16,260 research outputs found
Study of the spin- Hubbard-Kondo lattice model by means of the Composite Operator Method
We study the spin- Hubbard-Kondo lattice model by means of the
Composite Operator Method, after applying a Holstein-Primakov transformation.
The spin and particle dynamics in the ferromagnetic state are calculated by
taking into account strong on-site correlations between electrons and
antiferromagnetic exchange among spins, together with usual Hund
coupling between electrons and spins
Effects of two-site composite excitations in the Hubbard model
The electronic states of the Hubbard model are investigated by use of the
Composite Operator Method. In addition to the Hubbard operators, two other
operators related with two-site composite excitations are included in the
basis. Within the present formulation, higher-order composite excitations are
reduced to the chosen operatorial basis by means of a procedure preserving the
particle-hole symmetry. The positive comparison with numerical simulations for
the double occupancy indicates that such approximation improves over the
two-pole approximation.Comment: 2 pages, 1 figur
Role of the attractive intersite interaction in the extended Hubbard model
We consider the extended Hubbard model in the atomic limit on a Bethe lattice
with coordination number z. By using the equations of motion formalism, the
model is exactly solved for both attractive and repulsive intersite potential
V. By focusing on the case of negative V, i.e., attractive intersite
interaction, we study the phase diagram at finite temperature and find, for
various values of the filling and of the on-site coupling U, a phase transition
towards a state with phase separation. We determine the critical temperature as
a function of the relevant parameters, U/|V|, n and z and we find a reentrant
behavior in the plane (U/|V|,T). Finally, several thermodynamic properties are
investigated near criticality.Comment: 7 pages, 7 figures. EPJB Topical Issue on Novel Quantum Phases and
Mesoscopic Physics in Quantum Gase
Symmetries in the Physics of Strongly Correlated Electronic Systems
Strongly correlated electron systems require the development of new
theoretical schemes in order to describe their unusual and unexpected
properties. The usual perturbation schemes are inadequate and new concepts must
be introduced. In our scheme of calculations, the Composite Operator Method, is
possible to recover, through a self-consistent calculation, a series of
fundamental symmetries by choosing a suitable Hilbert space.Comment: 11 pages, LaTeX, Cmp2e.sty used, submitted to Condensed Matter
Physic
The N-Chain Hubbard model in the Composite Operator Method
We propose a theoretical framework to describe the ladder systems. The
N-chain Hubbard model has been studied within the Composite Operator Method. In
this scheme of calculations the single-particle Green's function for any number
of coupled chains is obtained by solving self-consistently a system of integral
equations.Comment: 6 pages, 1 embedded Postscript figure, LaTeX, to be published in
Physica
Bosonic sector of the two-dimensional Hubbard model studied within a two-pole approximation
The charge and spin dynamics of the two-dimensional Hubbard model in the
paramagnetic phase is first studied by means of the two-pole approximation
within the framework of the Composite Operator Method. The fully
self-consistent scheme requires: no decoupling, the fulfillment of both Pauli
principle and hydrodynamics constraints, the simultaneous solution of fermionic
and bosonic sectors and a very rich momentum dependence of the response
functions. The temperature and momentum dependencies, as well as the dependency
on the Coulomb repulsion strength and the filling, of the calculated charge and
spin susceptibilities and correlation functions are in very good agreement with
the numerical calculations present in the literature
Frustration-driven QPT in the 1D extended anisotropic Heisenberg model
By using Density Matrix Renormalization Group (DMRG) technique we study the
1D extended anisotropic Heisenberg model. We find that starting from the
ferromagnetic phase, the system undergoes two quantum phase transitions (QPTs)
induced by frustration. By increasing the next-nearest-neighbor (NNN)
interaction, the ground state of the system changes smoothly from a completely
polarized state to a NNN correlated one. On the contrary, letting the in-plane
interaction to be greater than the out-of-plane one, the ground state changes
abruptly.Comment: 4 pages, 4 figures, to be presented at CSMAG-07 Kosice, Slovakia,
July 200
Equations of motion approach to the spin-1/2 Ising model on the Bethe lattice
We exactly solve the ferromagnetic spin-1/2 Ising model on the Bethe lattice
in the presence of an external magnetic field by means of the equations of
motion method within the Green's function formalism. In particular, such an
approach is applied to an isomorphic model of localized Fermi particles
interacting via an intersite Coulomb interaction. A complete set of
eigenoperators is found together with the corresponding eigenvalues. The
Green's functions and the correlation functions are written in terms of a
finite set of parameters to be self-consistently determined. A procedure is
developed, that allows us to exactly fix the unknown parameters in the case of
a Bethe lattice with any coordination number z. Non-local correlation functions
up to four points are also provided together with a study of the relevant
thermodynamic quantities.Comment: RevTex, 29 pages, 13 figure
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