Theory Of Resonant Inelastic X-ray Research Articles

Real Space Green's Function Approach to Resonant Inelastic X-Ray Scattering and HERFD XAS

We present a theory of resonant inelastic x-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) x-ray absorption spectra (XAS) based on the real space multiple scattering Green's function formalism. The Kramers-Heisenberg formula is transformed into an expression involving Single particle Green's functions. Simplifying assumptions give the RIXS cross section in terms of a convolution of an effective absorption spectrum with the x-ray emission spectrum. Given the RIXS cross section, calculations of the HERFD XAS are also possible. Self-energy effects are calculated using an efficient many-pole model. Further many-body effects can be treated via a convolution with an effective spectral function. The approach is implemented as an extension of the FEFF9 code. Calculated results are compared with experimental RIXS of WO2.

Theory of resonant inelastic x-ray scattering in iridium oxide compounds: Probing spin-orbit-entangled ground states and excitations

We determine how the elementary excitations of iridium-oxide materials, which are dominated by a strong relativistic spin-orbit coupling, appear in Resonant Inelastic X-ray Scattering (RIXS). Whereas the RIXS spectral weight at the L2 x-ray edge vanishes, we find it to be strong at the L3-edge. Applying this to Sr2IrO4, we observe that RIXS, besides being sensitive to local doublet to quartet transitions, meticulously maps out the strongly dispersive delocalized excitations of the low-lying spin-orbit doublets.

Theory of resonant inelastic X-ray scattering spectrum for Ni impurities in cuprates

We perform the numerically exact diagonalization calculation for small Cu–O clusters with a Ni impurity site, representing the Ni-substituted cuprate, to examine the single-particle excitation spectra as well as the resonant inelastic X-ray scattering (RIXS) spectra. We clarify relations between low-energy electronic structures near the Ni site and excitations seen in the RIXS spectra.

Theory of resonant inelastic x-ray scattering at theKedge inLa2CuO4: Multiple scattering effects

We develop a theory of resonant inelastic x-ray scattering (RIXS) at the $K$ edge in La$_2$CuO$_4$ on the basis of the Keldysh Green's function formalism. In our previous analysis (Phys. Rev. B 71, 035110 (2005)), the scattering by the core-hole potential was treated within the Born approximation, and a crude-model density of states was used for the $4p$ band. We improve the analysis by taking account of the multiple scattering in Cu3d-O$2p$ bands and by using a realistic $4p$ DOS obtained from a band calculation. The multiple scattering effect is evaluated with the use of the time representation developed by Nozi\`eres and De Dominicis. It is found that the multiple scattering effect makes the $K$-edge peak in the absorption coefficient shift to the lower energy region as a function of photon energy, that is, the photon energy required to excite the $1s$ electron to the $K$-edge peak reduces. It is also found that the multiple-scattering effect does not change the two-peak structure in the RIXS spectra but modifies slightly the shape as a function of energy loss. These findings suggests that the multiple scattering effect could mainly be included into a renormalization of the core-level energy and partly justify the Born approximation, leading to a future application to the RIXS in three-dimensional systems.

Theory of resonant inelastic X-ray scattering in cuprates

Abstract We analyze theoretically resonant inelastic X-ray scattering near the Cu absorption K edge in some cuprates (La 2 CuO 4 , Sr 2 CuO 3 , SrCuO 2 ). We explain the drastic momentum dependence of the spectra observed recently in experiments. The antiferromagnetic ground state is described by using the Hartree–Fock approximation, and scattering by the Cu1s core hole is treated within the Born approximation, and the electron correlation effects are taken into account within the random phase approximation, using the time-dependent perturbation theory. The main spectral weight originates predominantly from the charge transfer process. The electron correlations between Cu3d electrons are essential for the drastic momentum dependence.

Theory and simulation of resonant inelastic X-ray scattering in s–p bonded systems: graphite, hexagonal boron nitride, diamond, and cubic boron nitride

Abstract This work discusses the theory of resonant inelastic X-ray scattering (RIXS) in the s–p bonded solids, graphite, hexagonal boron nitride (hBN), diamond, and cubic boron nitride (cBN). In RIXS, core electrons are resonantly excited to unoccupied states, and X-ray emission (because of radiative core hole–valence electron recombination) is studied. In particular, evolution of the emission spectrum with increasing incident photon energy is of interest. This work begins with the independent-electron approach originally set forth by Ma and co-workers, which is based on the Kramers–Heisenberg formula for light scattering. However, simulations presented also examine effects of the inclusion of the attraction between the promoted electron and core hole in intermediate states. The results suggest that, while RIXS emission spectra can be influenced by that attraction, many features of the independent-electron picture are essentially preserved. The interplay between electron interactions, band-structure effects, and the effects being omitted is discussed.