624 research outputs found
Antiferromagnetic ordering of itinerant systems in modified mean-field theory
This is an analysis of the itinerant model for antiferromagnetism, in which
is included both on-site and inter-site electron correlations. We also consider
the band degeneration, which brings into the Hamiltonian the on-site exchange
interactions. The Green function technique is used and the coherent potential
approximation (CPA) decoupling for the on-site Coulomb repulsion. This
decoupling combined with the modified Hartree-Fock approximation for the
inter-site interactions generates the change in shape of the spin bands with
growing interaction constants, which is described by the correlation factors
and which decreases the kinetic energy of the system. The effective Hartree
field and the gain in kinetic energy due to the on-site and inter-site
correlation factors drive the antiferromagnetism. The on-site and inter-site
interactions act together towards the antiferromagnetism. Their cooperation
decreases the interaction constants required for the antiferromagnetic
ordering. This new approach allows for the antiferromagnetic instability in the
purely itinerant model at the half-filling in the split band limit. This
situation describes the high temperature superconducting cuprates.Comment: 26 pages, 9 figure
Magnetic ordering of itinerant systems in modified mean field theory: antiferromagnetism
We analyze the itinerant model for antiferromagnetism, which was developed
previously by Plischke, Mattis, Brouers and Mizia. In this model we include
both; single-site and two-site electron correlations. Including additionally
band degeneration into the model allows for considering intra-atomic exchange
interactions in the Hamiltonian. The modified Hartree-Fock approximation for
the two-site interactions is used. This approximation gives the spin band
narrowing, which is the same for both spin directions and affects possibility
of antiferromagnetic ordering. We use the Green function technique and CPA
decoupling. This allows for the change in shape of the spin bands, described by
the correlation factors, which decreases kinetic energy of the system. The
effective Hartree field and the gain in kinetic energy due to the on-site and
inter-site correlation factors drive the antiferromagnetism.Comment: 8 pages, 1 figur
Hubbard Model with Inter-Site Kinetic Correlations
We introduced the inter-site electron-electron correlation to the Hubbard III
approximation. This correlation was excluded in the Hubbard III approximation
and also in the equivalent coherent potential approximation. Including it
brings two spin dependent effects: the bandwidth correction and the bandshift
correction, which both stimulate the ferromagnetic ground state. The bandshift
correction factor causes an exchange splitting between the spin-up and
spin-down spectrum, and its role is similar to the exchange interaction in the
classic Stoner model. The spin dependent bandwidth correction lowers the
kinetic energy of electrons by decreasing the majority spin bandwidth for some
electron occupations with respect to the minority spin bandwidth. In certain
conditions it can lead to ferromagnetic alignment. A gain in the kinetic energy
achieved in this way is the opposite extreme to the effect of a gain in
potential energy due to exchange splitting. The bandshift factor is a dominant
force behind the ferromagnetism. The influence of the bandwidth factor is too
weak to create ferromagnetism and the only result is the correction to the
classic coherent potential approximation in favor of ferromagnetism.Comment: 29 pages, 5 figure
Alternative equation of motion approach to the single-impurity Anderson model
Solving the single-impurity Anderson model (SIAM) is a basic problem of solid
state physics. The SIAM model is very important, at present it is also used for
systems with quantum impurities, e.g. semiconductor quantum dots and molecular
transistors. Its main application is in the scheme of dynamical mean field
theory (DMFT) describing strong correlation electron systems. To solve the SIAM
problem we use the equation of motion (EOM) Green function approach. In this
report we present the novel EOM approximation in which we differentiate the
Green function over both time variables. This differs from the commonly used
EOM solution by Appelbaum, Penn and Lacroix where the authors take time
derivative only over primary time variable. After extending calculations to
higher order Green functions we find the new approximate dynamical solution of
SIAM. The results are compared with the solutions to the SIAM problem at
intermediate Coulomb repulsion U such as the Modified Iterative Perturbation
Theory. Our approach is suitable for describing quantum dots.Comment: 6 pages, 3 figure
Phase separation near half-filling point in superconducting compounds
We present the model of superconducting ceramics using the single band
extended Hubbard Hamiltonian. We investigate the simultaneous presence of
antiferromagnetism (AF) and d-wave superconductivity (SC) in the coherent
potential (CP) approximation applied to the on-site Coulomb repulsion . We
consider the hopping interaction, , the inter-site charge-charge
interaction, , (creating SC), and the single site Hund's type exchange
interaction, , (creating AF). The influence of these interactions on
the separation of superconducting and antiferromagnetic phases near the
half-filling point is investigated. Results are compared with the experimental
data for YBaCuO and NdCeCuO compounds.Comment: 4 pages, 4 figure
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