Query: core hole effect

title: Ultrafast X Ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect.
abstract: In recent years, ultrafast pump probe spectroscopy has provided insightful information about the nonequilibrium dynamics of excitations in materials. In a typical experiment of time resolved x ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x ray probe pulse after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x ray absorption spectroscopy in one dimensional strongly correlated systems. The core hole created by the x ray is modeled as an additional effective potential of the core hole site, which changes the spectrum qualitatively. In equilibrium, the spectrum reveals the charge gap at half filling and the metal insulator transition in the presence of the core hole effect. Furthermore, a pump probe scheme is introduced to drive the system out of equilibrium before the x ray probe. The effects of the pump pulse with varying frequencies, shapes, and fluences are discussed for the dynamics of strongly correlated systems in and out of resonance. The spectrum indicates that the driven insulating state has a metallic droplet around the core hole. The rich structures of the nonequilibrium x ray absorption spectrum give more insight into the dynamics of electronic structures.

title: Title Core hole effect on dipolar and quadrupolar transitions of SrTiO3 and BaTiO3 at TiK edge

title: Influence of the core hole effect on optical properties of magnesium oxide (MgO) near the Mg L edge region.
abstract: The influence of the core hole effect on optical properties of magnesium oxide (MgO) is established through experimental determination of optical constants and first principles density functional theory studies. Optical constants (d and b) of MgO thin film are measured in the spectral region 40 300 eV using reflectance spectroscopy techniques at the Indus 1 synchrotron radiation source. The obtained optical constants show strong core exciton features near the Mg L edge region, causing significant mismatch with Henke's tabulated values. On comparing the experimentally obtained optical constants with Henke's tabulated values, an edge shift of ~3.0 eV is also observed. Distinct evidence of effects of core exciton on optical constants (d and b) in the near Mg L edge absorption spectra are confirmed through first principles simulations.

title: CORE HOLE EFFECT IN THE Ce L3 X RAY ABSORPTION SPECTRA OF CeO2 AND CeFe2: NEW EXAMINATION BY USING RESONANT X RAY EMISSION SPECTROSCOPY
abstract: We consider two different resonant X ray emission spectra for Ce compounds: Ce 3d to 2p X ray emission (denoted by 3d RXES) and valence to 2p X ray emission (v RXES) both of which follow the Ce 2p to 5d resonant excitation. We propose that the comparison of the 3d and v RXES spectra is a new powerful method of directly detecting the core hole effect in the final state of Ce L3 X ray absorption spectra (XAS) We applied this method to recent experimental RXES spectra for CeO2 and CeFe2, and showed unambiguously that the core hole effect should be essential in the XAS of both materials. This result is confirmed by theoretical calculations, which reproduce well the experimental RXES and XAS spectra. We conclude that the ground state of CeO2 is in the mixed state of 4f0 and configurations, where is a ligand hole, instead of a pure 4f0 configuration which was proposed recently by first principles energy band calculations. Also, we conclude that the double peaks observed in L3 XAS of CeFe2 are caused by the 4f0 and 4f1 configurations, which are mixed in the ground state but separated in energy by the large core hole potential in the final state of XAS.

title: Core hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure
abstract: The authors investigated theoretically core hole effects on X ray absorption near edge structures (XANES) of Np and Am LIII in neptunium dioxide (NpO2) and americium dioxide (AmO2) with CaF2 type crystal lattices using the all electron full potential linearized augmented plane wave (FP LAPW) method. The peak creation mechanism of XANES was shown by examining the electronic structures of these oxides, which indicated that core hole screening was more marked for AmO2 than for NpO2 because of the difference in the charge transfer between these oxides. Furthermore, the results of charge density analysis suggested that the white line was assigned to the quasi bound state composed of the localized Np d or Am d components and O components, and that the tail structure was created as a result of delocalized standing waves between the Np or Am atoms.

title: Core hole effect on topological band order in cubic semiconductors: A first principles study
abstract: High energy x ray beams can excite the inner shell electrons out of the sample and simultaneously leave behind core holes. Using first principles calculations, we investigate the core hole effect on the topological band order in cubic semiconductors Ge and InSb. The band orders around the Fermi level are significantly changed by the presence of core holes, and consequently a topological phase transition from trivial to nontrivial states occurs with the increasing of the density of the core hole. Our work reveals an underlying relation between the core hole effect and the topological band order. It also provides a novel method for engineering three dimensional topological insulators by optical means.

title: Oxygen 1s excitation and tetragonal distortion from core hole effect in BaTiO3
abstract: The accurate description of the O 1s excitation in BaTiO3 has been elusive so far. In this Rapid Communication, the electronic structure and the high resolution electron energy loss near edge structures of the O K edge in tetragonal BaTiO3 are investigated using first principles calculations. The results demonstrate a clear correlation between the broadening of the lower energy fine structure and the anisotropic effects induced by the core hole potential, which are directly related to the structural distortion occurring in tetragonal BaTiO3. Moreover, we show that a significant improvement of the description of higher lying structures can be obtained when correcting the energy position of the Ba 4f states. This demonstrates that the O 1s spectrum can be a very effective probe of the distortion and changes in the local electronic structure, and be used as a sensitive tool for studying new materials and ferroelectric transitions.

title: A new method of directly determining the core hole effect in the Ce L3 XAS of mixed valence Ce compounds An application of resonant X ray emission spectroscopy
abstract: Using a simplified model, we calculate resonant X ray emission (RXE) spectra with Ce 2p to 5d excitation and Ce nl to 2p de excitation for mixed valence Ce compounds, where nl is 3d or valence stat

title: Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene
abstract: Carbon K edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS) indicating significant core hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core hole effect, which affects both transition energy and intensity. During the course of this analysis, the core hole effect in benzene was also analyzed. It was found that the magnitude of the core hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final stat.

title: Core hole effect on dipolar and quadrupolar transitions of SrTiO3 and BaTiO3 at Ti K edge
abstract: Experimental pre edge fine structures in the x ray absorption spectra of \mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3} and \mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3} at Ti $K$ edge are successfully reproduced within the framework of first principles one electron theory using the full potential augmented plane wave plus local orbitals method. The importance of the following two points has been demonstrated: (1) proper inclusion of a Ti $1s$ core hole using a large supercell, and (2) consideration of both dipolar and quadrupolar transitions. The core hole effect is found to be more significant for quadrupolar transitions that form the first two peaks of the pre edge structure, i.e. $p1$ to $p2$ They correspond to {t}_{2g} and {e}_{g} bands of the core holed Ti. The other two peaks, $p3$ and $p4$ are found to be entirely due to the dipolar transition.