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Abstract
The article reviews the rich phenomena of metal-insulator transitions, anomalous metalicity, taking as examples iron and titanium oxides. The diverse phenomena include strong spin and orbital fluctuations, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. Another important phenomena presented in the article is a valence fluctuation which occur often in rare-earth compounds. We consider some Ce, Sm, Eu, Tm, and Yb compounds such as Ce, Sm and Tm monochalcogenides, Sm and Yb borides, mixed-valent and charge-ordered Sm, Eu and Yb pnictides and chalcogenides R4X3and R3X4(R = Sm, Eu, Yb; X = As, Sb, Bi), intermediate-valence YbInCu4and heavy-fermion compounds YbMCu4(M = Cu, Ag, Au, Pd). Issues addressed include the nature of the electronic ground states, the metal-insulator transition, the electronic and magnetic structures. The discussion includes key experiments, such as optical and magneto-optical spectroscopic measurements, x-ray photoemission and x-ray absorption, bremsstrahlung isochromat spectroscopy measurements as well as x-ray magnetic circular dichroism.
Highlights
Determination of the energy band structure of solids is a many-body problem
In order to separate the influence of the lattice constant from the influence of the ionic potential of the pnictide component on the electronic structure of Sm4X3 (X = As, Sb, and Bi), we present in Figure 83(b) the density of states (DOSs) of Sm4Bi3 evaluated with the lattice constant of Sm4Sb3 (a = 9.308 A ) and for a = 9.561 A
A fundamental aspect of this observation is that we find the pinning of the 4 f 14 state at EF to be a genetic property of YbInCu4 compound: it happens irrespective of the precise value of Ueff
Summary
Determination of the energy band structure of solids is a many-body problem. Band theory, a mean-field theory to treat this problem, in the framework of the local spin density approximation (LSDA), has been successful for many kinds of materials and has become the de facto tool of firstprinciples calculations in solid-state physics. It is necessary to distinguish between homogeneously mixed-valence compounds and inhomogeneously mixedvalence compounds In the former, all the rare-earth ions occupy crystallographically equivalent sites, and, this is essentially a single ion property where the magnetic ion hybridizes with the sea of the conduction electrons, causing an exchange of the inner 4 f electron with the conduction band at the Fermi level. Such effects are expected to arise in systems where two electron configurations corresponding to 4 f occupation numbers n and n − 1 have nearly degenerate energies.
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