Charge ordering, charge fluctuations and lattice effects in strongly correlated electron systems

Abstract

Charge fluctuation and charge-ordering phenomena in compounds based on the 3d electrons of transition-metal ions and 4f electrons of rare-earth ions are reviewed with particular emphasis on the mutual coupling of charge and lattice degrees of freedom. For the description of charge ordering in inhomogeneously mixed-valence compounds of localized 3d or 4f-electron systems, we employ Landau's phenomenological theory for second-order phase transitions. By use of the group-theoretical method, the charge fluctuation mode corresponding to the active representation for the second-order transition is determined. The localization of 3d and 4f electrons makes the valence for the specific ions an integer number of charge units e in the ordered phases at low temperatures. The elastic soft mode observed by the ultrasonic method is often a useful indication for the charge fluctuation mode that is frozen below the charge-ordering point T C. The transverse c44 mode exhibiting a considerable softening in the rare-earth com...