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J. -P. Rueff

Publications and source records attributed to J. -P. Rueff.

At least 19 recordsLinked to original sources
arXiv2026

Pressure-induced electronic and structural evolution of EuIrGe3

We investigated the pressure-induced evolution of the electronic and crystal structure of the noncentrosymmetric BaNiSn3-type antiferromagnet EuIrGe3 using x-ray absorption spectroscopy, synchrotron x-ray diffraction complemented by density functional theory calculations, and electrical resistivity measurements. The Eu L3-edge spectra reveal a continuous increase in the mean Eu valence under compression, accompanied by modifications of the Ge and Ir electronic states. High- pressure x-ray diffraction shows anisotropic lattice compression in the tetragonal (I4mm) phase and provides evidence for a structural phase transition above 38 GPa. The experimentally deter- mined lattice and equation-of-state parameters are in good agreement with the DFT calculations. Electrical resistivity measurements reveal a monotonic increase in the antiferromagnetic ordering temperatures up to 18 GPa, indicating that the antiferromagnetic ground state remains robust despite the increasing contribution of the nonmagnetic Eu3+ configuration to the intermediate va- lence state. These results demonstrate that EuIrGe3 exhibits a pressure response distinct from EuCoGe3 and EuRhGe3, highlighting the important role of the transition metal d-electron states in the pressure-induced electronic and structural evolution of the EuT Ge3 family

cond-mat.str-el↗
arXiv2026

Structural and magnetic phases of topological kagome metal Fe$_3$Sn$_2$ under pressure

We investigate the pressure-induced evolution of crystal structure and magnetism in the kagome ferromagnet Fe$_3$Sn$_2$ by combining X-ray diffraction, X-ray Emission Spectroscopy, X-ray Magnetic Circular Dichroism, and spin-polarized density functional theory calculations. X-ray diffraction reveals a structural phase transition above $\sim$20~GPa, which coincides with a pronounced reduction of the local Fe magnetic moment evidenced by X-ray emission spectroscopy, indicating a high-spin to low-spin transition. While XES probes the amplitude of the local moment, XMCD provides direct information on the orientation of the ordered magnetic moments and uncovers a rich pressure--temperature magnetic phase diagram. At room temperature, a collinear ferromagnetic phase with moments aligned along the $c$ axis persists up to the structural transition. At low temperature, a tilted magnetic configuration remains stable to significantly higher pressures, while at intermediate temperatures pressure stabilizes the low-temperature magnetic phase at the expense of the high-temperature one. Spin-polarized first-principles calculations show that, although isotropic ferromagnetic exchange interactions remain robust under compression, pressure enhances spin--orbit--driven magnetic anisotropy and Dzyaloshinskii--Moriya interactions, favoring non-collinear magnetic configurations. Our results demonstrate that pressure reshapes the magnetic energy landscape of Fe$_3$Sn$_2$ by coupling lattice, spin state, and relativistic magnetic interactions, establishing hydrostatic pressure as an effective control parameter to engineer magnetic anisotropy and potentially topological phases in kagome materials.

cond-mat.str-el↗
arXiv2026

Anomalous pressure dependence of the bulk modulus and Yb valence in cubic YbPd

We investigate the Yb valence instabilities in the strongly correlated YbPd compound using resonant X-ray emission spectroscopy as a function of pressure across the charge-order (CO) transition. At a low temperature (T = 30 K) in the CO phase, the Yb $4f$ valence remains nearly constant up to a pressure P$_L$ = 1.5 $\pm {0.2}$ GPa, and then increases gradually at higher pressures. In contrast, at room temperature in the normal phase, an anomalous decrease of the Yb $4f$ valence is observed, without any accompanying structural phase transition. This behavior is corroborated by a systematic pressure-dependent decrease of the unit-cell volume. Based on a Birch-Murnaghan analysis, the compressibility indicates hardening of the lattice with applied pressure up to a distinct kink seen at P$_K$ = 1.6 $\pm {0.2}$ GPa. In contrast, for P $>$ P$_K$, the Yb $4f$ valence saturates and the compressibility reveals a counterintuitive pressure-induced softening. The results show a minimum in the compressibility of YbPd (with $f^{0}$-$f^{1}$ hole-type mixed-valence), reminiscent of the maximum in compressibility seen in the $γ$-$α$ first-order isostructural phase transition in cerium (with $f^{0}$-$f^{1}$ electron-type mixed-valence).

cond-mat.str-el↗
arXiv2025

Surface Plasmon-Enhanced X-ray Ultraviolet Nonlinear Interactions

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.

physics.optics↗
arXiv2024

Identification by Inelastic X-Ray scattering of bulk alteration of solid dynamics due to Liquid Wetting

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 microns 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.

cond-mat.mes-hall↗
arXiv2023

Pressure evolution of electronic and crystal structure of non-centrosymmetric EuCoGe$_3$

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.

cond-mat.str-el↗
arXiv2019

Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction

The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically, and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, non-destructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV energy range to probe the structure of an epitaxially-grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO (001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 $\pm$ 4 angstrom, with minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and x-ray reflectivity measurements. Through the STEM measurements we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 angstrom. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few angstrom precision.

cond-mat.mtrl-sci↗
arXiv2018

Interface chemical and electronic properties of LaAlO3/SrVO3 heterostructures

We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect.

cond-mat.mtrl-sci↗
arXiv2018

The Galaxies Beamline at SOLEIL Synchrotron: Inelastic X-ray Scattering and Photoelectron Spectroscopy in the Hard X-ray Range

The GALAXIES beamline at the SOLEIL synchrotron is dedicated to inelastic x-ray scattering (IXS) and photoelectron spectroscopy (HAXPES) in the 2.3-12 keV hard x-ray range. These two techniques offer powerful, complementary methods of characterization of materials with bulk sensitivity, chemical and orbital selectivity, resonant enhancement and high resolving power. After a description of the beamline components and endstations, we address the beamline performances through a selection of recent works both in the solid and gas phases and using either IXS or HAXPES approaches. Prospects for studies on liquids are discussed.

physics.ins-det↗
arXiv2018

Intermediate valence in single crystalline Yb$_2$Si$_2$Al

Yb$_2$Si$_2$Al may be a prototype for exploring different aspects of the Shastry-Sutherland lattice, formed by planes of orthogonally coupled Yb ions. Measurements of the magnetic susceptibility find incoherently fluctuating Yb$^{3+}$ moments coexisting with a weakly correlated metallic state that is confirmed by measurements of the electrical resistivity. Increasing signs of Kondo coherence are found with decreasing temperature, including an enhanced Sommerfeld coefficient and Kadowaki-Woods ratio that signal that the metallic state found at the lowest temperatures is a Fermi liquid where correlations have become significantly stronger. A pronounced peak in the electronic and magnetic specific heat indicates that the coupling of the Yb moments to the conduction electrons leads to an effective Kondo temperature that is approximately 30 K. The valence of Yb$_2$Si$_2$Al has been investigated with electron spectroscopy methods. Yb$_2$Si$_2$Al is found to be strongly intermediate valent ($v_F=2.68(2)$ at 80 K). Taken together, these experimental data are consistent with a scenario where a coherent Kondo lattice forms in Yb$_2$Si$_2$Al from an incoherently fluctuating ensemble of Yb moments with incomplete Kondo compensation, and strong intermediate valence character.

cond-mat.str-el↗
arXiv2018

Emergent high-spin state above 7 GPa in superconducting FeSe

The local electronic and magnetic properties of superconducting FeSe have been investigated by K$β$ x-ray emission (XES) and simultaneous x-ray absorption spectroscopy (XAS) at the Fe K-edge at high pressure and low temperature. Our results indicate a sluggish decrease of the local Fe spin moment under pressure up to 7~GPa, in line with previous reports, followed by a sudden increase at higher pressure which has been hitherto unobserved. The magnetic surge is preceded by an abrupt change of the Fe local structure as observed by the decrease of the XAS pre-edge region intensity and corroborated by ab-initio simulations. This pressure corresponds to a structural transition, previously detected by x-ray diffraction, from the $Cmma$ form to the denser $Pbnm$ form with octahedral coordination of iron. Finally, the near-edge region of the XAS spectra shows a change before this transition at 5~GPa, corresponding well with the onset pressure of the previously observed enhancement of $T_c$. Our results emphasize the delicate interplay between structural, magnetic, and superconducting properties in FeSe under pressure.

cond-mat.supr-con↗
arXiv2017

Revisiting the origin of satellites in core level photoemission of transparent conducting oxides: the case of $n$-doped SnO$_2$

The longstanding problem of interpretation of satellite structures in core level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO$_2$. Comparison of {\it ab initio} many-body calculations with experimental hard X-ray photoemission spectra of the Sn 4$d$ states shows that strong satellites are produced by coupling of the Sn core hole to the plasma oscillations of the free electrons introduced by doping. Within the same theoretical framework, spectral changes of the valence band spectra are also related to dynamical screening effects. These results demonstrate that, for the interpretation of electron correlation features in the core level photoelectron spectra of such narrow-band materials, going beyond the homogeneous electron gas electron-plasmon coupling model is essential.

cond-mat.str-el↗
arXiv2017

Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator

The electric field control of functional properties is a crucial goal in oxide-based electronics. Non-volatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab-initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects.

cond-mat.mtrl-sci↗
arXiv2017

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 correlations between transition metal and oxygen ions. Strong correlations thus 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 metaloxygen 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.

cond-mat.str-el↗
arXiv2017

The structure and evolution of semiconducting buffer graphene grown on SiC(0001)

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing waves analysis we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26\% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is the likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature.

cond-mat.mtrl-sci↗
arXiv2016

Ultrafast evolution and transient phases of the prototype out-of-equilibrium Mott-Hubbard material V2O3

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behavior, including non-thermal phases and photoinduced phase transitions. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states of matter inaccessible by quasi-adiabatic pathways. Here we present a study of the ultrafast non-equilibrium evolution of the prototype Mott-Hubbard material V2O3, which presents a transient non-thermal phase developing immediately after photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a marked hardening of the A1g coherent phonon. This configuration is in stark contrast with the thermally accessible ones - the A1g phonon frequency actually softens when heating the material. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are of particular relevance for the optical manipulation of strongly correlated systems, whose electronic and structural properties are often strongly intertwinned.

cond-mat.str-el↗
arXiv2016

Magnetovolume effect, macroscopic hysteresis and moment collapse in the paramagnetic state of cubic MnGe under pressure

Itinerant magnets generally exhibit pressure induced transitions towards non magnetic states. Using synchrotron based X-ray diffraction and emission spectroscopy, the evolution of the lattice and spin moment in the chiral magnet MnGe was investigated in the paramagnetic state and under pressures up to 38 GPa. The collapse of spin-moment takes place in two steps. A first-order transition with a huge hysteresis around 7 GPa transforms the system from the high-spin at ambient pressure to a low-spin state. The coexistence of spin-states and observation of history-depending irreversibility is explained as effect of long-range elastic strains mediated by magnetovolume coupling. Only in a second transition, at about 23 GPa, the spin-moment collapses.

cond-mat.mtrl-sci↗