X-ray Photoemission Experiments Research Articles

First Principles Studies on the Electronic Structure and Band Structure of Paraelectric SrTiO<sub>3</sub> by Different Approximations

The electronic structure, energy band structure, total density of states (DOS) and electronic density of perovskite SrTiO3 in the cubic phase are calculated by the using full potential-linearized augmented plane wave (FP-LAPW) method in the framework density functional theory (DFT) with the generalized gradient approximation (GGA) by WIEN2k package. The calculated band structure shows a direct band gap of 2.5 eV at the Γ point in the Brillouin zone.The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, I have conclude that the bonding between Sr and TiO2 is mainly ionic and that the TiO2 entities bond covalently.The calculated band structure and density of state of SrTiO3 are in good agreement with theoretical and experimental results.

Ab initio study of 5d-shells Ir substitution for Fe-based SmOFe1−xIrxAs

The lattice and electronic properties for 5d-shells Ir substituted Fe-based superconductor SmOFe1−xIrxAs (x=0,0.2,0.25,0.3) are investigated based on the density functional theory (DFT) with a spin generalized gradient approximation SGGA+U method. The electronic density of states (DOS) of SmOFe1−xIrxAs is studied and well compared with the results of experimental X-ray photoemission spectroscopy (XPS). The calculation indicated that iridium substitution at the Fe site induced a modification of the FeAs4 tetrahedron and suppressed the magnetic ordering corresponding to the Fe-3d, which may be the main cause of inducing superconductivity in Ir-doped SmOFeAs system.

Carbon in palladium catalysts: A metastable carbide

The catalytic activity of palladium toward selective hydrogenation of hydrocarbons depends on the partial pressure of hydrogen. It has been suggested that the reaction proceeds selectively toward partial hydrogenation only when a carbon-rich film is present at the metal surface. On the basis of first-principles simulations, we show that carbon can dissolve into the metal because graphite formation is delayed by the large critical nucleus necessary for graphite nucleation. A bulk carbide Pd(6)C with a hexagonal six-layer fcc-like supercell forms. The structure is characterized by core level shifts of 0.66-0.70 eV in the core states of Pd, in agreement with experimental x-ray photoemission spectra. Moreover, this phase traps bulk-dissolved hydrogen, suppressing the total hydrogenation reaction channel and fostering partial hydrogenation.

X-ray photoemission study in Re0.7Ca0.3MnO3 epitaxial films

We have performed the x-ray photoemission experiments in the colossal magnetoresistive manganites Re0.7Ca0.3MnO3 with Re=La, Pr, and Nd in order to study the Re dependence of electronic structures in this system. The samples are epitaxial films grown on LaAlO3 substrates with pulsed laser deposition system. It is found that the binding energies of Mn 2p3/2 are 641.7 and 643.1 eV for La0.7Ca0.3MnO3 and Nd0.7Ca0.3MnO3, respectively. The shift of this energy level is related to the change in bond angle and/or bond length between Mn and oxygen. Moreover, two lines of O 1s core level are assigned as Mn–O (530 eV) and Re–O (533 eV). The intensity ratio of these two lines in Nd0.7Ca0.3MnO3 is different from those in La0.7Ca0.3MnO3 and Pr0.7Ca0.3MnO3, suggesting that the ion redistribution between Re–O and Mn–O sites occurs with the Re substitution in these films.

Effect of Cr substitution on the electronic structure ofCuAl1−xCrxO2

The geometries and electronic structures of ${\text{CuAl}}_{1\ensuremath{-}x}{\text{Cr}}_{x}{\text{O}}_{2}$ have been investigated using density-functional theory with on-site corrections for strongly correlated systems $(\text{GGA}+U)$ for $x=0,\frac{1}{2},1$. Al is found to be well described within the ionic model, with a valence charge corresponding to a $+3$ oxidation state. Substituting Cr for Al is predicted to increase the density of states at the top of the valence band, in agreement with experimental x-ray photoemission spectroscopy data. Analysis of atom-projected densities of states and valence charges suggests that this is due to increased covalency between Cr and O; the valence charge for O in the Cu-O-(Al,Cr) subunits changes from $\ensuremath{-}1.74$ to $\ensuremath{\sim}\ensuremath{-}1.25$ when Cr replaces Al. This produces an indirect oxygen-mediated change to the $\text{Cu}\text{ }d$ states.

Etching enhanced annealing of GaMnAs layers

Low temperature annealing of GaMnAs layers has been established as a standard tool for improvement of their magnetic and transport properties. We report on a strong enhancement of the annealing efficiency achieved by chemical removal of the surface oxide layer during the annealing of GaMnAs layers. These data together with the results of angular resolved X-ray photoemission experiments are used for elucidation of processes during the annealing of GaMnAs layers.

Electronic structure and effects of dynamical electron correlation in ferromagnetic bcc Fe, fcc Ni, and antiferromagnetic NiO

We have constructed a $\text{LDA}+\text{DMFT}$ method in the framework of the iterative perturbation theory with local density approximation (full-LDA) Hamiltonian without mapping onto the effective Wannier orbitals. We then apply this $\text{LDA}+\text{DMFT}$ method to ferromagnetic bcc Fe and fcc Ni as a test of transition metal, and to antiferromagnetic NiO as an example of transition metal oxide. In Fe and Ni, the width of occupied $3d$ bands is narrower than those in LDA and Ni 6 eV satellite appears. In NiO, the resultant electronic structure is of charge-transfer insulator type and the band gap is 4.3 eV. These results are in good agreement with the experimental x-ray photoemission spectroscopy. The configuration mixing and dynamical correlation effects play a crucial role in these results.

Electronic spectrum of a helically Hund-coupled [formula omitted]- MnO 2

X-ray diffraction (XRD) and X-ray photoemission (XPS) were used to investigate a high-quality β - MnO 2 thin film epitaxially grown on oriented MgO(001) substrate by plasma-assisted molecular beam epitaxy (PAMBE). Electronic spectrum and energy gap of the β - MnO 2 were computed by local density approximation plus dynamical mean-field theory (LDA+DMFT) in Wannier function basis for a material-specific multiband Hubbard-like Hamiltonian. In the vicinity of Fermi surface, the shape of the calculated electronic spectrum is in good overall agreement with experimental XPS figure.

Phase Change in (GeTe)1-x(Sb2Te3)x Investigated by Hard X-ray Photoemission Spectroscopy

ABSTRACTThe mechanism of the fast reversible change between the amorphous and crystalline phases in (GeTe)1−x(Sb2Te3)x (GST) has not yet been fully understood. The crystalline phase has been identified as having a NaCl-type cubic structure with random occupation of the A sites by Ge, Sb and vacancies, and 100% occupation of B sites by Te. This fact calls our attention to a possible close relation to the inherent crystal bonding instability observed for the average five valence electrons (<5>) family. We present here the results of systematic hard X-ray photoemission experiments on GST films with various compositions in both the amorphous and crystalline phases, and discuss that a similar chemical bonding instability does indeed play an essential role in the phase change mechanism in GST. We propose a model for the fast phase change, in which 6 fold to 3 fold transition of p-like bonding play an essential role, in this class of materials.

Investigation of the electronic structure of the BiSBr and BiSeBr clusters by density functional method

The energy levels of valence bands (VB) of the BiSBr and BiSeBr crystals have been calculated for investigation of the photoelectron emission spectra of BiSBr, BiSeBr and BiSI crystals. The molecular model of this crystal has been used for the calculation of VB by the Density Functional Theory (DFT) method. The molecular cluster, consisting of 20 molecules of BiSBr, BiSeBr, has been used for calculations of averaged total density of states, including atom vibrations. The spectra of the averaged total density of states from VB of BiSBr and BiSeBr clusters have been compared with the experimental photoelectron emission spectra from VB of BiSI crystals. The results clarify that the atomic vibrations in A 5B 6C 7 type crystals with chain structure create a smoother appearance of the averaged total density of state spectrum and the experimental X-ray photoemission spectra (XPS).

Ab initiowave function-based methods for excited states in solids: Correlation corrections to the band structure of ionic oxides

Ab initio wave function-based methods are applied to the study of electron correlation effects on the band structure of oxide systems. We choose MgO as a prototype closed-shell ionic oxide. Our analysis is based on a local Hamiltonian approach and performed on finite fragments cut from the infinite solid. Localized Wannier functions and embedding potentials are obtained from prior periodic Hartree-Fock (HF) calculations. We investigate the role of various electron correlation effects in reducing the HF band gap and modifying the bandwidths. On-site and nearest-neighbor charge relaxation as well as long-range polarization effects are calculated. Whereas correlation effects are essential for computing accurate band gaps, we found that they produce smaller changes on the HF bandwidths, at least for this material. Surprisingly, a broadening effect is obtained for the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ valence bands. The ab initio data are in good agreement with the energy gap and bandwidth derived from thermoreflectance and x-ray photoemission experiments. The results show that the wave function-based approach applied here allows for well controlled approximations and a transparent identification of the microscopic processes which determine the electronic band structure.

Spectroscopic studies of nanocrystalline diamond materials

The structural and electronic properties of nanocrystalline diamond films synthesized by a modified hot-filament chemical vapour deposition process were investigated by both bulk- and surface-sensitive techniques. Diffraction and microscopy data show the films to consist of diamond grains with an average crystallite size of about 10–15 nm and a root-mean-square roughness of similar size. Carbon core-level excitations in transmission electron energy-loss spectroscopy reveal an sp 2 content below 5%. The low energy loss spectra are quite similar to that of diamond crystal. The high sp 3 content in the films was also confirmed by C 1 s photoelectron plasmon energy loss features in X-ray photoemission experiments and by X-ray excited Auger-electron spectroscopy. We find that the hydrogen covered diamond surface gets contaminated after storage for several months under ambient conditions. Heating up to 500 °C in vacuo is required to desorb the adsorbate layer.

Growth mode and molecular orientation of phthalocyanine molecules on metal single crystal substrates: A NEXAFS and XPS study

We report on near-edge X-ray absorption and X-ray photoemission experiments for the large π-conjugated organic molecule metal-free phthalocyanine (H 2Pc) on two different single crystal surfaces. Results from H 2Pc deposited on Ag(1 1 1) at 300 K show a clear linear dichroism indicating that the film grows as islands with the molecules lying essentially flat. On a Ni 3Al(1 1 1) and a cooled Ag(1 1 1) substrate, however, the molecules form a homogeneous film and have an average tilt angle of 45–60°. The different film structure is the result of a different molecule–substrate interaction.

Electronic structure and spectroscopic properties of thulium monochalcogenides

The electronic structure of the thulium monochalcogenides TmS, TmSe, and TmTe is studied with several theoretical approaches. The total energy is evaluated with the self-interaction corrected local-spin density approximation, whereby the Tm ions are described with either twelve or thirteen localized f electrons with the remaining electrons forming bands. The comparisons of these two scenarios reveal the valency shift of the Tm ion through the series. The spectral functions of TmX compounds are calculated including multiplet effects, and they are compared to experimental x-ray photoemission spectra. The basic tool is the Hubbard-I approximation in which the embedding of an isolated f(n) ion in a solid is performed by modifying the crystal Hamiltonian as obtained from the local-density approximation with the atomic self-energy of the ion. The parameters of the model are obtained from the self-consistent band structure calculation. The agreement with experiment is excellent, reproducing all significant multiplet structures.

Core-Level Shift of Si Nanocrystals Embedded in a SiO2 Matrix

It is expected from existing theories that the core level of Si nanocrystals (nc-Si) embedded in a SiO2 matrix should shift toward a higher binding energy as compared to that of bulk crystalline Si due to quantum size effect. Indeed, it is observed in X-ray photoemission experiments that the Si 2p core level shifts to a higher apparent binding energy by 1−2 eV for all five oxidation states of Sin+ (n = 0, 1, 2, 3, and 4) in the material system of SiO2 containing nc-Si. However, it is found that the core-level shift is due to a charging effect in the material system. After correction for the charging effect by using C 1s binding energy due to contamination on the SiO2 surface, the core level of the oxidation state Si4+ is the same as that of pure SiO2, whereas the core level of the isolated nc-Si with an average size of about 3 nm shifts by ∼ 0.6 eV to a lower binding energy as compared to that of bulk crystalline Si. It is suspected that the core-level shift of the nc-Si toward a lower binding energy is due to the influence of the differential charging between the SiO2 surface layer and the nc-Si underneath.

Bonding in skutterudites: Combined experimental and theoretical characterization ofCoSb3

We report on the bonding in skutterudites using the ${\mathrm{CoSb}}_{3}$ example. The theoretical electronic structure is calculated in both density-functional theory and empirical tight-binding frameworks. It is shown to be in good agreement with x-ray photoemission and photoabsorption experiments, and with ${}^{121}\mathrm{Sb}$ M\"ossbauer spectroscopy. It is then used to demonstrate that (i) the ${\mathrm{Sb}}_{4}$ rings are not the main structural features to analyze the highest valence-band behavior, (ii) the structure must be described in terms of corner-connected ${\mathrm{CoSb}}_{6}$ octahedra, which are tilted to form the Sb rings.

Electronic structure, chemical bonding, and optical properties of paraelectricBaTiO3

The electronic-energy band structure, site, and angular-momentum decomposed density of states (DOS) and charge-density contours of perovskite ${\mathrm{BaTiO}}_{3}$ in the paraelectric phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with the atomic-sphere approximation using density-functional theory in its local-density approximation. The calculated band structure shows a direct band gap of 1.2 eV at the \ensuremath{\Gamma} point in the Brillouin zone. The total DOS is compared to the experimental x-ray photoemission spectrum. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ba and ${\mathrm{TiO}}_{3}$ is mainly ionic and that the ${\mathrm{TiO}}_{3}$ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of ${\mathrm{BaTiO}}_{3}.$ The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient, absorption coefficient, and the electron energy-loss spectrum) are calculated. The calculated spectra are compared with the experimental results for ${\mathrm{BaTiO}}_{3}$ at room temperature in the ferroelectric phase and are found to be in good agreement with the experimental data in the low-energy regions. The role of band-structure calculation as regards the optical properties of ${\mathrm{BaTiO}}_{3}$ is discussed.

Structural and optical properties of paraelectric SrTiO3

The electronic energy band structure, site- and angular-momentum-decomposed densities of states (DOS) and charge-density contours of perovskite SrTiO3 in the paraelectric cubic phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows a direct band gap of ~1.4 eV at the gamma point in the Brillouin zone. The total DOS is compared with experimental x-ray photoemission spectra. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Sr and TiO3 is mainly ionic and that the TiO3 entities bond covalently. Using the projected DOS and band structure we have analysed the interband contribution to the optical properties of SrTiO3. The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient and absorption coefficient) and the electron energy-loss spectrum are calculated. The calculated spectra are compared with the experimental results for SrTiO3 in the cubic phase and are found to be in good agreement with the experimental results in low-energy regions. The role of band-structure calculation as regards the optical properties of SrTiO3 is discussed.

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2−xSnxO8+δ System

The Bi2Sr2CaCu2O8+δ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi2Sr2CaCu2O8+δ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.

The soft X-ray spectroscopy beamline at the LNLS: technical description and commissioning results

The soft X-ray spectroscopy beamline installed at a bending-magnet source in the LNLS is presented. A technical description of the main elements is given and some selected commissioning results are shown. The beamline optics was designed to cover the soft X-ray energy range from 790 up to 4000 eV. The bending-magnet source has a critical energy of 2.08 keV and delivers ∼1012 photons s−1 mradH−1 (0.1% bandwidth)−1 (100 mA)−1. The focusing element is a gold-coated toroidal mirror operating at an angle of incidence of 1°. The double-crystal monochromator has three pairs of crystals which can be selected by a lateral translation. The UHV experimental station is equipped with an ion gun, an electron gun, a LEED optics and an electron analyser. The beamline is intended for X-ray absorption, photoemission, reflectivity and dichroism experiments. The beamline has been installed, commissioned, and is now open to the external users community.