Mixed Valence Ce Compounds Research Articles

A new method of directly determining the core–hole effect in the Ce L 3 XAS of mixed valence Ce compounds—An application of resonant X-ray emission spectroscopy

Using a simplified model, we calculate resonant X-ray emission (RXE) spectra with Ce 2 p to 5 d excitation and Ce nl to 2 p de-excitation for mixed valence Ce compounds, where nl is 3 d or valence states. If the core–hole effect is large to play an important role in the Ce L 3 XAS, the RXE spectral features for nl = 3 d and valence states are significantly different, but if it is negligibly small, they are essentially the same. Therefore, these RXE spectra are useful as a new powerful method of directly determining the core–hole effect in the L 3 XAS. More detailed theoretical calculations of these RXE spectra are made for CeO 2, and compared with new experimental data, indicating the importance of the core–hole effect. Similar investigations are desirable for some metallic mixed valence Ce compounds in which the core–hole effect is controversial. Finally, we point out that for nl = 5 d we can observe the resonant inelastic X-ray scattering spectra corresponding to the charge-transfer excitations or Kondo resonance excitations.

Unified theory of x-ray magnetic circular dichroism atL2,3absorption edges for a series of Ce compounds

A theoretical study is presented with an extended single impurity Anderson model for the spectra of x-ray magnetic circular dichroism (XMCD) at the $\text{Ce}\text{ }{L}_{2,3}$ edges of mixed-valence Ce compounds under an external magnetic field. The effects of magnetic polariztion of the $\text{Ce}\text{ }4f$ states, the exchange interaction between the $\text{Ce}\text{ }4f$ and $5d$ states, and the hybridization between the $\text{Ce}\text{ }4f$ and ligand states are taken into account. Recent experimental results for ${\text{CePd}}_{3}$ are well explained. The theory can be applied to the XMCD of a localized ${\text{Ce}}^{3+}$ system, weakly and strongly mixed-valence Ce systems by changing the hybridization strength, and can also be applied to ferromagnetic Ce compounds by replacing the external magnetic field with the ferromagnetic exchange field. The effects of the hybridization in the final state, the $\text{Ce}\text{ }4{f}^{2}$ configuration, and the spin polarization of the $\text{Ce}\text{ }5d$ states through the hybridization with a spin-polarized ligand band are studied in detail, and a systematic change in the ${L}_{2}$ and ${L}_{3}$ XMCD intensities with the change in the mixed-valence strength is discussed. The present theory gives a framework to understand varied XMCD behavior of a series of Ce compounds in a unified fashion.

Theory of X-ray Magnetic Circular Dichroism at L2,3Absorption Edges of Ce Compounds

Theoretical study is made, with an extended single impurity Anderson model, for the spectra of X-ray magnetic circular dichroism (XMCD) at Ce L2;3 edges of trivalence, weakly mixed valence and strongly mixed valence Ce compounds under external magnetic field. Recent experimental results for CePd3 are well explained. The present theory gives a framework to understand a variety of XMCD behavior of various kinds of ferromagnetic Ce compounds in a unified fashion.

Theory of hard X-ray resonant photoelectron emission and related spectra in mixed valence Ce compounds

Analyses of hard X-ray resonant photoelectron emission spectroscopy (HAX-RPES) and related spectroscopies such as X-ray absorption spectroscopy (XAS) and resonant X-ray emission spectroscopy (RXES) are made for mixed valence Ce compounds with single impurity Anderson model. First, the spectra of Ce 2 p 3 / 2 XAS ( L 3 XAS) and the conventional Ce 3d X-ray photoelectron emission spectroscopy (XPS) are analyzed for CePd 7 , taking into account the bulk/surface sensitivity in XPS. Then it is shown that the bulk properties are more directly obtained from the Ce 3d HAX-RPES spectra, in which the Ce 2p–5d excitation is followed by the Ce 2p3d5d Auger decay. Furthermore, the Ce 3d HAX-RPES can reveal some hidden structures in the Ce L 3 XAS. From the analysis of the Ce HAX-RPES for CeRh 3 , we reveal the existence of the Ce 4 f 2 contribution in the L 3 XAS, and confirm an important role of the Coulomb interaction U fd between Ce 4f and 5d elctrons in the XAS structure. For ferromagnetic mixed valence Ce compound CeFe 2 , the spectra of the CeL 3 XAS and its magnetic circular dichroism (MCD-XAS) are analyzed, and the effect of U fd is also important in explaining the relative peak positions of the XAS and MCD-XAS. Finally, the MCD in RXES (MCD-RXES) is calculated for CeFe 2 , and it is predicted theoretically that the excitation spectrum of MCD-RXES can also reveal the hidden structure of the 4 f 2 configuration in the Ce L 3 XAS (and MCD-XAS) spectrum.

Theory of resonant spectroscopies in nickel, cerium compounds and other systems

Abstract In this paper, we discuss theoretically information about electronic structures in strongly correlated systems obtained from resonant X-ray spectroscopies. From a many-body point of view on the basis of an impurity Anderson model or a cluster model including the intra-atomic multipole interaction between electrons, we calculate spectra by exactly diagonalizing our Hamiltonian. First, for ferromagnetic Ni, through an analysis of magnetic circular dichroism in 3 d resonant photoemission spectroscopy (RPES) and resonant inverse photoemission spectroscopy (RIPES), it is shown that, in addition to 3 d 9 and 3 d 10 , the 3 d 8 configuration with weight of about 20% is contained as electron configuration of a Ni atom in Ni metal. The 3 d 8 configuration is furthermore shown to be mainly composed of 3 F and its relation to the origin of ferromagnetism is discussed. Second, for mixed valent Ce compounds, including CeNi 2 , it is shown, through an analysis of RIPES, tht the 4 f -conduction band hybridization must be reduced by the presence of a core hole. Third, for some 3 d transition metal (TM) oxides, 3 d RPES at TM ion 2 p threshold is shown to be a powerful method for estimating the magnitude of 3 d –3 d interaction strength and the charge transfer energy.

High energy resonant photoemission and resonant Auger spectroscopy in mixed valent Ce compounds

Abstract In CeRh3 and Ce7Rh3 compounds, strong resonant effects are observed in the Ce 3d and 4d photoemission spectra at the Ce L3 and M4,5 absorption edges respectively. These resonances show unambiguously that interference effects exist in core level spectroscopies. In the CeRh3 mixed valent compound, the resonances of photoemission peaks corresponding to 4f0, 4f1 and 4f2 final state character occur at different photon energies, making it possible to unravel the X-ray absorption profiles. Moreover, resonant L3M4,5N4,5 Raman Auger lines are observed up to 10 eV above the L3 absorption edge, showing the strong localized character of the core excited state.

Surface effects in the spectroscopy of mixed-valent Ce compounds

The electronic properties of correlated mixed-valent Ce compounds have been studied in detail over the last two decades by means of a variety of electron spectroscopies. This work has been carried out in connection with their peculiar ground state properties, deriving from the hybridization of the Ce 4f wavefunctions with the conduction band states. However, the relevance of surface effects, in the interpretation of the spectral line-shapes has been recognized only very recently. These are related to the reduction of 4f hybridization at the surface. In this review I will give a summary of the main achievements in this area, showing the procedures to separate the surface from the bulk spectral contributions for both the Ce 4f spectral functions and the Ce 3d core levels. In addition, a systematic analysis of the surface and bulk electronic properties of a number of different compounds is reported. This new information can in some cases reconcile the results obtained by different spectroscopies in terms of theoretical models based on the Anderson single impurity Hamiltonian. However, when comparing spectroscopies which rise to considerably different final states this may no longer be valid and some possible solutions are outlined. Furthermore, preliminary results on single crystals are presented. They seem to show somewhat unexpected features which need to be studied in deeper detail in the future.

Circular Dichroism in the 3dand 4dCore Photoabsorption for Ferromagnetic Ce Compounds -Interplay of Hybridization, Spin Orbit Interaction and Crystal Field-

The Ce 3 d - and 4 d -core photoabsorption spectra (3 d XAS and 4 d XAS) of ferromagnetic mixed valent Ce compounds are calculated on the basis of the impurity Anderson model. The model takes into account the Coulomb interaction producing multiplet structures, the spin-orbit interaction, the crystal field effect and the molecular field acting on the 4 f spin. The calculation shows a strong circular dichroism even for the Ce magnetic moment of ∼0.5 µ B . The dichroism is furthermore shown to be a powerful method to measure the spin and orbital contributions to the 4 f moment. On the basis of the calculation, the 4 f magnetic state of the ferromagnet CeRh 3 B 2 is discussed.

Circular dichroism in the 3d- and 4d-core photoabsorption for ferromagnetic mixed valent Ce compounds

Effects of symmetry breaking in the 4f orbital occupancy of Ce atom in ferromagnetic Ce compounds on the multiplet structures in the Ce 3d- and 4d-core photoabsorption spectra are discussed based on the impurity Anderson model. The calculation predicts a strong circular dichroism in the spectra even for a molecular field producing the 4f magnetic moment of ~0.5µB. The interplay of the 4f-conduction band hybridization and the 4f spin-orbit interaction in the ground state is discussed in detail and the present dichroism is shown to be a good method to measure the spin and orbital contributions to the 4f magnetic moment.

Electronic Spectroscopies of the Mixed Valence Ce Compounds: A Slave Boson Approach to the Impurity Model11Project supported in part by National Natural Science Foundation of China under Grant No. 1891101 and Grant No. 1870702.

A slave boson approach to the electronic spectroscopies for the mixed valence Ce compounds in the impurity model is presented. Three valence configurations of Ce ion ( and ) are considered so the framework of the theory can be used for culculating both XPS and BIS. The spectrum under mean field approximation is presented.

Pressure-induced changes inLIIIx-ray-absorption near-edge structure of CeO2and CeF4: Relevance to4f-electronic structure

X-ray-absorption near-edge structure (XANES) studies were performed at the ${L}_{\mathrm{III}}$ thresholds of Ce${\mathrm{O}}_{2}$ and Ce${\mathrm{F}}_{4}$ as a function of external pressure up to \ensuremath{\cong}267 kbar at room temperature. In both cases, the ${4f}^{0}$-related higher-energy component of the essentially double-peaked XANES structure decreases in relative intensity with pressure, opposite to what is generally observed for metallic mixed-valent Ce compounds. This supports the proposed peak assignment on the basis of core-hole induced many-body effects due to $4f$-ligand covalency compatible with traditional tetravalency of these compounds.

Multiplet structure of core photoabsorption spectra in mixed valent Ce compounds

The effect of the 4f-valence band hybridization on the multiplet structure in core X-ray absorption spectra (XAS) of mixed valence systems is discussed on the basis of the impurity Anderson model incorporated with the 4f-core hole exchange interaction. A qualitative interpretation for 4d-XAS in the prethreshold region og CeCo2 and metallic Ce compounds is presented.

Theory of Photoemission, Photoabsorption and Inverse Photoemission in CeO2 and Metallic Ce Compounds

Ce 3d core X-ray photoemission (3d-XPS), 3d core photoabsorption (3d-XAS), 3d core resonant photoemission (3d-RXPS) and bremsstrahlung isochromat spectra (BIS) in a mixed valent insulator CeO2 and typical metallic mixed valent Ce compounds such as CeRh3 are studied theoretically on the basis of the Anderson model. By assuming that the weight of 4f0 configuration in the ground state, the 4f-valence band mixing and the 4f-4f Coulomb interaction for CeO2 are considerably larger than those for metallic Ce compounds, a remarkable difference in 3d-XPS and an apparent similarity in 3d-XAS between the two systems are consistently explained. Furthermore differences in 3d-RXPS and in BIS between the two systems expected from the present analysis are predicted. A preliminary discussion about the effect of 4f-core hole and 4f-4f exchange interactions on core absorption spectra in mixed valent systems is also presented.

Ground-state properties of Kondo lattice systems

Recent result concerning the ground-state properties of Ce-based Kondo-lattice systems are reviewed. For the antiferromagnet CeAl 2, a long-lasting controversy concerning the pressure dependence of the Néel temperature is resolved. For the enhanced Pauli paramagnet CeAl 3 evidence is reported of a coherence-derived pseudo-gap in the “Kondo-resonance” density of states at the Fermi level. Finally, the possible role of valence fluctuations on “heavy-feremion” superconductivity in CeCu 2Si 2 is briefly addressed by a comparison of the superconducting transition temperature ( T c) of CeCu 2Si 2 and several mixed-valent Ce compounds withthe T c values of the related La homologs.

Author index to volumes 47 & 48

Recent result concerning the ground-state properties of Ce-based Kondo-lattice systems are reviewed. For the antiferromagnet CeAl2, a long-lasting controversy concerning the pressure dependence of the Néel temperature is resolved. For the enhanced Pauli paramagnet CeAl3 evidence is reported of a coherence-derived pseudo-gap in the “Kondo-resonance” density of states at the Fermi level. Finally, the possible role of valence fluctuations on “heavy-feremion” superconductivity in CeCu2Si2 is briefly addressed by a comparison of the superconducting transition temperature (Tc) of CeCu2Si2 and several mixed-valent Ce compounds withthe Tc values of the related La homologs.

Correlation effects in the electronic structure and photoemission spectra of mixed-valence cerium compounds

The electronic ground states and core-hole and valence-hole excited states of so-called tetravalent and mixed-valence Ce compounds are studied taking into account strong correlations among $4f$ electrons. By an analysis of the core-level photoemission satellites, Ce${\mathrm{O}}_{2}$, which has been thought to be tetravalent, is identified as a mixed-valence system with $\ensuremath{\sim}0.6 4f$ electron, where the magnetic moment of the $4{f}^{1}$ configuration is quenched via singlet coupling between the $4f$ electron and the O $2p$ hole. $4f$-derived satellite features in the resonant photoemission spectrum of Ce${\mathrm{O}}_{2}$ are calculated, which qualitatively explain resonant photoemission spectra of tetravalent Ce intermetallic compounds. The core-level and $4f$-derived photoemission spectra of mixed-valence CeN are also studied by considering only the $4f$-electron---N $2p$-hole coupling as in Ce${\mathrm{O}}_{2}$ and neglecting the $4f\ensuremath{-}5d$ mixing. It is also suggested that for the $\ensuremath{\alpha}$ phase of Ce metal the absence of a localized magnetic moment, volume collapse, and a $4{f}^{0}$ signal in the core-level photoemission spectra are explained by this type of configuration-mixing description.