65,281 research outputs found
Competition between Hund-Rule Coupling and Kondo Effect
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 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
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
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
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
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
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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