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Onishi, Hiroaki; Hotta, Takashi
AIP Conference Proceedings 850, p.1075 - 1076, 2006/09
We investigate the ground-state property of a 
-orbital Hubbard model on a triangular lattice at electron density 5.5 by using numerical techniques. There appears several types of paramagnetic phases, but we observe in common that one or two orbitals among three orbitals become relevant due to the effect of orbital ordering. It is found that charge stripe stabilized by the nearest-neighbor Coulomb interaction consists of antiferromagnetic/ferro-orbital chains for small Hund's coupling, while there occurs stripe charge ordering with ferromagnetic/antiferro-orbital chains for large Hund's coupling.
Onishi, Hiroaki; Hotta, Takashi
Physica B; Condensed Matter, 378-380, p.589 - 591, 2006/05
Times Cited Count：1 Percentile：6.64(Physics, Condensed Matter)We investigate the ground-state property of an 
-orbital Hubbard model at quarter filling on a zigzag chain by exploiting the density matrix renormalization group method. When two orbitals are degenerate, the zigzag chain is decoupled to a doble-chain spin system to suppress the spin frustration due to the spatial anisotropy of the occupied orbital. On the other hand, when the level splitting is increased and the orbital anisotropy disappears, a characteristic change in the spin incommensurability is observed due to the revival of the spin frustration.
Hotta, Takashi
Physical Review Letters, 96(19), p.197201_1 - 197201_4, 2006/05
Times Cited Count：40 Percentile：81.37(Physics, Multidisciplinary)A mechanism of non-magnetic Kondo effect is proposed on the basis of a multiorbital Anderson model coupled with dynamical Jahn-Teller (JT) phonons. An electron system coupled dynamically with JT phonons has a vibronic ground state with double degeneracy due to clockwise and anti-clockwise rotational modes with entropy of 
. When a temperature is lower than a characteristic energy to turn the rotational direction, the rotational degree of freedom is eventually suppressed and the corresponding entropy is released, leading to quasi-Kondo behavior. We discuss possible relevance of this quasi-Kondo phenomenon to electronic properties of filled skutterudites.
-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.
-electron systemsHotta, Takashi
Physica B; Condensed Matter, 359-361, p.1003 - 1005, 2005/04
Times Cited Count：1 Percentile：6.23(Physics, Condensed Matter)In order to understand magnetic properties of filled skutterudite materials, we evaluate magnetic susceptibility of the Anderson model with orbital degeneracy by using the numerical renormalization group method. Even when the local ground state is 
, magnetic fluctuations significantly remain, if 
is the excited state with small excitation energy, as experimentally proposed for Pr-based filled skutterudite compounds.
Onishi, Hiroaki; Hotta, Takashi
Physica B; Condensed Matter, 359-361, p.669 - 671, 2005/04
Times Cited Count：3 Percentile：17(Physics, Condensed Matter)By using the density matrix renormalization group method, we investigate ground- and excited-state properties of the -orbital degenerate Hubbard model at quarter filling for two kinds of lattices, zigzag chain and ladder. In the zigzag chain, the system is effectively regarded as a decoupled double chain of the 
=1/2 antiferromagnetic Heisenberg model, and the spin gap is approximately zero, similar to the case of weakly coupled Heisenberg chains. On the other hand, in the ladder, the spin correlation on the rung remains robust and the spin gap exists.
Onishi, Hiroaki; Hotta, Takashi
New Journal of Physics (Internet), 6(1), p.193_1 - 193_16, 2004/12
We discuss a crucial role of orbital degree of freedom to understand the complex magnetic structure of actinide compounds from a microscopic viewpoint. Typical examples which we pick up here are UTGa
and NpTGa, where T denotes a transition metal ion. An orbital degenerate Hubbard model constructed from the 
- coupling scheme is provided to investigate the magnetic structure of actinide compounds. By analyzing the model Hamiltonian with the use of unbiased numerical technique such as exact diagonalization, we obtain the phase diagram including several kinds of magnetic states. An orbital-based scenario is proposed to understand the change in the magnetic structure from G- to A-type antiferromagntic (AF) phases, experimentally observed in UNiGa and UPtGa. We also discuss the appearance of three kinds of AF phases, C-, A-, and G-type, for NpTGa (T=Fe, Co, and Ni).
-orbital degeneracyOnishi, Hiroaki; Hotta, Takashi
Physical Review B, 70(10), p.100402_1 - 100402_4, 2004/09
Times Cited Count：3 Percentile：19.92(Materials Science, Multidisciplinary)In order to clarify a key role of orbital degeneracy in the spin =1 Haldane system, we investigate ground-state properties of the -orbital degenerate Hubbard model on the linear chain by numerical techniques such as DMRG. When the Hund's rule coupling is small, an antiferromagnetic phase with ferro-orbital ordering occurs, which corresponds to the gapped Haldane phase. On the other hand, increasing the Hund's rule coupling, a ferromagnetic phase with antiferro-orbital correlation appears. This phase is described by the pseudo-spin 
=1/2 antiferro-orbital Heisenberg model, i.e., there exists gapless excitation with respect to orbital degree of freedom.
- coupling schemeHotta, Takashi
Physical Review B, 70(5), p.054405_1 - 054405_10, 2004/08
Times Cited Count：17 Percentile：61.26(Materials Science, Multidisciplinary)Key role of orbital degree of freedom to understand the magnetic structure of uranium compounds is discussed from a microscopic viewpoint by focusing on typical examples such as UMGa (M=Ni and Pt) and UGa
. In order to clarify the meanings of orbital and spin in -electron systems, first we examine the validity of a - coupling scheme to compose local -electronic states. Then, an orbital degenerate Hubbard model constructed from the - coupling scheme is provided to investigate the magnetic structure of uranium compounds. By analyzing the model Hamiltonian in the cubic system with the use of numerical technique, we obtain the phase diagram consistent with the experimental results for AuCu-type uranium compounds. Furthermore, taking account of tetragonal effects, an orbital-based scenario is proposed to understand the change in the magnetic structure from G- to A-type antiferromagnetic phases, experimentally observed in UNiGa and UPtGa. Finally, we discuss future problems included in the microscopic theory along the direction presented here.
Hotta, Takashi; Dagotto, E.*
Physical Review Letters, 92(22), p.227201_1 - 227201_4, 2004/06
Times Cited Count：40 Percentile：80.89(Physics, Multidisciplinary)Ground-state properties of layered nickelates are investigated based on the orbital-degenerate Hubbard model coupled with lattice distortions, by using numerical techniques. The N
el state composed of spin =1 ions is confirmed in the undoped limit 
=0. At =1/2, novel antiferromagnetic states, called CE- and E-type phases, are found by increasing the Hund's coupling. (
/
)-type orbital ordering is predicted to occur in a checkerboard-type charge-ordered state. At =1/3, 
Coulombic and phononic interactions are found to be important, since the former stabilizes the spin stripe, while the latter leads to the striped charge-order.
Takimoto, Tetsuya; Hotta, Takashi; Ueda, Kazuo*
Physical Review B, 69(10), p.104504_1 - 104504_9, 2004/03
Times Cited Count：153 Percentile：96.83(Materials Science, Multidisciplinary)In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the 
-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppression of the spin and orbital fluctuations, respectively. It is proposed that the orbital splitting energy is a controlling parameter changing from the paramagnetic to the antiferromagnetic phase with the -wave superconducting phase in between.
Takimoto, Tetsuya; Hotta, Takashi; Ueda, Kazuo*
Journal of Physics; Condensed Matter, 15(28), p.S2087 - S2093, 2003/07
Times Cited Count：14 Percentile：57.88(Physics, Condensed Matter)Rich physics is expected to appear due to the coupling between orbital and spin degrees of freedom in strongly correlated electron system with orbital degeneracy. Especially, it is considered that the superconducting transition in orbital degenerate system is one of the most interesting phenomena. For instance, triplet superconductivity occurs in Sr
RuO
where three cylindrical Fermi surfaces are observed. Also, heavy fermion superconductors are considered to have potentially orbital degree of freedom due to large degeneracy of -electron states. We discuss the effect of orbital fluctuations on superconductivity by using random phase approximation and fluctuation exchange (FLEX) approximation. It is also expected that the FLEX approximation is useful to study the correlation between spin and orbital fluctuations for the orbital degenerate system.
-electron systems on the basis of - coupling schemeHotta, Takashi; Ueda, Kazuo
Physical Review B, 67(10), p.104518_1 - 104518_16, 2003/03
Times Cited Count：87 Percentile：93.31(Materials Science, Multidisciplinary)We construct a microscopic model for -electron systems, composed of -electron hopping, Coulomb interaction, and crystalline electric field terms. based on the - coupling scheme. The Hamiltonian obtained is regarded as an orbital degenerate Hubbard model, since it includes two pseudo-spin and three pseudo-orbital degrees of freedom. For practical purposes, it is further simplified into a couple of two-orbital models by discarding one of the three orbitals. One of those simplified models is here analyzed using the exact diagonalization method to clarify ground-state properties by evaluating several kinds of correlation functions. We attempt to discuss a possible relation of the present results with experimental observations for recently discovered heavy fermion superconductors CeMIn (M=Ir, Co, and Rh), and a comprehensive scenario to understand superconducting and antiferromagnetic tendencies in the so-called ``115'' materials such as CeMIn, UMGa, and PuCoGa from the microscopic viewpoint.
