5f-Electron Phenomena in the Metallic State

In this chapter, the properties of actinides in the metallic state will be reviewed with an emphasis on those properties which are unique or predominantly found in the metallic solid state. Such properties include magnetism, superconductivity, enhanced mass, spin and charge-density waves, as well as quantum critical points. An introduction to fundamental condensed matter principles is included to focus the discussion on the properties in the metallic state. Systematics of the actinide 5f electronic structure will be presented for elements, alloys, metallic, and semi-metallic compounds so as to elucidate the unique characteristics that arise from the properties of actinides and 5f electrons in a periodic potential.

There are two defining characteristics to materials in the metallic state: first, the material exhibits a periodic potential which controls much of the electronic structure, and second, there is a finite density of electronic states at the chemical potential which influences, among other properties, the thermodynamic andtransport characteristics. For the early actinide metals, these two characteristics are often manifested as narrow bands containing a substantial 5f electron component. Complexity in material properties often arises when competing or overlapping energy scales are available. In the metallic state, with a continuum of electron energy levels available, there is the possibility for interaction of charge with spin and lattice degrees of freedom. Because the actinides have an open 5f electron shell which, in the metallic state, often straddles the boundary between localized and itinerant character, the interplay between spin, charge, and lattice degrees of freedom leads to varied and interesting properties. In order to better understand the controlling role of the 5f electrons in the metallic state, one should look beyond the elements and beyond standard temperature and pressure. To elucidate the fundamental properties of 5f electrons in the metallic state, we consider the actinide elements at low temperature and high pressure. An additional dimension to the understanding of the 5f metals can be attained by considering the actinide elements in a metallic host matrix, e.g. alloys and compounds.