44,721 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
Magnetic behavior of a spin-1 Blume-Emery-Griffiths model
I study the one-dimensional spin-1 Blume-Emery-Griffiths model with bilinear
and biquadratic exchange interactions and single-ion crystal field under an
applied magnetic field. This model can be exactly mapped into a tight-binding
Hubbard model - extended to include intersite interactions - provided one
renormalizes the chemical and the on-site potentials, which become temperature
dependent. After this transformation, I provide the exact solution of the
Blume-Emery-Griffiths model in one dimension by means of the Green's functions
and equations of motion formalism. I investigate the magnetic variations of
physical quantities - such as magnetization, quadrupolar moment, susceptibility
- for different values of the interaction parameters and of the applied field,
focusing on the role played by the biquadratic interaction in the breakdown of
the magnetization plateaus.Comment: 4 pages, 5 figures. ICM 2009 (Karlsruhe) Conference proceeding
Different orderings in the narrow-band limit of the extended Hubbard model on the Bethe lattice
We present the exact solution of a system of Fermi particles living on the
sites of a Bethe lattice with coordination number z and interacting through
on-site U and nearest-neighbor V interactions. This is a physical realization
of the extended Hubbard model in the atomic limit. Within the Green's function
and equations of motion formalism, we provide a comprehensive analysis of the
model and we study the phase diagram at finite temperature in the whole model's
parameter space, allowing for the on-site and nearest-neighbor interactions to
be either repulsive or attractive. We find the existence of critical regions
where charge ordering (V>0) and phase separation (V<0) are observed. This
scenario is endorsed by the study of several thermodynamic quantities.Comment: 17 pages, 20 figure
One-dimensional extended Hubbard model in the atomic limit
We present the exact solution of the one-dimensional extended Hubbard model
in the atomic limit within the Green's function and equation of motion
formalism. We provide a comprehensive and systematic analysis of the model by
considering all the relevant response and correlation functions as well as
thermodynamic quantities in the whole parameter space. At zero temperature we
identify four phases in the plane (U,n) [U is the onsite potential and n is the
filling] and relative phase transitions as well as different types of charge
ordering. These features are endorsed by investigating at T=0 the chemical
potential and pertinent local correlators, the particle and double occupancy
correlation functions, the entropy, and by studying the behavior in the limit T
going to zero of the charge and spin susceptibilities. A detailed study of the
thermodynamic quantities is also presented at finite temperature. This study
evidences that a finite-range order persists for a wide range of the
temperature, as shown by the behavior of the correlation functions and by the
two-peak structure exhibited by the charge susceptibility and by the entropy.
Moreover, the equation of motion formalism, together with the use of composite
operators, allows us to exactly determine the set of elementary excitations. As
a result, the density of states can be determined exactly and a detailed
analysis of the specific heat allows for identifying the excitations and for
ascribing its two-peak structure to a redistribution of the charge density.Comment: 28 pages;added references and corrected typos. This paper is an
extended version of Phys. Rev. E 77, 061120 (2008
The Hubbard model: bosonic excitations and zero-frequency constants
A fully self-consistent calculation of the bosonic dynamics of the Hubbard
model is developed within the Composite Operator Method. From one side we
consider a basic set of fermionic composite operators (Hubbard fields) and
calculate the retarded propagators. On the other side we consider a basic set
of bosonic composite operators (charge, spin and pair) and calculate the causal
propagators. The equations for the Green's functions (GF) (retarded and
causal), studied in the polar approximation, are coupled and depend on a set of
parameters not determined by the dynamics. First, the pair sector is
self-consistently solved together with the fermionic one and the zero-frequency
constants (ZFC) are calculated not assuming the ergodic value, but fixing the
representation of the GF in such a way to maintain the constrains required by
the algebra of the composite fields. Then, the scheme to compute the charge and
spin sectors, ZFCs included, is given in terms of the fermionic and pair
correlators
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
Different realizations of tomographic principle in quantum state measurement
We establish a general principle for the tomographic approach to quantum
state reconstruction, till now based on a simple rotation transformation in the
phase space, which allows us to consider other types of transformations. Then,
we will present different realizations of the principle in specific examples.Comment: 17 pages, Latex file, no figures, accepted by J. of Mod. Op
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