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Abstract

The electronic structure of the heavy fermion compound CeB6 is probed by resonant inelastic soft X-ray scattering using photon energies across the Ce 3d and 4d absorption edges. The hybridization between the localized 4f orbitals and the delocalized valence-band states is studied by identifying the different spectral contributions from inelastic Raman scattering and normal fluorescence. Pronounced energy-loss structures are observed below the elastic peak at both the 3d and 4d thresholds. The origin and character of the inelastic scattering structures are discussed in terms of charge-transfer excitations in connection to the dipole allowed transitions with 4f character. Calculations within the single impurity Anderson model with full multiplet effects are found to yield consistent spectral functions to the experimental data.

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... Composition, structural properties, and microstructure of the investigated borides were thus directly connected to their mechanical properties. Hexaborides are formed by configuration interaction of 4f narrow-band states of heavy-fermion elements La and Ce, LaB6 [45] and CeB6 [46] with charge-transfer to B. These materials exhibit a variety of interesting properties, e.g., high melting points, resistance to cathode poisoning and the lowest known work function ~2.5 eV, useful as electron emitters in electron microscopes, microwave tubes, electron lithography, electron beam welding, x-ray tubes, and free electron lasers [47]. In 2015, Andrievski [48] reviewed the Group 4 TMB2 both as bulk and thin films, focusing on their synthesis and the resulting microstructure and mechanical properties. ...
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We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.
... The energy resolution at the Zr 1s edge of the beamline monochromator was 0.5 eV. The X-ray absorption spectra were recorded in reflection mode by detecting the fluorescence yield [21], using a passivated implanted planar silicon (PIPS) detector from 150 eV below to 1200 eV above the Zr 1s absorption edge energy (E0=17.993 keV) with 0.5 eV energy steps. ...
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The chemical bonding in an epitaxial ZrB2 film is investigated by Zr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies and compared to the ZrB2 compound target from which the film was synthesized as well as a bulk {\alpha}-Zr reference. Quantitative analysis of X-ray Photoelectron Spectroscopy spectra reveals at the surface: ~5% O in the epitaxial ZrB2 film, ~19% O in the ZrB2 compound target and ~22% O in the bulk {\alpha}-Zr reference after completed sputter cleaning. For the ZrB2 compound target, X-ray diffraction (XRD) shows weak but visible -111, 111, and 220 peaks from monoclinic ZrO2 together with peaks from ZrB2 and where the intensity distribution for the ZrB2 peaks show a randomly oriented target material. For the bulk {\alpha}-Zr reference no peaks from any crystalline oxide were visible in the diffractogram recorded from the 0001-oriented metal. The Zr K-edge absorption from the two ZrB2 samples demonstrate more pronounced oscillations for the epitaxial ZrB2 film than in the bulk ZrB2 attributed to the high atomic ordering within the columns of the film. The XANES exhibits no pre-peak due to lack of p-d hybridization in ZrB2, but with a chemical shift towards higher energy of 4 eV in the film and 6 eV for the bulk compared to {\alpha}-Zr (17.993 keV) from the charge-transfer from Zr to B. The 2 eV larger shift in bulk ZrB2 material suggests higher oxygen content than in the epitaxial film, which is supported by XPS. In EXAFS, the modelled cell-edge in ZrB2 is slightly smaller in the thin film (a=3.165 {\AA}, c=3.520 {\AA}) in comparison to the bulk target material (a=3.175 {\AA}, c=3.540 {\AA}) while in hexagonal closest-packed metal ({\alpha}-phase, a=3.254 {\AA}, c=5.147 {\AA}).
Article
Resonant Lα emission spectra of Ce compounds, CeF3, CeO2 and CeB6, were measured around the LIII absorption threshold. As the energy of the incident photon is tuned at the pre-edge region and at the absorption peaks of the Ce LIII absorption spectrum, the Raman peaks are resonantly enhanced. Obtained spectra are decomposed into Raman and normal Lα emission peaks by line shape analysis. The results show that the Raman spectra provide more detailed information than the absorption spectra about the 4f configuration of Ce compounds.
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High resolution X-ray absorption near-edge structures (XANES) of rare earth hexaboride (LaB6 and CeB6) single crystals have been studied at the regions of the B K-edge and the La and Ce N5,4-edge. The difference in the occupancy of 4f electrons between the ground state of La (4d104f0) and Ce (4d104f1) causes different features in the La and Ce N5,4-edge XANES (giant resonance), while B K-edge XANES exhibit similar features for the two materials indicating that the influence of the 4f electron is confined in the vicinity of the lanthanide ion. Theoretical simulations based on density functional theory using the WIEN2k code have been conducted to compare with the XANES at B K-edge. The results and their implications are discussed.
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The valence shell electron configurations within a few electron volts above the Fermi level in cerium, ytterbium, europium and samarium compounds were probed by resonant X-ray emission spectroscopy (RXES) at the L3 absorption pre-edge. The rare earth systems show distinct spectral signatures depending on the f-electron configuration. The high energy resolution experimental results reported here are well reproduced by atomic multiplet calculations confirming the localized character of the 4f electrons. The magnitude of the electron–electron interactions within the 4f shell and between 3d and 4f electrons is analyzed. The present technique is a powerful tool for the study of the 4f valence electron configuration that, unlike L3 absorption spectroscopy at the main edge, is little influenced by valence electron relaxation following core hole creation.
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Theoretical predictions are given for low energy excitations, such as crystal field excitations and Kondo resonance excitations, to be detected by high-resolution measurements of resonant inelastic x-ray scattering (RIXS) of rare-earth materials with Yb compounds as typical examples. Crystal field excitations in the Yb 3d RIXS of a Yb3+ ion in the cubic crystal field are formulated, and the calculation of RIXS spectra for YbN is done. Kondo resonance excitations revealed in the Yb 3d RIXS spectra are calculated for mixed-valence Yb compounds, Yb1-xLuxAl3, in the leading term approximation of the 1/Nf expansion method with a single impurity Anderson model. It is emphasized that the high-resolution RIXS with polarization dependence is a powerful tool to study the crystal field levels together with their symmetry and also the Kondo bound state in rare-earth compounds. Some in-depth discussions are given on the polarization effects of RIXS, including 4d and 2p RIXS spectra, the coherence effect of the Kondo bound states, and the importance of the high-resolution RIXS spectra for condensed matter physics under extreme conditions.
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The electronic structure and chemical bonding of the recently discovered inverse perovskite Sc3AlN, in comparison to those of ScN and Sc metal, have been investigated by bulk-sensitive soft-x-ray emission spectroscopy. The measured Sc L, N K, Al L1, and Al L2,3 emission spectra are compared with calculated spectra using first-principles density-functional theory including dipole transition-matrix elements. The main Sc 3d–N 2p and Sc 3d–Al 3p chemical bond regions are identified at −4 and −1.4 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states in the Al L2,3 emission from Sc3AlN in comparison to that for pure Al metal is found, which reflects the Sc 3d–Al 3p hybridization observed in the Al L1 emission. The differences between the electronic structures of Sc3AlN, ScN, and Sc metal are discussed in relation to the change in the conductivity and elastic properties.
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We report a resonant inelastic x-ray scattering (RIXS) study of crystalline CeB(6). Ce L(α1,2) RIXS was measured with excitation energies resonant with the Ce L(3)-edge. A lifetime-broadening suppressed x-ray absorption near-edge structure (LBS-XANES), which successfully reproduced the L(α1,2) RIXS spectra over wide ranges of excitation and emission energies, was simulated using the SIM-RIXS program. A pre-edge structure in the LBS-XANES can be resolved, and many-body effects were suggested in the L(α1,2) RIXS around the Ce L(3)-edge energy. No convincing signs of Ce (II) or Ce (IV) states were observed in the LBS-XANES. Ce L(γ4) RIXS was measured at 302 K and 28 K with excitation energies across the Ce L(1)-edge. The interactions of p-valence electrons between Ce and B(6) were found to be considerably small, regardless of temperature. Thus, the electronic state of CeB(6) was concluded to be suitably described as a nominally Ce(4f(1))(3+)(e(-))(B(6))(2-) system with some hybridization among all valence orbitals of Ce and B.
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Electron-energy-loss spectra of sputtered clean cerium metal and of cerium dioxide were measured in the valence-band region and in the vicinities of the 4d, 4p, 4s, 3d, and 3p excitation thresholds. The energies of the primary electrons ranged from 80 to 3000 eV. Bulk and surface plasmons were identified in the valence-band region of the metal. For the dioxide, the surface plasmon vanished and "plasmonlike" peaks were observed. Also, an intra-conduction-band transition which appears in the metal and is highly sensitive to the chemical state of the surface was found. Core-level excitations were identified as transitions to the 4f levels located above, within, and below the conduction band. Transitions of the 4d electrons displayed "giant resonances" due to the higher multiplet splittings of the final states. The intensities of some of the excitation lines were enhanced at low incident electron energies due to the contribution of higher multipole transitions. The core-level excitations were also found in the oxide. Chemical shifts of some of the oxide peaks relative to those of the metal were observed, especially for transitions into final states located above the metallic conduction band.
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X-ray fluorescence spectroscopy with monochromatic photon excitation is presented as a tool for studies of charge-transfer excitations in correlated systems, using CeO2 and UO3 as examples. Ce 4f-->3d and U 5f-->3d x-ray fluorescence, with excitation near the 3d thresholds, probes states as eigenvalues for the ground state Hamiltonian from the Anderson impurity model. Sweeping the excitation energy across 3d absorption edges enhances contributions of different electronic configurations to fluorescence so that observed resonances indicate the charge-transfer origin of the absorption satellites.
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Photoemission studies of the γ-α phase transition in cerium show changes in two 4f-related features in the valence band (0.3 and 2 eV below EF) and broadening of the multiplet structure in the region of the 4d-4f transitions. We show that the observed features can be explained by increased hybridization of the 4f wave function upon entering the α phase.
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X-ray emission spectroscopy (XES) is undergoing remarkable progress with high brightness synchrotron radiation sources. Here I discuss some theoretical aspects of XES in rare earth compounds and transition metal compounds. It is to be noted that XES is formulated as a coherent second order quantum process, in contrast to the fact that X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are first-order optical processes, because the X-ray emission follows the preceding X-ray excitation of a core electron. When the incident X-ray photon energy, Ω, is well above the threshold of the core electron excitation, the XES corresponds to the normal XES (NXES), but when Ω is near the threshold, it is called the resonant XES (RXES). The final states of the first-order processes, XPS and XAS, are the same as the intermediate states of the second-order processes, NXES and RXES, respectively. I show some examples of RXES, where the atomic multiplet structure, the charge transfer effect and the crystal field level excitation play important roles. The calculated results are compared with recent experimental data, and it is discussed what information on the electronic states can be obtained from the analysis of XES data.
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Electron energy loss spectroscopy in reflection mode has been used to probe low energy excitations (ΔE < 20 eV) at the surface of clean polycrystalline rare earth metals, the first comprehensive study through the range of elements La to Lu. From Eu to Lu there are well defined bulk plasmon excitations of the valence electrons at energies close to free electron plasmon energies i.e. assuming two free electrons per atom for divalent Eu and Yb (7 < ΔE < 9 eV), and three per atom for the rest (11 < ΔE < 14 eV). La, Ce, Pr and Nd have more complex structure between 7 and 12 eV with peaks whose relative intensities are sensitive to both incident energy and exposure to very small doses of oxygen, indicating that for these elements surface plasmon excitation may be separated from bulk excitation. On the low energy side of the plasmon region there is always a sharp feature (2.5 < ΔE < 4 eV), which is particularly intense at low primary energies and for the higher members of the rare earth series. This excitation has no direct parallel in the optical studies of rare earths. The peak intensity is very sensitive to oxygen exposure, and preliminary studies on single crystal Ce(001) give some support for interpreting it as an intrinsic one-electron surface excitation.
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A review is presented of theoretical studies on core-level spectroscopy in solids. In core level spectra, the dynamical response of outer electrons to the core hole is reflected through the screening of core hole potential. Impurity Anderson model (IAM) or cluster model is successfuly applied to the analysis of X-ray photoemission spectra (XPS), X-ray absorption spectra (XAS) and X-ray emission spectra (XES) in f and d electron systems, where the f and d electron states are hybridized with the other valence or conduction electron states. The effect of a core-hole potential plays an important role in these spectra, as well as the hybridization and intra-atomic multiplet coupling effects. Some typical examples for core-level spectra in rare-earth compounds and transition-metal compounds are shown with some discussions.
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We analyze fluorescence yield spectraof rare earth oxides at 3d threshold regionby means of ionic model with full multiplet effects.The calculation is based on the second-order optical formula,and the effect of the polarization of incident and emitted X-raysis taken into account.The calculated results are in satisfactory agreement with recentexperimental data.The difference between fluorescence yield spectra and 3d X-ray absorptionspectra is explained.
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Electronic structures of rare-earth hexaborides (RB6)have been investigated by photoemission and inverse photoemissionspectroscopy. The X-ray photoemission (XPS) features of the 3d and 4dcore states of heavy R elements are interpreted in comparison with themultiplet calculation. The valence band spectra probed by XPS andX-ray bremsstrahlung isochromat spectroscopy (X-BIS) methods are fullyutilized to interpret the optical conductivity spectra.
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Ce 3d XPS and 4d-4f resonant photoemission spectra were measured and adopted to study the electronic structure of CeB6. Observed spectra were analyzed in terms of the single impurity Anderson model, which manifests the dense Kondo behavior of the electronic structure of CeB6.The low energy excitations which appear in photoemission spectra were discussed based on the 4d-4f resonant photoemission spectra measured with an energy resolution of 78 meV.
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Photoelectron spectra of CeB6 and PrB6 have been measured over the excitation-energy range from 30 to 140 eV. From the locations of the peaks which grow resonantly upon excitation in the giant absorption bands above the 4d thresholds of rare earths, the binding energies of 4f electrons in resonance are found to be 2.5 eV in CeB6 and 4.7 eV in PrB6. The shapes of energy distribution curves observed by excitation below resonance agree with the reported energy distribution curve of LaB6 and with the density-of-states curve calculated on LaB6 by the Xalpha-APW method. A sharp peak is resolved at the Fermi edge of CeB6. Its origin is discussed in relation to valence fluctuation.
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A Comment on the Letter by M. Garnier et al., Phys. Rev. Lett. 78, 4127 (1997). The authors of the Letter offer a Reply.
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We give an overview of the use of the impurity Anderson Hamiltonian to describe the spectroscopic and low-energy thermodynamic properties of cerium intermetallics, with emphasis on interpreting 4f photoemission spectra. We show Ce valence-band resonant photoemission, Bremsstrahlung isochromat and 3d X-ray photoemission spectra for CeRu2, CeNi2, CeIr2 and CeAl, and give a complete theoretical analysis of the spectra. We summarize the relation between the large and small energy scale properties. For each system, all the spectra, as well as the static magnetic susceptibility and Kondo temperature, can be described by the model using essentially the same parameters. We also present details of resonant photoemission spectra for five other cerium compounds, CeSi2, CeOs2, CePd3, CeCo2 and CeNi5, and discuss generally the problem of obtaining the experimental 4f spectrum. Alternative theories of 4f photoemission are examined critically and we give applications of the Anderson Hamiltonian theory to CeAl2, PrAl2, and NdAl2.
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This book looks at the physics of electronic fluctuations (noise) in solids. The author emphasizes many fundamental experiments that have become classics: physical mechanisms of fluctuations, and the nature and magnitude of noise. He also includes the most comprehensive and complete review of flicker (1/f) noise in the literature. It will be useful to graduate students and researchers in physics and electronic engineering, and especially those carrying out research in the fields of noise phenomena and highly sensitive electronic devices--detectors, electronic devices for low-noise amplifiers, and quantum magnetometers (SQUIDS).
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Photoemission spectra measured with an energy resolution of 5 meV from Ce-based heavy fermion materials are presented. The spectra add strong support to the interpretation of the peak at the Fermi level in terms of the spectral function calculated within the single impurity model. This work clearly demonstrates that if reliable information on heavy fermion compounds is to be obtained using photoemission spectroscopy, it is necessary to work with an instrumental resolution approaching kTK, where TK is the Kondo temperature.
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The N4,5 emission spectra of LaB6 and CeB6 were measured to investigate the radiative decay of the 4d94fn+1 excited states, where n is the number of 4f electrons in the ground state. These spectra show an intense and broad emission peak above the N4,5 threshold in addition to sharp peaks below the threshold. The energy positions of these emission peaks are compared with those of the peaks in the absorption spectra and the existing results of resonant photoemission. The comparison shows that the fine structure below the threshold coincides in energy with the multiplet structures in the absorption spectrum, and that the intense emission peak is located in the energy region where the electron-emission peak due to the direct recombination of the 4f electron with the 4d hole has been observed in resonant photoemission. The peaks observed in the energy region below and above the N4,5 threshold are attributed to the radiative decay of the 4d94fn+1 excited states.
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Comparative high-resolution photoemission measurement was performed for CeB6 and CeCu6 to study the electronic structure near EF. The photoemission spectra near EF of both compounds show a characteristic satellite at 270 meV from EF due to the spin-orbit splitting of a Ce 4f electron (4f7/2-4f5/2), while the spectrum of CeB6 exhibits an additional small feature around 70 meV. Model calculations based on the noncrossing approximation show that this satellite is due to the crystal-field splitting (Γ7-Γ8) of the Ce 4f5/2 state in CeB6.
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Both the interpretation of atomic spectra and the application of atomic spectroscopy to current problems in astrophysics, laser physics, and thermonuclear plasmas require a thorough knowledge of the Slater-Condon theory of atomic structure and spectra. This book gathers together aspects of the theory that are widely scattered in the literature and augments them to produce a coherent set of closed-form equations suitable both for computer calculations on cases of arbitrary complexity and for hand calculations for very simple cases.
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Measurements of the magnetization process at 4.2 K and 1.3 K for magnetic field up to about 150 kOe and of the temperature dependence of the susceptibility between 4.2 K and room temperature have been done in order to investigate Kondo states of dilute and dense Kondo system in single crystalline CexLa1-xB6 (0.03{≤q}x{≤q}1.0). The magnetic phase diagram is constructed above 0.6 K for Ce0.5La0.5B6, Ce0.75La0.25B6 and CeB6. A largely anisotropic magnetization and the temperature dependence of susceptibility in dilute alloys indicate the \varGamma8 quartet Kondo state contrary to the previous report. Dependence of Kondo temperature on the Ce concentration is very weak.
Article
The performance of ALS beamline 7.0 is described. This is an integrated system for delivering radiation from a 5 cm period undulator to spectroscopy and microscopy experiments across the range of photon energies from 60 to 1200 eV. The beamline is engineered to deliver the highest possible flux, with negligible deformation of the optic surfaces due to heating. Two experiment stations are served with rapid interchangeability. The measured operational parameters, the resolution and flux delivered, and the refocus of the light into a small spot at the experiment are all discussed. © 1995 American Institute of Physics.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
We analyze 4f → 3d resonant X-ray emission spectra (4f → 3d RXES) of Ce compounds, CeF3 and CeO2, by means of impurity Anderson model with multiplet effects. The calculation is based on the second-order optical formula, and the results are in satisfactory agreement with recent experimental data. It is to be emphasized that the effect of configuration-dependent hybridization is important in order to explain the experimental results of 3d-XAS and 4f → 3d RXES of CeO2, consistently. The calculations of 3d-XPS, valence-XPS(v-XPS), BIS, 4d-XPS and 4d-XAS are also made for CeO2 with the new parameter values including the configuration-dependent hybridization, and the results are in better agreement with experimental data than previous analyses, especially for the insulating energy gap determined by the v-XPS and BIS.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
Theoretical calculations of resonant X-ray emission spectrum (RXES) in rare earth systems and transition metal compounds are presented and compared with recent experimental data. RXES is formulated as a coherent second-order optical process, in contrast to that X-ray absorption spectrum (XAS) and X-ray photoemission spectrum (XPS) are the first-order optical process, and is calculated with atomic model or impurity Anderson model. From the analysis of RXES we can obtain important information on electronic excitations in rare earth systems and transition metal compounds, such as the multiplet excitation, the charge transfer excitation and the crystal field level excitation. The cluster size dependence of XAS, XPS and RXES is also studied theoretically with a one-dimensional d–p model, which describes qualitatively the effect of translational symmetry for nominally d° (or f°) compounds such as TiO2 (CeO2).
Article
The heavy-fermion compound [Formula: see text] has been studied using the fully relativistic spin-polarized mean muffin-tin orbital method within the local density approximation. Two separate calculations, one where the f electron is treated as a valence electron and the other where it is treated as part of the core, have been performed and the Fermi surface is obtained. The angular-dependent de Haas - van Alphen (dHvA) frequencies are calculated in both cases and they are compared with the experimental dHvA frequencies. We also calculated the electron momentum densities and compared them with the electron - positron momentum densities measured from the two-dimensional angular correlation of electron - positron annihilation radiation. The spin polarization of the Fermi surface is analysed and we present a new interpretation of the experimental data of Harrison et al.
Article
In this paper we review the present status of electron spectroscopy of Ce and Yb Kondo systems, with particular emphasis on photoemission and inverse photoemission. Our starting point is the situation in the mid-1980s, characterized by the development of a 'Kondo scenario' for the thermodynamic and spectral properties, as described in the review by Allen et al. (1986, Adv. Phys., 35, 275). That picture, based on room-temperature results and on a T=0 approach to the impurity Anderson Hamiltonian, has been generally confirmed and better defined over the past decade by a number of more elaborate experiments, and by new calculational schemes. The use of high-energy resolution and cryogenic temperatures has led to the direct observation of the characteristic low-energy excitations and has opened the way to stringent tests of the most fundamental aspect of model, namely its scaling properties. Despite the success of the Kondo approach, more work remains to be done. The improved quality of the data has revealed some quantitative discrepancies with the predictions of the model, stressing the need for more realistic theoretical schemes. Moreover, some recent experiments questioning the validity of the model have stirred a fierce controversy within the spectroscopic community. We review those conflicting results and discuss the possible reasons for the discrepancies, and how new experiments could solve the present difficulties.
Article
Reflectivity spectra of trivalent rare-earth hexaborides (RB6; R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, and Y) and mixed-valent SmB6 have been measured in the photon energy region from 1 meV to 40 eV at 300 and 9 K. We discuss the absorption structure due to conduction electrons. Even in LaB6 and YB6, which have no 4f electrons, the optical conductivity spectra were not fitted by a simple Drude model and a frequency-dependent relaxation time was observed. This was attributed to the electron-phonon and electron-electron scattering. In RB6 with 4f electrons, a common absorption structure was observed at the energy position of about 0.6 eV. The intensity was found to be almost proportional to the 4f spin angular momentum. From the f-sum rule, the absorption was found to be due to the conduction electrons. The origin was assigned to be due to the transition or to the relevant exciton absorption from the saddle points of Sigma1 at the neck point to the saddle points of Gamma12 and Gamma25 assisted by the scattering of the intra-atomic 5d-4f Coulomb exchange interaction, in particular enhanced in the heavy RB6 compounds by the lattice instability. In SmB6, the absorption that was seen in trivalent RB6's was also observed in addition to absorptions from 4f states of Sm2+ to 5d states and from 5d to 4f in Sm3+. This is thought to mean that the Fermi level of SmB6 is located at the same energy position in the band structure as that of the trivalent RB6.
Article
Evidence is provided of the role of the different hybridization strengths between the surface and the bulk in determining the magnitude of the surface shift for the shallow Ce 4{ital f} levels, with respect to the deeper core levels. This was achieved by comparing the photoemission core levels for a weakly hybridized case (CeAl) to a case of intermediate hybridization ({gamma}-Ce). For CeAl a 4{ital f} surface shift of 0.45 eV was observed, similar to that for the 5{ital p} core level, whereas a smaller (if any) 4{ital f} surface shift was observed for {gamma}-Ce. Model calculations based on the Anderson impurity Hamiltonian are shown to give a correct evaluation of this effect, which can be exploited as a way of testing the results of such a description for the Ce {ital f} states. {copyright} {ital 1996 The American Physical Society.}
Article
4{ital f} levels in Ce heavy-fermion compounds are examined using resonant photoemission. We find the following inconsistencies with the predictions of the Kondo model: (a) All temperature dependence can be accounted for simply by phonon broadening and the Fermi function; (b) the spectral weights of the features near {ital E}{sub {ital F}} do not scale with {ital T}{sub {ital K}}; and (c) the line shape of the feature previously identified as the Kondo resonance is Lorentzian and about an order of magnitude broader than predictions. Instrument resolution is not a limiting factor.
Article
Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds. Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions. High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique. Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref. 6), it should become feasible to probe the bulk electronic states in these materials. Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2. Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences.
  • M Butorin
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  • Y Ma
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  • D K Shuh
M. Butorin, D. C. Mancini, J.-H. Guo, N. Wassdahl, J. Nordgren, M. Nakazawa, S. Tanaka, T. Uozumi, A. Kotani, Y. Ma, K. E. Myano, B. A. Karlin, D. K. Shuh; Phys. Rev. Lett. 77, 574 (1996).
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J. Nordgren, G. Bray, S. Cramm, R. Nyholm, J.-E. Rubensson, and N. Wassdahl, Rev. Sci. Instrum. 60, 1690 (1989).
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