ∼11 K at ambient pressure. Pressure resistant magnetic ordering and stable divalent Eu state have been reported in EuRhGe3. Here, we study the pressure evolution of the Eu valence of EuRhGe3 by high resolution x-ray absorption spectroscopy using the partial fluorescence yield method. Our study reveals a successive increase of the Eu valence with increasing pressure without any valence transition. The obtained mean Eu valence approaches ∼2.4 around 40 GPa at 300 K. The experimental data are also analyzed by a full multiplet configuration interaction calculation based on the single impurity Anderson model. The analysis reveals a decrease of the Eu 4f orbital occupation by applying pressure. Pressure evolution of the electronic structure studied by density functional theory suggests that the Rh ions have little contribution to the pressure evolution of the Eu valence, while it implies an active involvement of the Ge ions.

A multiedge study of the local structure of lithium borate glasses and melts has been carried out using X-ray Raman scattering (XRS) as a function of temperature. Thanks to a wide range of compositions, from pure B2O3 up to the metaborate composition, we are able to finely interpret the modifications of the local environment of both the boron and oxygen atoms in terms of boron coordination number, formation of nonbridging oxygens (NBOs), and polymerization degree of the borate framework as a function of temperature and composition. A temperature-induced [4]B to [3]B conversion is observed above the glass transition temperature (Tg) from the glass to the melt from the triborate composition up to the metaborate composition. Two distinct melt structures are reported: a well-polymerized borate network—with few NBOs—below the triborate composition and a depolymerized borate network above the diborate composition with a rapid increase of the number of NBOs when Li2O is added. These two structurally distinct melts allow explaining the two dynamic regimes observed for lithium ion diffusion.

We present a temperature and doping dependent resonant inelastic X-ray scattering experiment at the V L$_{2,3}$ and O K edges in La$_{1-x}$Sr$_x$VO$_3$ for $x=0$ and $x=0.1$. This material is a canonical example of a compound that exhibits a filling control metal-insulator transition and undergoes orbital ordering and antiferromagnetic transitions at low temperature. Temperature dependent measurements at the V L$_{3}$ edge reveal an intra-t$_{2g}$ excitation that blueshifts by 40 meV from room temperature to 30 K at a rate that differs between the para- and antiferromagnetic phases. The lineshape can be partially explained by a purely local model using crystal field theory calculations. At $x=0.1$ the doping is shown to affect the local electronic structure primarily on the O sites, which is in disagreement with a simple Mott-Hubbard picture. We reveal the presence of phonon overtone features at the O K edge, which evidences that the low energy part of the spectrum is dominated by phonon response.

Negative thermal expansion is an unusual phenomenon appearing in only a handful of materials, but pursuit and mastery of the phenomenon holds great promise for applications across disciplines and industries. Here we report use of x-ray spectroscopy and diffraction to investigate the 4f-electronic properties in Y-doped SmS and employ the Kondo volume collapse model to interpret the results. Our measurements reveal an unparalleled decrease of the bulk Sm valence by over 20% at low temperatures in the mixed-valent golden phase, which we show is caused by a strong coupling between an emergent Kondo lattice state and a large isotropic volume change. The amplitude and temperature range of the negative thermal expansion appear strongly dependent on the Y concentration and the associated chemical disorder, providing control over the observed effect. This finding opens avenues for the design of Kondo lattice materials with tunable, giant, and isotropic negative thermal expansion.

The first colour photographs were created by a process introduced by Edmond Becquerel in 1848. The nature of these photochromatic images colours motivated a debate between scientists during the XIXth century, which is still not settled. We present the results of chemical analysis (EDX, HAXPES and EXAFS) and morphology studies (SEM, STEM) aiming at explaining the optical properties of the photochromatic images (UV-visible spectroscopy and low loss EELS). We rule out the two hypotheses (pigment and interferences) that have prevailed since 1848, respectively based on variations in the oxidation degree of the compound forming the sensitized layer and periodically spaced photolytic silver planes. A study of the silver nanoparticles dispersions contained in the coloured layers showed specific localizations and sizes distributions of the nanoparticles for each colour. These results allow us to formulate a plasmonic hypothesis on the origin of the photochromatic images colours.

Edmond Becquerel invented in 1848 the first colour photographic process. The "photochromatic images" he produced raised several questions, among which the photochromic nature of the sensitized layer. Here, we present the first characterization of the sensitized layer created at the surface of a silver plate or a silver foil according to Becquerel's process, which XIX$^{\text{th}}$ century scientists called "silver photochloride". It is constituted by silver nanoparticles dispersed in a micrometric silver chloride grains matrix. It is thus similar to the widely studied Ag/AgCl composites, which suggests new synthesis routes for these photocatalysts. The chemical composition of the sensitized layer has been identified by complementary spectroscopies (EDX, XPS, HAXPES and EXAFS), while its morphology has been studied by electron microscopies (SEM and STEM). These techniques involve X-ray or electron beams, which can have an impact on the silver chloride-based sensitized layer; a large part of this article hence introduces a study of the beam effects and of the way of reducing them.

While Fe-N-C materials are a promising alternative to platinum for catalyzing oxygen reduction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe-N-C catalysts initially comprising two distinct FeNx sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure-activity correlations drawn from end-of-test 57Fe Mössbauer spectroscopy reveal that both sites initially contribute to the ORR activity but only S2 significantly contributes after 50 h of operation. From in situ 57Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeNx moieties in different electronic states, we identify S1 to be a high-spin FeN4C12 moiety and S2 a low- or intermediate spin FeN4C10 moiety. These insights lay the ground for rational approaches towards Fe-N-C cathodes with improved durability in acidic fuel cells.

Electron-core hole interactions are critical for proper interpretation of core-level spectroscopies commonly used as analytical tools in materials science. Here we utilize resonant Auger-electron spectroscopy to uniquely identify exciton, shake, and charge-transfer processes that result from the sudden creation of the core hole in both x-ray-absorption and photoemission spectra. These effects are captured for the transition-metal compounds SrTiO3 and MoS2 by fully ab initio, combined real-time cumulant, and Bethe-Salpeter equation approaches to account for core hole dynamics and screening. Atomic charges and excited-state electron-density fluctuations reflect materials' solid-state electronic structure, loss of translational symmetry around the core hole, and breakdown of the sudden approximation. They also demonstrate competition between long- and short-range screening in a solid.

Detecting and quantifying non-bridging oxygen (NBO) atoms is of particular interest for understanding the physical properties of melts or compressed materials, and requires an unequivocal spectral signature usable during in situ measurements. In this paper, we evidence a low-energy feature of NBO in lithium silicate crystals using X-ray Raman scattering (XRS) spectroscopy around the energy losses of the oxygen K-edge. A specific peak at 534-535 eV in the edge onset is unequivocally attributed to the presence of NBO. Its intensity is used to quantify NBO in lithium silicate glasses. A similar feature at low energy has also been evidenced in Na2SiO3 and MgSiO3, generalizing the method to other alkali and alkali-earth silicates. This non-destructive method of NBO quantification, which is based on an X-ray inelastic scattering technique, can be extended to other spectroscopies such as electron-energy loss spectroscopy, and soft X-ray absorption spectroscopy at the oxygen K edge.