Structural distortion and magnetism in transition metal oxides: crucial roles of orbital degrees of freedom

A characteristic feature of the physics in transition-metal oxides is that the charge, spin, and lattice degrees of freedom are strongly coupled. The key to understanding these strong mutual couplings is the orbital degree of freedom (ODF), which plays a crucial role in controlling the phases and various physical properties. We have been working on TMO extensively in recent years. Examples are FeO and MnO, La1-xSrxMnO3, Ca2-xSrxRuO4, Sr2FeMoO6, and SrTiO3, not only for the bulk but also for the surfaces. A review will be given in this article, with concentration on the strong coupling between the structural distortion and the magnetism mediated by ODF. Most of the studies were conducted by our STATE (simulational tool for atom technology) code, which is particularly designed for the transition-metal systems. Some particular aspects of STATE code, such as LDA + U method and virtual crystal approximation, will be also discussed.