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Abstract

The spin transition in LaCoO3 is investigated by temperature-dependent resonant soft X-ray emission spectroscopy near the Co 2p absorption edges. This element-specific technique is more bulk sensitive with respect to the temperature induced spin-state of the Co3+ ions in LaCoO3 than other high-energy spectroscopic methods. The spin transition is interpreted and discussed with ab-initio density-functional theory within the fixed-spin moment method, which is found to yield consistent spectral functions to the experimental data. The spectral changes for LaCoO3 as a function of temperature suggest a change in spin-state as the temperature is raised from 85 to 300 K while the system remains in the same spin state as the temperature is further increased to 510 K.

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... The Heyd-Scuseria-Ernzerh (HSE06) hybrid functional formalism [151] has been employed for a variety of different spin configurations and also provides an AFM groundstate ordering [72,60,152]. Furthermore, by making use of expanded unit cells (i.e., the supercell approach [153]), in order to remove possible size constraints, more complex magnetic ordering has been investigated for Cr 2 AC, with A = Al, Ge, and Ga. This alternative method provided for Cr 2 AlC a ground-state magnetic ordering with an in-plane AFM spin configuration [154]. ...
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... (ii) The XAS spectrum of Fe metal (x = 0) has narrower 2p 3/2 , and 2p 1/2 absorption peaks, whereas the XAS spectra of the carbon-containing films exhibit small peak A comparison of the integrated peak areas at different carbon contents shows that the 2p 3/2 /2p 1/2 peak ratio is lowest for x = 0.72 (1.9), and x = 0.45 (2.0) in comparison to the lower carbon contents x = 0.26 (2.2), and 0.21 (2.3), while it is higher for bulk Fe x = 0 (2.4). A lower 2p 3/2 /2p 1/2 peak ratio is an indication of higher ionicity (lower conductivity) for the higher carbon contents [26,27]. Note that the 2p 3/2 /2p 1/2 peak ratio is a result of the ionicity for Fe, in the FeC y carbide phase, and not for the entire film that is the sum of both the Fe-rich (∼ FeC y ), and carbon-rich domains in the matrix. ...
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... eV. The spectral feature is similar to the Co L 3 -edge RIXS spectra of LS Co 3+ in LaCoO 3 20,21 , but its crystal-field excitation energy is much larger 21 . Tomiyasu et al. reported Co L 3 -edge RIXS of LaCoO 3 single crystal with high energy resolution (~80 meV). ...
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... The Heyd-Scuseria-Ernzerh (HSE06) hybrid functional formalism [151] has been employed for a variety of different spin configurations and also provides an AFM groundstate ordering [72,60,152]. Furthermore, by making use of expanded unit cells (i.e., the supercell approach [153]), in order to remove possible size constraints, more complex magnetic ordering has been investigated for Cr 2 AC, with A = Al, Ge, and Ga. This alternative method provided for Cr 2 AlC a ground-state magnetic ordering with an in-plane AFM spin configuration [154]. ...
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... For the XAS spectra, we used the experimental values for 2p 3/2 /2p 1/2 branching ratio (1.1:1) and for the L 2, 3 peak splitting (8.0 eV), which is somewhat larger than our calculated ab initio spin-orbit splitting of 7.6 eV. Absorption spectra were calculated within the same theoretical scheme used for XES, but with the addition of core-hole effects [29]. Specifically, we generated such a self-consistentfield potential using a × × 2 2 1 hexagonal supercell of 32 atoms containing one core-hole on the investigated element. ...
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High resolution experimental data for resonant soft x–ray scattering of transition metal compounds are shown. The compounds studied are the ionic transition metal di–fluorides, ionic and covalent orthovanadates and members of the La1–xSrxCoO3 perovskite family. In all compounds we studied the transition metal L2,3 edge and also the ligand (oxygen or fluorine) K edge. For the ionic compounds the transition metal data are in good agreement with atomic multiplet ligand field calculations that include charge transfer effects. Density functional calculations give very useful information to interpret the ligand x–ray emission data. The experimental metal Lα emission data show that the region between valence and conduction bands in the di–fluorides has several d–excited states. At the L2edge of the ionic orthovanadates we found the signature of a fast Coster–Kronig decay process that results in a very localized emission peak. Changes in the oxidation sate in the La1–xSrxCoO3 compounds are observed at both the metal L2,3edge and the oxygen K edge absorption spectra.
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Chapter
MXenes (transition metal carbides and nitrides/carbonitrides) are a novel type of two-dimensional (2D) materials that are derived from MAX-phase precursors. Their structural and compositional diversity, outstanding electrical conductivity, and tunable surface properties have made MXenes one of the best and most versatile families of 2D materials, with rapidly growing fields of application. Since their inception, MXenes have attracted enormous research interest in various research fields, including physics, chemistry, materials science, nanotechnology, nanomedicine, and environmental science. This chapter briefly introduces fundamental aspects of MXenes such as MAX-phase precursors, MXene derivatives, and methods of synthesis and processing. Various MXene properties such as electronic, optical, magnetic, and mechanical properties are then briefly discussed. The next section briefly describes the current advances of MXene-based composites in various application fields. Finally, the chapter highlights the current challenges and research prospects for the development of MXene-based advanced composite materials for specific applications.
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Chapter
Experimental electronic structure of LaCoO\(_3\) and other RCoO\(_3\) (R = rare earth) in connection with the spin crossover phenomena in LaCoO\(_3\) is reviewed. As the experimental probes, we mostly deal with electron spectroscopic methods, namely photoemission spectroscopy and x-ray absorption spectroscopy. Experimental results are compared with ab initio band-structure calculations and configuration-interaction cluster-model calculations. First, we discuss how electronic-structure measurements can contribute to studying spin crossover phenomena, particularly of LaCoO\(_3\). Then in Sect. 2.2, we give a brief historical review of electron spectroscopic studies on LaCoO\(_3\) in order to highlight the conflict between the two major models, namely the low spin-high spin model and the low spin-intermediate spin-high spin model. Sections 2.3 to 2.5 are devoted to discussing this conflict and how we may be able to resolve it beyond this binarism. In Sect. 2.6, we remark the probing depth of the methods and the surface magnetism of LaCoO\(_3\). In Sect. 2.7, we briefly discuss a part of recent advances both in experiment and theory in this field. We run through the electronic structure of RCoO\(_3\) and other related Co oxides in Sect. 2.8, and finally we give the concluding remarks in Sect. 2.9.
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The spin states of Co$^{3+}$ ions in perovskite-type LaCoO$_3$, governed by complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can detect directly. In this letter, we revealed the tensile-strain dependence of spin states, $i. e.$ the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO$_3$ via resonant inelastic soft x-ray scattering. The tensile-strain as small as 1.0% was found to realize different spin states from that in the bulk.
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The electronic structure of the perovskite LaCoO3 for different spin states of Co ions was calculated in the local-density approximation LDA+U approach. The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculations show that Co 3d states of t2g symmetry form narrow bands which could easily localize, while eg orbitals, due to their strong hybridization with the oxygen 2p states, form a broad σ* band. With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs. This intermediate-spin (occupation t2g5eg1) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO3 at 90 K<T<500 K. Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested. © 1996 The American Physical Society.
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Homogeneous samples of LaCoO 3 were prepared from coprecipitated precursors; they reveal that single-phase samples are restricted to a narrow range of the [La]/[Co] ratio of about 1.0. A first-order phase change near 1200 K only appears where Co 3O 4 is present as an impurity phase. Evolution with temperature from all low-spin Co(III) at T = 0 K to 50% high-spin Co 3+ and 50% intermediate-spin Co(III) above 650 K is monitored with magnetic susceptibility, resistance, and thermopower measurements. Evidence is found for a dynamic ordering of high-spin Co 3+ and low-spin Co(III) in the intermediate-temperature interval 110 K < T < 350 K. The lifetime of a Co 3+ ion decreases with increasing temperature to a τ < 10 -8 sec for T > 200 K. The intermediate-temperature phase is semiconductive with an Eg ≈ 0.14 eV for excitations of hole-electron pairs. The holes are mobile; the electrons are trapped as Co 2+ on high-spin sites until they recombine with a mobile hole. The metallic high-temperature phase for T > 650 K is nucleated and grows with increasing temperature in the interval 350 K < T < 650 K.
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The oxygen nonstoichiometry (δ) of La 1- xSr xCoO 3- δ (0 < x ≤ 0.6) has been studied as a function of temperature (573-1673 K) and oxygen partial pressure (1-10 -3 atm) using TGA and coulometric titration techniques. The absolute values of oxygen nonstoichiometry were determined by the direct reduction in a flux of hydrogen (TGA) or by decreasing PO 2 in a coulometric titration cell. The boundaries of phase stability of La 0.7Sr 0.3CoO 3-δ and (La 0.7Sr 0.3) 2CoO 4± v were evaluated. A possible mechanism of disordering processes has been discussed.
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Neutron-diffraction measurements have revealeda new anomalous thermal lattice expansion of LaCoO3 near 500 K thatindicates the existence of a second spin-state transition, in addition to theone previously established near 100 K. The anomalous expansion and thetemperature dependence of the Co magnetic moments are successfullyinterpreted in a wide temperature range based on a simple model assuminglow-spin (LS, S=0), intermediate-spin (IS, S=1), and high-spin(HS, S=2) states of Co atoms.The first spin transition, near 100 K, is from LS to IS, andthe second, near 500 K, is from IS to a mixed state of IS and HS.The fitted model parameters indicate that the initially-large energy differencebetween IS and HS states decreases towards zero as the secondtransition proceeds. The large drop in resistivity associated with the lattertransition appears to be correlated with the population of the HS state.
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Precision x-ray measurements of the motions of the atoms in LaCoO3 every 50°C from room temperature to 1000°C indicate that the crystal space group is R3¯c below 375°C and R3¯ above 375°C. In the symmetry R3¯ there are two distinguishable octahedral cobalt positions, CoI having larger crystalline fields than CoII. Since high- and low-spin cobalt ions are simultaneously present, this indicates preferential long-range ordering of low-spin cobalt at CoI sites, and of high-spin cobalt at CoII sites for T>375°C. Calorimetric data show a first-order transition at Tt=937°C and a higher order transition in the temperature interval 125
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59Co and 139La NMR measurements have been performedto study the microscopic magnetic properties closely relatedwith the spin state of a trivalent cobalt ion in LaCoO3.The temperature-independent 59Co and 139La Knightshifts below ˜ 30 K clearly revealed the presence of anonmagnetic ground state at low temperatures. We calculated thetemperature dependence of the magnetic susceptibility chiand the 59Co Knight shift 59K in a single ion modelwith the low-spin 1A1 ground state above which thehigh-spin 5T2 state is located. Then, both the trigonalcrystal-field and spin-orbit interactions are taken intoaccount. From this analysis, we infer the presence of anantiferromagnetic exchange interaction between the high-spinstates. The characteristic temperature dependence of the59Co hyperfine coupling constant is also discussed.
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The crystal structure and properties of La1 − xSrxCoO3 − y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co4+ (cobalt ions with one positive effective charge, CoCo.) and oxygen vacancies (Vo..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.
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X-ray photoemission measurements of the core levels and valence electronic structure of LaCoO3 and La0.5Ca0.5CoO3 high quality epitaxial films are presented. Shifts of the core levels and main valence band features are consistent with a doping-induced change in the chemical potential. Oxygen states are found to significantly contribute to a peak in the valence band at 1 eV binding energy, verifying earlier results of cluster calculations. A Fermi level crossing of this same band upon doping is observed, yielding a high Fermi level density of states.
Article
We present soft-x-ray-absorption spectra (XAS) of LaCoO3 taken at different temperatures (80–630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The O 1s XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by t2g6(1A1) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by t2g4eg2(5T2) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80–630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.
Article
We have studied the electronic structure of LaCoO3 by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Co 2p core-level and valence-band photoemission spectra display satellite structures indicating a strong electron-correlation effect. The Co 2p core-level photoemission, the valence-band photoemission, and the O 1s XAS spectra have been analyzed using a configuration-interaction cluster model for the initial-state configurations of the low-spin (LS: 1A1), intermediate-spin (IS: 3T1) and high-spin (HS: 5T2) states and their mixtures. The ground state of LaCoO3 in the LS state is found to have heavily mixed d6 and d7L-̱ character, reflecting the strong covalency. The magnetic susceptibility has been analyzed for various level orderings of the LS, IS, and HS states. From the analyses of the photoemission spectra and the magnetic susceptibility data, the temperature-induced paramagnetism in LaCoO3 above ∼90 K is most likely due to a gradual LS-to-IS transition.
Article
We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co3+ ions with increasing temperature from a low-spin ground state (t2g6eg0) to an intermediate-spin state (t2g5eg1) without (100–500 K) and with (>500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.
Article
We report the valence-band and Co 2p photoemission spectra of ${\mathrm{LaCoO}}_{3}$ obtained at different temperatures (100, 300, and 573 K). Analysis of the core-level spectrum in terms of a configuration interaction model suggests that both low- and high-spin states coexist at the low temperature (100 K). It also indicates that there is a decrease in the low-spin contribution at 573 K related to local structural changes. Photoemission spectra of the valence-band region further support this interpretation.
Article
Crystallographic, magnetic, and electrical studies of the system La1−xSrxCoO3.00±0.01 for 0≤x≤0.5 give indirect evidence for the presence of chemical inhomogeneities separating strontium‐free regions, where localized ``3d'' electrons occur at thermally excited high‐spin Co3+ ions, from strontium‐rich regions, where the ``3d'' electrons are collective and give ferromagnetism at low temperatures. These different regions occur within the same rhombohedral perovskite crystal and appear to represent two different electronic phases within the same crystallographic phase. A schematic band model for the ferromagnetic phase is presented.
Article
The pressure dependence of the 100 K spin-state transition in LaCoO3 is investigated through magnetization measurement under pressures of up to 18.2 kbar. The energy gap, Δ, between the low-spin ground state and the excited magnetic state (intermediate spin) increases remarkably with increasing pressure. The linear part of the pressure dependence of Δ, which is dominant below 5 kbar, is interpreted consistently by the volume expansion due to the spin-state transition under ambient pressure. In the higher pressure region, a quadratic pressure dependence is dominant, which suggests a very steep volume dependence of the energy of the excited magnetic state under the highly compressed condition.
Article
The cubic Laves phase of YCo2 is near a transition from a paramagnetic to a magnetically ordered state. Self-consistent energy-band calculations yield the total energy and the magnetic moment as functions of volume. The new fixed spin-moment (FSM) method, which allows one to calculate the total energy as an explicit function of the magnetisation, is introduced. At the theoretical equilibrium volume YCo2 is found to be a strongly enhanced Pauli paramagnet, but at a slightly larger lattice constant a metamagnetic transition seems possible. Its occurrence can be understood on the basis of a double minimum in the total energy obtained from the present FSM calculations, which lead to an estimated critical field of about 350 T. In the magnetic state the Y moment is coupled antiferromagnetically to the Co moment.
Article
Co K-edge near-edge X-ray absorption spectra are reported for RECoO3 (RE=Y, Ho, Gd, Nd) at 300 K and for LaCoO3 in the temperature range (140<or=T<or=830 K). The pre-edge structure associated with Co 1s to 3d excitation is found to be comparable for all the cobaltates at room temperature, consistent with the proposed similarity of their electronic structure. For LaCoO3, temperature-dependent changes are observed in the pre-edge structure. These data, when compared with previously published UV photoelectron spectra and calculated density of states, are found to be consistent with a proposed model for the higher-order semiconductor-to-metal transition of LaCoO3 which involves increased sigma *- pi * band overlap with temperature.
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
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
Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by the correlation effects is categorized as the Mott Insulator. Near the transition point the metallic state shows fluctuations and orderings in the spin, charge, and orbital degrees of freedom. The properties of these metals are frequently quite different from those of ordinary metals, as measured by transport, optical, and magnetic probes. The review first describes theoretical approaches to the unusual metallic states and to the metal-insulator transition. The Fermi-liquid theory treats the correlations that can be adiabatically connected with the noninteracting picture. Strong-coupling models that do not require Fermi-liquid behavior have also been developed. Much work has also been done on the scaling theory of the transition. A central issue for this review is the evaluation of these approaches in simple theoretical systems such as the Hubbard model and t-J models. Another key issue is strong competition among various orderings as in the interplay of spin and orbital fluctuations. Experimentally, the unusual properties of the metallic state near the insulating transition have been most extensively studied in d-electron systems. In particular, there is revived interest in transition-metal oxides, motivated by the epoch-making findings of high-temperature superconductivity in cuprates and colossal magnetoresistance in manganites. The article reviews the rich phenomena of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ru compounds. The diverse phenomena include strong spin and orbital fluctuations, mass renormalization effects, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. These parameters are experimentally varied by doping, pressure, chemical composition, and magnetic fields. Much of the observed behavior can be described by the current theory. Open questions and future problems are also extracted from comparison between experimental results and theoretical achievements.
Article
The spin gap energy ($\approx${} 30 meV) associated with the low-spin ($S=0$) to high-spin ($S=2$) transition in LaCo${\mathrm{O}}_{3}$ was proved to be considerably smaller than the charge gap energy ($\approx${} 0.1 eV) which was estimated by optical spectroscopy. Hole doping in the low-spin ground state of LaCo${\mathrm{O}}_{3}$ leads to formation of localized magnetic polarons with unusually high spin number ($S=10$-${}16$), which can be viewed as a precursor of the doping-induced ferromagnetic metallic state.
Article
A method to determine the absorption coefficient near the onset of core-electron transitions for concentrated samples using fluorescence-yield (FY) detection is presented. Measuring the FY signal for different experimental geometries, we are able to calculate the true absorption coefficient. Thus we are able to correct fully for saturation effects present in FY spectra of concentrated samples. The technique is demonstrated for Co and a buried layer of CoSi2.
Article
The electron emission spectrum resulting from thermal collisions of He*(23S) atoms with LaCoO3 was measured as well as the hnu-dependent photoemission spectra. The He* spectrum shows that the lower-lying Co 3d derived (main) bands are strongly suppressed relative to the O 2p derived bands, reflecting the spatial distribution of the initial Co 3d wave functions. On the other hand, the higher-lying Co 3d derived (satellite) band is anomalously enhanced in the He* spectrum. These findings are discussed on the basis of the initial and final configuration interactions, using a Co2O11 cluster.
Article
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Spin transition in LaCoO3investigated etc
  • J B Goodenough
  • Magnuson
Goodenough J. B., Progress in Solid State Chemistry, edited by Reiss H., Vol. 5 (Pergamon Press Inc., New York) 1967, p. 145. rM. Magnuson et al.: Spin transition in LaCoO3investigated etc. 295
  • K Asai
  • P Gehring
  • H Chou
  • G Shirane
K. Asai, P. Gehring, H. Chou and G. Shirane; Phys. Rev. B 40, 10982 (1989).
  • S Masuda
  • M Aoki
  • Y Harada
  • H Hirohashi
  • Y Watanabe
  • Y Sakisaka
  • H Kato
S. Masuda, M. Aoki, Y. Harada, H. Hirohashi, Y. Watanabe, Y. Sakisaka and H. Kato; Phys. Rev. Lett. 71, 4214 (1993).
  • A R Williams
  • V L Moruzzi
  • C D Jr
  • J Gelatt
  • K Kübler
  • Schwarz
A. R. Williams, V. L. Moruzzi, C. D. Jr. Gelatt, J. Kübler and K. Schwarz; J. Appl. Phys. 53, 2019 (1982).
  • G Thornton
  • I W Own
  • G P Diakun
G. Thornton, I. W. Own and G. P. Diakun; J. Phys. Condens. Matter 3, 417 (1991).
  • M Itoh
  • M Sugahara
  • I Natori
  • K Motoya
M. Itoh, M. Sugahara, I. Natori and K. Motoya; J. Phys. Soc. Jpn. 64, 3967 (1995).
  • T Möller
T. Möller; Synchrotron Radiat. News. 6, 16 (1993).
  • M Imada
  • A Fujimori
  • Y Tokura
M. Imada, A. Fujimori and Y. Tokura; Rev. Mod. Phys. 70, 1039 (1998) and references therein.
  • A Neckel
  • K Schwarz
  • R Eibler
A. Neckel, K. Schwarz, R. Eibler and P. Rastl; Microchim. Acta, Suppl. 6, 257 (1975).
  • M A Korotin
  • S Yu
  • I V Ezhov
  • V I Solovyev
  • D I Anisimov
  • G A Khomskii
  • Sawatzky
M. A. Korotin, S. Yu. Ezhov, I. V. Solovyev, V. I. Anisimov, D. I. Khomskii and G. A. Sawatzky; Phys. Rev. B 54, 5309 (1996).
  • J B Goodenough
J. B. Goodenough, Progress in Solid State Chemistry, Vol. 5, p 145, Ed. by H. Reiss (Pergamon Press Inc., New York, 1967);
  • K Schwarz
  • P Mohn
K. Schwarz and P. Mohn; J. Phys. F: Met. Phys., 14, L129 (1984).
  • K Asai
  • O Yokokura
  • M Suzuki
  • T Naka
  • T Matsumoto
  • H Takahashi
  • N Mori
  • K Kohn
K. Asai, O. Yokokura, M. Suzuki, T. Naka, T. Matsumoto, H. Takahashi, N. Mori and K. Kohn; J. Phys. Soc. Jpn; 66, 967 (1997).