32 research outputs found
Electronic structure and magnetic anisotropy of CrO_2
The problem of importance of strong correlations for the electronic
structure, transport and magnetic properties of half--metallic ferromagnetic
CrO_2 is addressed by performing density functional electronic structure
calculations in the local spin density approximation (LSDA) as well as using
the LSDA+U method. It is shown that the corresponding low--temperature
experimental data are best fitted without accounting for the Hubbard U
corrections. We conclude that the ordered phase of CrO$_2 is weakly correlated.Comment: 5 pages, 7 EPS figures, in RevTex forma
Spin and orbital excitation spectrum in the Kugel-Khomskii model
We discuss spin and orbital ordering in the twofold orbital degenerate
superexchange model in three dimensions relevant to perovskite transition metal
oxides. We focus on the particular point on the classical phase diagram where
orbital degeneracy is lifted by quantum effects exclusively. Dispersion and
damping of the spin and orbital excitations are calculated at this point taking
into account their mutual interaction. Interaction corrections to the
mean-field order parameters are found to be small. We conclude that
quasi-one-dimensional Neel spin order accompanied by the uniform
d_{3z^2-r^2}-type orbital ordering is stable against quantum fluctuations.Comment: 4 pages with 3 PS figures, 1 table, RevTeX, accepted to Phys. Rev. B.
Rapid Communicatio
Interpolative Approach for Solving the Anderson Impurity Model
A rational representation for the self--energy is explored to interpolate the
solution of the Anderson impurity model in general orbitally degenerate case.
Several constrains such as the Friedel's sum rule, positions of the Hubbard
bands as well as the value of quasiparticle residue are used to establish the
equations for the coefficients of the interpolation. We employ two fast
techniques, the slave--boson mean--field and the Hubbard I approximations to
determine the functional dependence of the coefficients on doping, degeneracy
and the strength of the interaction. The obtained spectral functions and
self--energies are in good agreement with the results of numerically exact
quantum Monte Carlo method.Comment: 15 pages, 9 figure
Dynamical correlations in multiorbital Hubbard models: Fluctuation-exchange approximations
We study the two band degenerate Hubbard model using the Fluctuation Exchange
approximation (FLEX) method and compare the results with Quantum Monte-Carlo
calculations. Both the self-consistent and the non-self-consistent versions of
the FLEX scheme are investigated. We find that, contrary to the one band case,
in the multiband case, good agreement with the Quantum Monte-Carlo results is
obtained within the electron-electron T-matrix approximation using the full
renormalization of the one-particle propagators. The crossover to strong
coupling and the formation of satellites is more clearly visible in the
non-self-consistent scheme. Finally we discuss the behavior of the FLEX for
higher orbital degeneracy.Comment: 18 pages with 12 PS figure
Orbital-selective Mott transitions in the degenerate Hubbard model
We investigate the Mott transitions in two-band Hubbard models with different
bandwidths. Applying dynamical mean field theory, we discuss the stability of
itinerant quasi-particle states in each band. We demonstrate that separate Mott
transitions occur at different Coulomb interaction strengths in general, which
merge to a single transition only under special conditions. This kind of
behavior may be relevant for the physics of the single-layer ruthenates,
CaSrRuO.Comment: 4 pages, 4 figure
Electronic spectrum in high-temperature cuprate superconductors
A microscopic theory for electronic spectrum of the CuO2 plane within an
effective p-d Hubbard model is proposed. Dyson equation for the single-electron
Green function in terms of the Hubbard operators is derived which is solved
self-consistently for the self-energy evaluated in the noncrossing
approximation. Electron scattering on spin fluctuations induced by kinematic
interaction is described by a dynamical spin susceptibility with a continuous
spectrum. Doping and temperature dependence of electron dispersions, spectral
functions, the Fermi surface and the coupling constant are studied in the hole
doped case. At low doping, an arc-type Fermi surface and a pseudogap in the
spectral function are observed.Comment: 13 pages (revtex), 18 figures, to be published in JET
Thermoelectric properties of the degenerate Hubbard model
We investigate the thermoelectric properties of a system near a pressure
driven Mott-Hubbard transition. The dependence of the thermopower and the
figure of merit on pressure and temperature within a degenerate Hubbard model
for integer filling n=1 is calculated using dynamical mean field theory.
Quantum Monte Carlo method is used to solve the impurity model. Obtained
results can qualitatively explain thermoelectric properties of various strongly
correlated materials.Comment: RevTex, 7 pages, 6 figure
Phase diagram of orbital-selective Mott transitions at finite temperatures
Mott transitions in the two-orbital Hubbard model with different bandwidths
are investigated at finite temperatures. By means of the self-energy functional
approach, we discuss the stability of the intermediate phase with one orbital
localized and the other itinerant, which is caused by the orbital-selective
Mott transition (OSMT). It is shown that the OSMT realizes two different
coexistence regions at finite temperatures in accordance with the recent
results of Liebsch. We further find that the particularly interesting behavior
emerges around the special condition and J=0, which includes a new type
of the coexistence region with three distinct states. By systematically
changing the Hund coupling, we establish the global phase diagram to elucidate
the key role played by the Hund coupling on the Mott transitions.Comment: 4 pages, 6 figure