Resonant Inelastic X-ray Scattering Process Research Articles

On the origin of low-valent uranium oxidation state

The significant interest in actinide bonding has recently focused on novel compounds with exotic oxidation states. However, the difficulty in obtaining relevant high-quality experimental data, particularly for low-valent actinide compounds, prevents a deeper understanding of 5f systems. Here we show X-ray absorption near-edge structure (XANES) measurements in the high-energy resolution fluorescence detection (HERFD) mode at the uranium M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two series exhibit clear differences, which we explain using electronic structure calculations of the 3d-4f resonant inelastic X-ray scattering (RIXS) process. To understand the changes observed, we implemented crystal field models with ab initio derived parameters and investigated the effect of reducing different contributions to the electron-electron interactions involved in the RIXS process. Our analysis shows that the electron-electron interactions weaken as the ligand changes from I to F, indicative of a decrease in ionicity both along and between the UX3 and UX4 halide series.

Time-resolved RIXS experiment with pulse-by-pulse parallel readout data collection using X-ray free electron laser

Time-resolved resonant inelastic X-ray scattering (RIXS) is one of the developing techniques enabled by the advent of X-ray free electron laser (FEL). It is important to evaluate how the FEL jitter, which is inherent in the self-amplified spontaneous emission process, influences the RIXS measurement. Here, we use a microchannel plate (MCP) based Timepix soft X-ray detector to conduct a time-resolved RIXS measurement at the Ti L3-edge on a charge-density-wave material TiSe2. The fast parallel Timepix readout and single photon sensitivity enable pulse-by-pulse data acquisition and analysis. Due to the FEL jitter, low detection efficiency of spectrometer, and low quantum yield of RIXS process, we find that less than 2% of the X-ray FEL pulses produce signals, preventing acquiring sufficient data statistics while maintaining temporal and energy resolution in this measurement. These limitations can be mitigated by using future X-ray FELs with high repetition rates, approaching MHz such as the European XFEL in Germany and LCLS-II in the USA, as well as by utilizing advanced detectors, such as the prototype used in this study.

Observing spin fractionalization in the Kitaev spin liquid via temperature evolution of indirect resonant inelastic x-ray scattering

Motivated by the ongoing effort to search for high-resolution signatures of quantum spin liquids, we investigate the temperature dependence of the indirect resonant inelastic x-ray scattering (RIXS) response for the Kitaev honeycomb model. We find that, as a result of spin fractionalization, the RIXS response changes qualitatively at two well-separated temperature scales, $T_L$ and $T_H$, which correspond to the characteristic energies of the two kinds of fractionalized excitations, $\mathbb{Z}_2$ gauge fluxes and Majorana fermions, respectively. While thermally excited $\mathbb{Z}_2$ gauge fluxes at temperature $T_L$ lead to a general broadening and softening of the response, the thermal proliferation of Majorana fermions at temperature $T_H \sim 10 \, T_L$ results in a significant shift of the spectral weight, both in terms of energy and momentum. Due to its exclusively indirect nature, the RIXS process we consider gives rise to a universal magnetic response and, from an experimental perspective, it directly corresponds to the $K$-edge of Ru$^{3+}$ in the Kitaev candidate material $\alpha$-RuCl$_3$.

Probing inter- and intrachain Zhang-Rice excitons inLi2CuO2and determining their binding energy

Cuprate materials, such as those hosting high-temperature superconductivity, represent a famous class of materials where the correlations between the strongly entangled charges and spins produce complex phase diagrams. Several years ago, the Zhang-Rice singlet was proposed as a natural quasiparticle in hole-doped cuprates. The occurrence and binding energy of this quasiparticle, consisting of a pair of bound holes with antiparallel spins on the same ${\mathrm{CuO}}_{4}$ plaquette, depends on the local electronic interactions, which are fundamental quantities for understanding the physics of the cuprates. Here, we employ state-of-the-art resonant inelastic x-ray scattering (RIXS) to probe the correlated physics of the ${\mathrm{CuO}}_{4}$ plaquettes in the quasi-one-dimensional chain cuprate ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$. By tuning the incoming photon energy to the O $K$ edge, we populate bound states related to the Zhang-Rice quasiparticles in the RIXS process. Both intra- and interchain Zhang-Rice singlets are observed and their occurrence is shown to depend on the nearest-neighbor spin-spin correlations, which are readily probed in this experiment. We also extract the binding energy of the Zhang-Rice singlet and identify the Zhang-Rice triplet excitation in the RIXS spectra.

Electron Dynamics in the Core-ExcitedCS2Molecule Revealed through Resonant Inelastic X-Ray Scattering Spectroscopy

We present an experimental and theoretical study of resonant inelastic x-ray scattering (RIXS) in the carbon disulphide CS 2 molecule near the sulfur K-absorption edge. We observe a strong evolution of the RIXS spectral profile with the excitation energy tuned below the lowest unoccupied molecular orbital (LUMO) absorption resonance. The reason for this is twofold. Reducing the photon energy in the vicinity of the LUMO absorption resonance leads to a relative suppression of the LUMO contribution with respect to the emission signal from the higher unoccupied molecular orbitals, which results in the modulation of the total RIXS profile. At even larger negative photon-energy detuning from the resonance, the excitation-energy dependence of the RIXS profile is dominated by the onset of electron dynamics triggered by a coherent excitation of multiple electronic states. Furthermore, our study demonstrates that in the hard x-ray regime, localization of the S 1s core hole occurs in CS 2 during the RIXS process because of the orientational dephasing of interference between the waves scattering on the two sulfur atoms. Core-hole localization leads to violation of the symmetry selection rules for the electron transitions observed in the spectra.

Enhanced charge excitations in electron-doped cuprates by resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) tuned for the Cu L edge is a possible tool to detect charge excitations in cuprate superconductors. We theoretically investigate the possibility for observing a collective charge excitation by the RIXS. The RIXS process via the intermediate state inevitably makes the spectral weight of charge excitation stronger in electron doping than in hole doping. Electron-hole asymmetry also appears in the dynamical charge structure factor, showing a new enhanced small-momentum low-energy mode in electron doping. These facts indicate a possibility of detecting the new charge mode by RIXS in electron-doped systems.

ANALYSIS OF CRYSTAL-FIELD MULTIPLETS OF V3+ ION IN PEROVSKITE OXIDES FOR RESONANT INELASTIC X-RAY SCATTERING SPECTROSCOPY

Spectral properties related to the low-energy d–d excitations in nearly cubic vanadium perovskites are examined for the expected L-edge resonant inelastic X-ray scattering (RIXS) measurements. Multiplet structure of V 3+ ion in the basic [Formula: see text] electron configuration is calculated with the help of ab initio quantum-chemical method applied to VO 6 cluster embedded in a crystalline environment. In the intermediate states of the RIXS process the induced 2p-core hole is taken into account by including strong 2p–3d electron interactions. A symmetry-group approach is applied to generate the basis set of many-electron wavefunctions of the intermediate core-hole states accessible in RIXS processes. Although a comprehensive description of the core-hole multiplets still remains a formidable task and requires using specially designed numerical codes, for particular resonant states the analysis is simplified and the calculation of RIXS amplitude can be carried out analytically to the end.

Significance of the inter-site Coulomb interaction between the O 2p and Cu 3d holes revealed by resonant inelastic x-ray scattering of Sr14Cu24O41

We have carried out resonant inelastic x-ray scattering (RIXS) measurements at the Cu K-edge on Sr14Cu24O41, which has two kinds of layers, namely, the ladder layer and the chain layer. The excitation from the Zhang–Rice singlet band to the upper Hubbard band (UHB) shows a large dispersion with a periodicity of the ladder layer. The excitation corresponding to the ligand-to-metal charge transfer is nearly dispersionless. Comparison between the RIXS data and theoretical calculations shows that the inter-site Coulomb interaction between the Cu 3d and O 2p holes plays a prominent role in the RIXS process. This interaction is expected to be important in the hole state in the ladder layer of this compound.

Observations on the resonant inelastic x-ray scattering cross section in copper oxide compounds

We report properties of the resonant inelastic x-ray scattering (RIXS) cross section in a number of cuprates. We find that the observed spectra have the periodicity of the underlying crystal lattice, that is, the excitations are a function of reduced momentum $\mathbf{q}$ and not the total momentum $\mathbf{Q}=\mathbf{G}+\mathbf{q}$, where $\mathbf{G}$ is a reciprocal lattice vector. Measurements with two different incident photon polarizations show that the line shape and momentum dependence of the observed spectra are independent of the particular state the $4p$ photoelectron resides in the intermediate state. These latter results support theoretical approaches in which the $4p$ is taken to be a spectator in the RIXS process. In addition, a discussion is presented of what might be expected to be generic features of the RIXS cross section with the aid of some crude models, and a comparison is made between RIXS and electron energy loss data for a number of cuprates both at $\mathbf{q}=0$ and at finite $\mathbf{q}$. Taken together, there is a suggestion in the data that RIXS measures an $S(\mathbf{q},\ensuremath{\omega})$-like response.

Bulk electronic structure ofK3C60as revealed by soft x-rays

We present $\mathrm{C}\phantom{\rule{0.2em}{0ex}}1s$ x-ray absorption, x-ray emission, and resonant inelastic x-ray scattering (RIXS) spectra of single-phase crystalline ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$. The comparison to valence-band photoelectron spectra from the same sample facilitates identification of the contribution from surface and bulk electronic states in the latter. Bulk-sensitive techniques show that the valence bands of ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ and pure ${\mathrm{C}}_{60}$ are characterized by spectral features of similar width, in agreement with the predictions of band-structure calculations. Symmetry selectivity in the RIXS process allows us to assign peaks in the $\mathrm{C}\phantom{\rule{0.2em}{0ex}}1s$ absorption spectrum, demonstrating a close correspondence with pure ${\mathrm{C}}_{60}$ also in the conduction band. The symmetry selectivity is as pronounced in ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ as in pure ${\mathrm{C}}_{60}$, indicating that the local ${\mathrm{C}}_{60}$ symmetry is not appreciably affected by the K doping, either in the ground state or intermediate state, on the time scale of $6\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$.

Resonant inelastic x-ray scattering ofMnO:L2,3edge measurements and assessment of their interpretation

The resonant inelastic x-ray scattering (RIXS) of $\mathrm{MnO}$ measured with high energy resolution across the ${\mathrm{L}}_{2,3}$ absorption edges of Mn is characterized by a very rich spectral structure due to the local electronic excitations. The spectra are dominated by dd and charge transfer excitations, both dipole allowed in the RIXS process. The spectra strongly depend on the energy and polarization of the incident photons. This vast experimental basis allows an accurate determination of the main parameters of theoretical models used to describe highly correlated electron systems like $\mathrm{MnO}$. We show the results for the single impurity Anderson model and the single ion crystal field model and we compare them. Both models reproduce well the dd excitation spectrum, but the former can also predict satisfactorily the charge transfer excitations.

Theory of Cu L-edge resonant inelastic X-ray scattering in insulating cuprates

Abstract We examine the momentum dependence of the Cu L -edge resonant inelastic X-ray scattering (RIXS) in insulating cuprates by using the numerically exact diagonalization technique on finite cluster systems. The two-band Hubbard model which has the orbitals with x 2 – y 2 and 3 z 2 – r 2 symmetries is adopted. We find that the charge transfer excitation spectrum has the large momentum dependence compared with d–d excitations, and there also appear additional charge excitation spectra characteristic of the Cu L -edge RIXS process.

Crystal-momentum-resolved electronic structure of solids using resonant soft-X-ray fluorescence spectroscopy

Resonant inelastic X-ray scattering (RIXS) has been observed in graphite and hexagonal boron nitride (hBN) above and below their K edges. Below the core threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation goes above the core binding energy, nonlinear dispersive effects are observed in graphite but not in hBN. We show that these two effects, which have previously been thought of as separate processes, i.e. resonant X-ray Raman scattering (below threshold) and RIXS (above threshold), are in fact described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. The role core-excitons play in the RIXS process is examined in finer detail, by using narrow-band excitation. Our results indicate that core-hole effects play a minor role in the RIXS observed from graphite but are more pronounced in hBN.

Soft-X-ray fluorescence studies of solids

Abstract Resonant inelastic X-ray scattering (RIXS) has been observed in many systems above and below their core threshold. Below threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation increases above the core binding energy, nonlinear dispersive effects are observed. These two effects are described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. However, significant questions have arisen concerning core-hole effects and their influence on RIXS. In this work, the role core-excitons play in the RIXS process is examined in finer detail in graphite, by using narrow-band excitation, and through comparison between experiment and simulated spectra which include the core-hole effects explicitly in the modeling. Based on these findings, we conclude that core-hole effects play a minor but detectable role in the RIXS observed from graphite.