Bremsstrahlung Isochromat Spectroscopy Research Articles

Electronic structure of Li-doped NiO.

The electronic structure of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$O has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and model-Hamiltonian cluster calculations. The measurements support the conclusion that the Li-doped holes have mainly oxygen character. The unoccupied electronic structure as probed with BIS shows broad structures growing in the gap. By comparing the XPS valence band to the results of the cluster calculations, we determine values for the Racah A parameter (=6.6 eV), charge transfer \ensuremath{\Delta} (=6.2 eV), and hybridization transfer integral (pd\ensuremath{\sigma}) (=1.3 eV). The first ionization state in the NiO valence-band cluster calculation is of $^{2}$${\mathit{E}}_{\mathit{g}}$ symmetry, a state in which the doped hole is antiferromagnetically coupled to the Ni spins. Hole-doped cluster calculations explain, because of a different fractional parentage upon annihilating a Ni 3d or an O 2p hole, the difference between the O 1s XAS results and the BIS results of heavily Li-doped NiO.

Electronic Band Structure and Unoccupied Density of Statesfor YbAs and Yb4As3

The one-electron energy band structure for YbAs and Yb 4 As 3 is calculated up to 1.5 Ry above the Fermi energy by a self-consistent augmented plane wave (APW) method with the local density approximation. The calculated unoccupied density of states is compared with the observed bremsstrahlung isochromat spectroscopy (BIS) spectra. In YbAs, two peaks appear at about 5.5 and 10 eV above the Fermi energy, in good agreement with the experimental result. These originate from the Yb 5 d and the Yb 5 d -As 4 p antibonding states, respectively. In Yb 4 As 3 , a broad peak originating from the Yb 5 d states combined with the antibonding state appears at about 5.5 eV above the Fermi energy. The discrepancy between calculated and observed peak positions is due to the valence fluctuation.

Angle dependence of matrix elements in X-ray bremsstrahlung isochromat spectroscopy

Transition matrix elements for bremsstrahlung isochromat spectroscopy (BIS) are investigated. Contrary to formulae presented before, they are found to be dependent on the angle between the incident electron beam and the emitted bremsstrahlung radiation. The effect of using angle-dependent BIS matrix elements is demonstrated for the case of Cu.

Electronic structure of CoO, Li-doped CoO, and LiCoO2.

The electronic structure of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{Co}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$O (0.01\ensuremath{\le}x\ensuremath{\le}0.2), ${\mathrm{LiCoO}}_{2}$, and ${\mathrm{Co}}_{3}$${\mathrm{O}}_{4}$(1% Li) has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and x-ray-absorption spectroscopy. The experimental results are compared with model cluster calculations. We find that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like. The first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character. Its local symmetry corresponds to $^{3}$${\mathit{T}}_{1\mathit{g}}$, similar to an intermediate-spin ${\mathrm{Co}}^{3+}$ state. For x\ensuremath{\le}0.2 the local Co electronic structure is similar to that of CoO. However, ${\mathrm{LiCoO}}_{2}$ has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a ${\mathrm{Co}}^{3+}$ low-spin ground state. ${\mathrm{LiCoO}}_{2}$ is an insulator with a gap of 2.7 eV. From a comparison of the XPS and BIS CoO spectra to the cluster calculations, we find values for U (=5.3 eV), \ensuremath{\Delta} (=5.5 eV), and (pd\ensuremath{\sigma}) (=1.3 eV).

Unoccupied density of states of La2-xSrxNiO4+ delta studied by polarization-dependent x-ray-absorption spectroscopy and bremsstrahlung isochromat spectroscopy.

Oxygen 1s x-ray-absorption measurements of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$ (0\ensuremath{\le}x\ensuremath{\le}1.15) are presented, together with O 1s polarization-dependent x-ray absorption on single-crystal ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{NiO}}_{4}$. It is concluded that the charge-compensating holes have mainly oxygen 2p character. The Ni 3${\mathit{d}}^{9}$ final states reached because of the p-d hybridization in the ground state are strongly polarized in the (a,b) plane. The Sr-induced O 2p holes are found to be distributed equally over in-plane ${\mathit{p}}_{\mathit{x}}$,${\mathit{p}}_{\mathit{y}}$ and out-of-plane ${\mathit{p}}_{\mathit{z}}$ orbitals. Possible explanations for this are presented. The intrinsic band gap of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$ is found to be at least 4 eV. The bremsstrahlung-isochromat-spectroscopy results show that the Sr-induced states are spread out over a large portion of the band gap and are not concentrated at the top of the valence band.

Empty states of the Ni/Si(111)7 × 7 interface

Up to now, the evolution of the empty states of the metal/silicon interface has received only little attention. It is however necessary to investigate both occupied and unoccupied states in order to get a complete picture of the interface electronic structure. We present a combined X-ray photoemission (XPS)/bremstrahlung isochromat spectroscopy (BIS) study of the Ni/Si(111)7 × 7 interface formation at room temperature. Valence and conduction band densities of states, as well as metal and Si core levels, were recorded for coverages (θ) ranging from 1 to 100 ML. Above 1 ML, the data reveal an intermixed phase. Its composition is close to Ni2Si at low coverage (θ = 3–6 ML), as testified by both XPS and BIS spectra. (For higher coverages, we found evidence for composition changes induced by the e-beam.) In the 1 ML range the situation seems to be more complex, with possible specific empty interface states.

Electronic structure of epitaxial β-FeSi2 on Si(111)

We have applied a combination of surface spectroscopy techniques (XPS, Auger, electron energy loss and bremsstrahlung isochromat spectroscopy) to investigate the chemical bonding of β-FeSi2 thin films epitaxially grown on the Si(111) surface. The local structure of the films has been investigated with the extended BIS fine structure technique. The semiconducting nature of the films is evidenced by a shift of the empty d-band to higher binding energies and by the lowering of the density of the states close to EF as compared to pure Fe films. The dielectric functions extracted from the electron energy loss spectra strongly support this picture. We measured an energy gap of 1.0 ± 0.2 eV which compares favourably with density of states calculations for the β-FeSi2 semiconducting phase.

Electron spectroscopy in the URh3Bx system.

High-resolution photoemission and bremsstrahlung-isochromat spectroscopy measurements have been carried out in the URh{sub 3}B{sub {ital x}} alloys. In this system, specific-heat and magnetic-susceptibility data suggest a progressive localization of 5{ital f} electrons with increasing boron content. Our photoemission experiments show that the 5{ital f} spectral function is nearly insensitive to the boron addition, in disagreement with existing band-structure calculations which predict a pronounced increase in the density of 5{ital f} states at the Fermi level between URh{sub 3} and URh{sub 3}B. We discuss these results in the framework of the Anderson impurity model developed for strongly correlated systems like cerium-based compounds. We show that multiplet effects strongly affect the 5{ital f} spectral function and could explain the spectroscopic experiments.

Investigations of transition-metal clusters deposited on solid substrates by photoelectron spectroscopy, bremsstrahlung isochromat spectroscopy and related techniques

The 2p core-level binding energies of Cu and Ni clusters increase while the kinetic energies of the LMM Auger lines decrease with decreasing cluster size. The shifts in the core-level binding energy and the Auger kinetic energy relative to the bulk metal are largest in the case of clusters deposited on an insulating substrate such as Al2O3 and least on amorphous carbon; shifts on TiO2 are intermediate between those on Al2O3 and carbon. The increase in core-level binding energies with decreasing cluster size signifies a metal to non-metal transition. The occurrence of such a transition is borne out more vividly by BIS studies. Accordingly, BIS studies on Pd clusters deposited on amorphous carbon clearly show that electron states are progressively introduced near the Fermi level with increase in cluster size. The chemical reactivity of transition-metal clusters varies with the size. Thus, the relative proportion of the atomic oxygen species increases with decreasing size of Ag clusters.

Reflection Valence- and Inner-Shell-Electrons Energy-Loss Spectra of SnS2and Momentum-Transfer Effects

The valence-electron and inner-shell-electron energy-loss spectra of SnS 2 are measured in reflection geometry. They are compared with the transmission electron-energy-loss spectroscopy (TEELS) spectra at various momentum transfers, the S K X-ray-absorption spectroscopy (XAS) spectrum, the reflectivity (R) spectrum, and the bremsstrahlung-isochromat spectroscopy (BIS) spectrum. The plasmon energy estimated at E 0 =1000 eV is 18.8 eV, which is equal to the one at the momentum transfer of 0.34 A -1 . The valence-electron REELS spectrum satisfies the dipole selection rules only to the first approximation. It seems that indirect and optically-forbidden transitions affect the energy-loss structures related to interband transitions. The inner-shell-electron REELS spectra are compatible to the XAS, the core R, and the BIS spectra except for the fact that optically-forbidden transitions are clearly observed in REELS when using low incident-energy beams.

Initial- versus final-state effects in the narrow-band spectra of heavy-fermion systems.

The {ital f}-electron spectral density of heavy-fermion systems, as observed by x-ray photoelectron spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS), is discussed in terms of initial- and final-state effects. The properties of the initial state, described by the periodic Anderson model, are obtained from the adiabatic perturbation theory, with the on-site correlation {ital U}{sub {ital f}{ital f}} taken as the expansion parameter. In the final state the transient effects following the sudden ionization of the system in XPS or BIS are discussed by using the time-dependent perturbation expansion. The overall spectral shapes, as would be observed experimentally, turn out to be given as a convolution of the adiabatic spectral function, which reflects the many-body interactions in the initial state, and the shape function, which accounts for the nonadiabatic effects following the destruction of the charge neutrality in XPS and BIS. Our treatment allows us to explain the position and the shape of the {ital f}-derived peaks in the XPS spectra of actinide intermetallics and to remove the conceptual difficulty in understanding the data acquired independently from thermodynamic (low-energy) and spectroscopic (high-energy) measurements of heavy fermions within a unified framework.

Bremsstrahlung-isochromat-spectroscopy and x-ray-photoelectron-spectroscopy investigation of the electronic structure of beta -FeSi2 and the Fe/Si(111) interface.

High-quality thin films of \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ have been obtained by annealing at 600 \ifmmode^\circ\else\textdegree\fi{}C Fe layers of thicknesses 50--100 \AA{} deposited in ultrahigh vacuum on clean Si(111) substrates. The semiconducting nature of the orthorhombic \ensuremath{\beta} phase has been checked by monitoring the empty states above ${\mathit{E}}_{\mathit{F}}$ with bremsstrahlung isochromat spectroscopy and the occupied states with x-ray-photoemission and Auger-electron spectroscopies. We have found that the \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ phase is semiconducting with a gap of about 1.0\ifmmode\pm\else\textpm\fi{}0.2 eV. Recent local-density-approximation calculations yielded two different values for the energy gap, i.e., 0.44 and 0.8 eV. Both calculations reproduce fairly well the experimental energy position of the valence states but only qualitatively that of the unoccupied states. The Fe/Si interface formation at room temperature has also been investigated. At very low coverages (\ensuremath{\Theta}=1--2 \AA{}) Fe and Si interdiffuse to form the \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$-like phase at the interface, while for increasing coverages (\ensuremath{\Theta}\ensuremath{\ge}3 \AA{}) the interfacial region is buried by the growth of a metallic Fe film in the form of clusters or discontinuous islands.

Photoemission spectroscopic study of copper-oxide superconductors and related materials

The electronic structure of high-Tc superconductors and related materials is studied by photoemission spectroscopy and Bremsstrahlung Isochromat spectroscopy. In the solid solution (La,Sr)2CuO4, (Nd,Ce)2CuO4, and (La,Sr)2NiO4 systems, the valence band spectra do not shift as a function of the doping concentration, which suggests that EF is pinned by the states within the charge-transfer gap and that the rigid band description is not appropriate in considering the metal-insulator transition in these systems.

Electronic structure of Bi2Sr2CaCu2O8 by photoemission and brehmsstrahlung isochromat spectroscopy

The electronic structure of Bi 2 Sr 2 CaCu 2 O 8 both above and below the Fermi level has been investigated by photoemission and Bremsstrahlung isochromat (BIS) spectroscopy. A state 2.2 eV above the Fermi level is assigned to the Cu 3d 10 band. A comparison with recent optical reflectivity studies shows that most interband transitions greater than ≈ 2 eV may be assigned to excitations within the Bi-O planes. Significant energy shifts are observed in the reflectivity data due to a final state effect.

Band-structure description of Mott insulators (NiO, MnO, FeO, CoO)

The unoccupied-states potential correction (USPC) in the local spin-density-functional (LSDF) formalism is proposed as a way to take correlation effects into account approximately. The method is based on using different potentials for the electrons in valence and conduction bands. When applied to antiferromagnetic transition-metal oxides (NiO, MnO, FeO, CoO), it solved the long-standing problem of reproducing the experimental values of the band gaps, which are underestimated in normal LSDF calculations. FeO and CoO become insulators, in contrast to the metallic ground state obtained in the usual LSDF calculations. Moreover, the calculated magnetic properties improved considerably due to the introduction of the USPC. It is shown that band-structure calculations using the USPC provide a correct description of available data from spectral measurements (ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, bremsstrahlung isochromate spectroscopy) for these compounds.

Palladium clusters on graphite: Evidence of resonant hybrid states in the valence and conduction bands.

The electronic structure of Pd clusters deposited on polycrystalline graphite has been investigated by x-ray photoemission, Auger spectroscopy, and bremsstrahlung isochromat spectroscopy. Initial- and final-state shifts of energy levels as a function of the average cluster size R are small, but important modifications of the density of filled and empty states at the Fermi level are observed. With decreasing particle size, the d portion of the conduction band broadens and shifts to higher energy while its valence counterpart shows the opposite shift. A simple analytical model is developed which accounts in some detail for the measured shifts and linewidths and also explains earlier x-ray-absorption data. The size dependences are explained in terms of mixing of Pd 4d and graphite * continua with a squared coupling constant V2 proportional to R-1. As a consequence a gap opens up across the Fermi level in the cluster density of states and unoccupied hybrid resonances arise as observed experimentally. Initial-state energy shifts and cluster charging are estimated and found to play a minor role for the present system.

Electron-spectroscopy study of YbXCu4 (X=Ag,Au,Pd).

We have studied the electronic structure of the heavy-electron compounds YbX${\mathrm{Cu}}_{4}$, with X =e/rAg,Au,Pd, using x-ray photoemission and bremsstrahlung isochromat spectroscopies. Consistent with other evidence of small Kondo temperatures, we find that Yb in this system is nearly trivalent and that Kondo features in the Yb 4f BIS spectra are either absent or weak.

Electronic structure of Ag2O.

The electronic structure of ${\mathrm{Ag}}_{2}$O has been investigated by photoelectron, Auger-electron, and bremsstrahlung isochromat spectroscopy. The experimental results are compared with model one-particle band-structure and two-particle cluster calculations. We extract values for the on-site Ag 4d and O 2p energies, the Ag 4d--O 2p transfer integrals, as well as the on-site Coulomb Ag 4d-4d and O 2p-2p interactions. A comparison with ${\mathrm{Cu}}_{2}$O and CuO is made, and the differences are discussed in view of high-${T}_{c}$ superconductivity and the instability of AgO.

Bremsstrahlung isochromat spectroscopy with spin-polarized electrons

We describe the construction and operation of a UHV system for bremsstrahlung isochromat spectroscopy in the x-ray regime (1463 eV) with spin-polarized electrons. The performance of the system is illustrated with measurements on Ni(110). The implications of this new technique to study magnetic properties through the spin-resolved electronic structure of the (near) surface are discussed.

Electron Spectroscopy of Heavy-Fermion Materials

The novel properties of heavy-fermion materials involve rare earth 4f electrons or actinide 5f electrons. For the 4f electrons the spectrum measured in photoemission and bremsstrahlung isochromat spectroscopies is dominated by single-site energetics and the impurity Anderson Hamiltonian gives at the semiquantitative level a unified description of the 4f spectrum and certain low energy properties characterized by a single-site Kondo temperature TK. Current efforts for 4f electron materials are directed at finding the quantitative limits of this impurity approach by detailed fitting of spectra using hybridization matrix elements calculated within the local density approximation (LDA). Examples presented herein are CeCu2Si2 and the α and γ phases of cerium metal. For the most commonly studied 5f case of uranium no such unified picture exists. Various current approaches to understanding uranium spectra include analysis and modeling of differences between measured spectra and LDA calculations, and purely experimental approaches such as alloy and dilution studies of UxY1-xPd3 and UxY1-xAl2.