65,281 research outputs found

    Competition between Hund-Rule Coupling and Kondo Effect

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    We investigate a problem about the competition between the hybridization and the Hund-rule coupling by applying the Wilson numerical renormalization-group method to the extended Kondo model where the impurity spin interacts via the Hund-rule coupling, with an extra spin which is isolated from the conduction electrons. It is shown that the Hund-rule coupling is an irrelevant perturbation against the strong coupling fixed point. However, the Hund-rule coupling decreases the characteristic energy TKT_K drastically to the lower side and the irrelevant operator, which describes the low energy physics, takes a form of ferromagnetic exchange interaction between the extra spin and the Kondo resonance states because of the existence of the Hund-rule coupling.Comment: 5 pages, LaTeX, 5 figures in one PS file, Submitted to J. Phys. Soc. Jp

    Heavy Quasi-Particle in the Two-Orbital Hubbard Model

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    The two-orbital Hubbard model with the Hund coupling is investigated in a metallic phase close to the Mott insulator. We calculate the one-particle spectral function and the optical conductivity within dynamical mean field theory, for which the effective impurity problem is solved by using the non-crossing approximation. For a metallic system close to quarter filling, a heavy quasi-particle band is formed by the Hubbard interaction, the effective mass of which is not so sensitive to the orbital splitting and the Hund coupling. In contrast, a heavy quasi-particle band near half filling disappears in the presence of the orbital splitting, but is induced again by the introduction of the Hund coupling, resulting in a different type of heavy quasi-particles.Comment: 6page, 7eps figures, to appear in J. Phys. Soc. Jp

    Effect of Hund coupling in the one-dimensional SU(4) Hubbard model

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    The one-dimensional SU(4) Hubbard model perturbed by Hund coupling is studied, away from half-filling, by means of renormalization group and bosonization methods. A spectral gap is always present in the spin-orbital sector irrespective of the magnitude of the Coulomb repulsion. We further distinguish between two qualitatively different regimes. At small Hund coupling, we find that the symmetry of the system is dynamically enlarged to SU(4) at low energy with the result of {\it coherent} spin-orbital excitations. When the charge sector is not gapped, a superconducting instability is shown to exist. At large Hund coupling, the symmetry is no longer enlarged to SU(4) and the excitations in the spin sector become {\it incoherent}. Furthermore, the superconductivity can be suppressed in favor of the conventional charge density wave state.Comment: 10 pages, 1 figur

    Microscopic origin of isotropic non-Heisenberg behavior in highly correlated systems

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    We have reanalyzed the microscopic origin of the isotropic deviations that are observed from the energy spacings predicted by the HDVV Hamiltonian. Usually, a biquadratic spin operator is added to the HDVV Hamiltonian to account for such deviations. It is shown here that this operator cannot describe the effect of the excited atomic non-Hund states which brought the most important contribution to the deviations. For systems containing more than two magnetic centers, non-Hund states cause additional interactions that are of the same order of magnitude as the biquadratic exchange and should have significant effects on the macroscopic properties of extended systems.Comment: 4 pages, 3 figure

    Stability of a metallic state in the two-orbital Hubbard model

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    Electron correlations in the two-orbital Hubbard model at half-filling are investigated by combining dynamical mean field theory with the exact diagonalization method. We systematically study how the interplay of the intra- and inter-band Coulomb interactions, together with the Hund coupling, affects the metal-insulator transition. It is found that if the intra- and inter-band Coulomb interactions are nearly equal, the Fermi-liquid state is stabilized due to orbital fluctuations up to fairly large interactions, while the system is immediately driven to the Mott insulating phase away from this condition. The effects of the isotropic and anisotropic Hund coupling are also addressed.Comment: 7 pages, 9 figure

    Spiral phase and phase separation of the double exchange model in the large-S limit

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    The phase diagram of the double exchange model is studied in the large-S limit at zero temperature in two and three dimensions. We find that the spiral state has lower energy than the canted antiferromagnetic state in the region between the antiferromagnetic phase and the ferromagnetic phase. At small doping, the spiral phase is unstable against phase separation due to its negative compressibility. When the Hund coupling is small, the system separates into spiral regions and antiferromagnetic regions. When the Hund coupling is large, the spiral phase disappears completely and the system separates into ferromagnetic regions and antiferromagnetic regions.Comment: 7 pages, 3 postscript figures. To be published in Phys. Rev.
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