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-electron systemsOnishi, Hiroaki; Hotta, Takashi
Journal of the Physical Society of Japan, 75(Suppl.), p.266 - 269, 2006/08
In order to clarify multipole correlations in -electron systems, we investigate a one-dimensional -electron model based on a 
- coupling scheme by exploiting a density-matrix renormalization group method. We discuss the effect of a crystalline electric field and the dependence on the change of -electron numbers in detail.
-orbital Hubbard modelXavier, J. C.*; Onishi, Hiroaki; Hotta, Takashi; Dagotto, E.*
Physical Review B, 73(1), p.014405_1 - 014405_9, 2006/01
Times Cited Count：9 Percentile：41.55(Materials Science, Multidisciplinary)We present the zero-temperature phase diagram of the one-dimensional -orbital Hubbard model, by using the density-matrix renormalization group and Lanczos techniques. Emphasis is given to the case for the electron density 
=5, of relevance to Co-based compounds. Several other cases for electron densities between =3 and 6 are also studied. At =5, our results indicate a first-order transition between a paramagnetic (PM) insulator phase and a ferromagnetic (FM) state by tuning the Hund's coupling. The results also suggest a transition from the =5 PM insulator phase to a metallic regime by changing the electron density. The behavior of the spin, charge, and orbital correlation functions in the FM and PM states are discussed.
Onishi, Hiroaki; Hotta, Takashi
Physical Review B, 71(18), p.180410_1 - 180410_4, 2005/05
Times Cited Count：5 Percentile：26.14(Materials Science, Multidisciplinary)In order to clarify a crucial role of orbital degree of freedom in geometrically frustrated systems, we investigate both ground- and excited-state properties of the 
-orbital degenerate Hubbard model on two kinds of lattices, ladder and zigzag chain, by using numerical techniques. In the ladder, spin correlation extends on the whole system, while the zigzag chain is decoupled to a double chain and spin excitation is confined in one side of the double chain due to the selection of a specific orbital. We envision a kind of self-organization phenomenon that the geometrically frustrated multi-orbital system is spontaneously reduced to a one-orbital model to suppress the spin frustration.
