559 research outputs found
New Comparisons for Local Quantities of the Two-Dimensional Hubbard Model
We have compared the results of our approximation scheme, the composite
operator method, for the double occupancy and the internal energy of the
two-dimensional Hubbard model with numerical data obtained by means of the
Lanczos and quantum Monte Carlo schemes. The agreement is very good at both
half-filling and away from it showing how reliable is the approximation scheme.Comment: 6 pages, 3 figure
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
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
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
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
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
The Hubbard model in the two-pole approximation
The two-dimensional Hubbard model is analyzed in the framework of the
two-pole expansion. It is demonstrated that several theoretical approaches,
when considered at their lowest level, are all equivalent and share the
property of satisfying the conservation of the first four spectral momenta. It
emerges that the various methods differ only in the way of fixing the internal
parameters and that it exists a unique way to preserve simultaneously the Pauli
principle and the particle-hole symmetry. A comprehensive comparison with
respect to some general symmetry properties and the data from quantum Monte
Carlo analysis shows the relevance of imposing the Pauli principle.Comment: 12 pages, 8 embedded Postscript figures, RevTeX, submitted to Int.
Jou. Mod. Phys.
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
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