Orbital ordering phenomena in d- and f-electron systems

d及びf電子系の軌道秩序現象

堀田 貴嗣

Hotta, Takashi

近年、d及びf電子系物質の特異な磁性現象が物性物理分野において活発に議論されている。これらの物質群は、強相関電子系という同一のカテゴリーに括られることが多い。たとえば、両方の物質に共通の性質として、活性な軌道自由度の存在が挙げられる。その一方で、f電子系では一般にスピン・軌道相互作用が強く、低エネルギーの物理はd電子系のそれとは異なって見える。そのためこれらの物質群は、これまで別々のコミュニティで独立して研究されることが多かった。しかし、自然は複雑ではあるが気まぐれではない。軌道自由度の存在のために、一見すると非常に複雑に見える磁性現象も、d及びf軌道によらない共通の原理で理解することができるのである。このレビューでは、d及びf電子系のそれぞれの複雑な磁気相図を理解するとともに、軌道秩序の概念によって、それらに共通の「軌道物理」の姿を浮き彫りにし、磁性物理学の新たなパラダイムへの扉を開く。

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.