摘要: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
R4X3 and R3X4 (R = Sm, Eu, Yb; X = As, Sb, Bi), intermediate-valence YbInCu4 and 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.