T. Jo

Many-body effects in core-level spectroscopy of rare-earth compounds

Abstract Many-body effects in core-level photoemission and core-level photoabsorption are discussed for rare-earth systems, especially for Ce and La compounds, both in metallic and insulating forms. Emphasis is put on effects of metallic mixed valency and insulating covalency of 4f electrons on these spectra. For the insulating compound CeO2, detailed analyses of the 3d core photoemission (3d-XPS) and the 2p core photoabsorption (L3-XAS) are presented by using the impurity Anderson model with a filled valence band. In order to give a consistent interpretation for 3d-XPS and L3-XAS, it is shown to be essential to take account of the Coulomb interactions U fd (between a 4f electron and a photoexcited 5d electron in the L3-XAS) and -U dc (between the 5d electron and a core hole), in addition to -U fc (between the 4f electron and the core hole). Discussions are given on the physical information derived from the analysis, on similarities and differences in spectral features between insulating and metallic syst...

Effects of hybridization on multiplet structures in 3d- and 4d-core photoemission spectra in light rare earth compounds

La or Ce 3d- and 4d-core photoemission spectra are discussed for La and Ce compounds on the basis of the impurity Anderson model taking into account the atomic multiplets due to electron-electron interaction. A consistent explanation of 3d and 4d XPS of La and Ce compounds is presented. We also give an interpretation of the multiplet structures in the spectra. By transforming the Hamiltonian, we show that conditions that promote the quenching of the multiplet structure in the solids are small electron-electron interaction, large hybridization between the localized orbit and the valence band, and small energy difference between different fn configurations in the final state.

Electronic structure and lattice constant of invar alloys

A theory of the volume striction in ferromagnetic transition metal alloys is developed on the basis of the Alexander‐Anderson‐Moriya theory of the exchange interaction. The volume derivative of the interaction between a pair of atoms is calculated under the assumption that a volume change affects the state density of the s band and 3d atomic levels. In fcc Ni‐Fe the number of 3d minority spin electrons of a Fe atom increases with increasing Fe concentration. The volume derivative of the interaction between two Fe atoms shows an anomalous dependence on the Fe concentration through the change of the number of 3d minority spin electrons which explains the large spontaneous volume striction, lattice constant anomaly and negative thermal expansion at low temperatures in invar alloys. The possibility of finding Fe atoms with magnetic moment antiparallel to bulk magnetization and its relevance to the lattice anomalies are discussed also.

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.

Hybridization, crystal field and magnetism in rare earths and core level spectroscopy

Abstract The usefulness and capability of core level spectroscopies for probing the 4f states of rare earth systems are discussed theoretically on the basis of atomic multiplet calculations including various “solid state effects”. By choosing some Ce compounds, the hybridization strength between the Ce 4f orbital and ligand orbitals are shown to be well reflected in 3d and 4d core X-ray photoemission spectra. The light polarization dependence of 3d and 4d core X-ray absorption, such as linear dichroism and magnetic circular dichroism, is shown to be a powerful characteristic which can be used to detect the distribution and symmetry of 4f electron occupation.

Magnetic states of cobalt oxide and ferrites and magnetic dichroism in 2p-3d X-ray absorption spectroscopy

Abstract It is shown that the recently observed magnetic circular dichroism (MCD) in Co 2p XAS for CoFe 2 O 4 can be explained by assuming an orbital-moment and spin contribution of ≈0.5 to 0.8 μ B and ≈2.8 μ B , respectively to the atomic moment for the Co 2+ ion at 300 K under either an octahedral or a trigonal crystalline field on the basis of an atomic calculation. At lower temperatures, the effect of trigonal symmetry is predicted to be reflected in the MCD of CoFe 2 O 4 . The discussion is extended to the magnetic linear dichroism in Co 2p XAS for the antiferromagnet CoO with tetragonal distortions.

Hybridization, Crystal Anisotropy and Magnetism in Ce Compounds and Core X-Ray Absorption

We demonstrate a usefulness of the Ce 3d· and 4d·core X-ray absorption spectra (XAS) in probing electronic states of Ce compounds on the basis of an Anderson impurity model. The model includes multiplet effects arising from electrostatic interactions between electrons, a crystalline anisotropy and a molecular field acting on the 4/ spin. We first show that our model well elucidates the effect of the hybridization on the multiplet structure in the 4d XAS for rCe, CeRha and CeO.. A difference in the 3d or 4d XAS between the two incident beams with polarizations parallel and perpendicular to the hexagonal c-axis of CeRhaB2 is then shown to be a promising characteristic to study the 4! occupancy among azimuthal quantum numbers under the hexagonal crystalline field. Finally a difference in the spectrum between the right and left circularly polarized beams in the ferromagnetic CeRhaB2 is shown to be very remarkable even for an observed Ce 4! moment of 0.3~0.5 f.La and is furthermore powerful in measuring the spin and orbital contributions to the 4/ moment.

Multiplet structure of core photoabsorption spectra in mixed valent Ce compounds

Abstract 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 CeCo 2 and metallic Ce compounds is presented.

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.

Theory of core photoemission and absorption spectra in CeO2 and other 4f compounds

Abstract The difference in spectrum between the 3d-core photoemission and L III -edge in an insulating mixed valence compound CeO 2 is explained by introducing 4f-5d Coulomb interaction on the basis of the filled-band Anderson model. A similar method is also applied to discuss the 3d-photoemission and L III -edge spectra in various La and Ce compounds.