Itinerant electron coupling

The mixed-valence iron oxides provide an experimental test-bed for studying the evolution of charge-transfer processes from the localized-electron to the itinerant-electron regimes. Moreover, it is possible to monitor the influence of the charge transfer on the interatomic magnetic coupling since the iron ions in oxides carry localized magnetic moments. 

The same difference can be shown to distinguish between cyclic systems with 4m and 4m + 2 carbon atoms respectively. The n = 0 energy level has degeneracies of m and 2m +1, for these respective systems. It is apparent that the zero level of 4m cyclics cannot accommodate more than 2m itinerant electrons, which means that the 2m odd couples cannot be delocalized to yield an aromatic system. The best known example of a non-aromatic conjugated cyclic molecule is cyclo-octatetraene, shown in Figure 6.5. 

The electronic structure of rare-earth atoms with the configuration (Xe)4f"5d 6s gives rise to conduction bands with 5d and 6s character in the intermetallic compounds. The d bands can have a rather large density of states. Therefore the coupling of phonons to these itinerant electrons can be quite strong. In this chapter we review experimental effects caused by the interaction of conduction electrons with sound waves. These experiments mainly concern the anomalous temperature dependence of symmetry elastic constants and the presence of magnetoacoustic quantum oscillations observed in sound velocity and attenuation. 

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