Orbital ordering phenomena in d- and f-electron systems

Hotta, Takashi

In recent decades, novel magnetism of d- and f-electron compounds has been discussed very intensively both in experimental and theoretical research fields of condensed matter physics. It has been recognized that those material groups are in the same category of strongly correlated electron systems, while the low-energy physics of d- and f-electron compounds has been separately investigated rather in different manners. One of common features of both d- and f-electron systems is certainly the existence of active orbital degree of freedom, but in f-electron materials, due to the strong spin-orbit interaction in rare-earth and actinide ions, the physics seems to be quite different from that of d-electron systems. It is important to seek for a simple principle persisting complicated phenomena in common with d- and f-electron materials, which opens the door to a new stage in orbital physics. In this sense, it is considered to be an important task of this article to explain common features of magnetism in d- and f-electron systems from a microscopic viewpoint, using a key concept of orbital ordering, in addition to the review of the complex phase diagram of each material group. It is emphasized that the same orbital physics works both in d- and f-electron complex materials in spite of the difference between d and f orbitals.