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A. Avella

Publications and source records attributed to A. Avella.

33 records · Page 2Linked to original sources
arXiv2007

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.

cond-mat.str-el↗
arXiv2006

Exact solution of the one-dimensional spin-$\frac32$ Ising model in magnetic field

In this paper, we study the Ising model with general spin $S$ in presence of an external magnetic field by means of the equations of motion method and of the Green's function formalism. First, the model is shown to be isomorphic to a fermionic one constituted of $2S$ species of localized particles interacting via an intersite Coulomb interaction. Then, an exact solution is found, for any dimension, in terms of a finite, complete set of eigenoperators of the latter Hamiltonian and of the corresponding eigenenergies. This explicit knowledge makes possible writing exact expressions for the corresponding Green's function and correlation functions, which turn out to depend on a finite set of parameters to be self-consistently determined. Finally, we present an original procedure, based on algebraic constraints, to exactly fix these latter parameters in the case of dimension 1 and spin $\frac32$. For this latter case and, just for comparison, for the cases of dimension 1 and spin $\frac12$ [F. Mancini, Eur. Phys. J. B \textbf{45}, 497 (2005)] and spin 1 [F. Mancini, Eur. Phys. J. B \textbf{47}, 527 (2005)], relevant properties such as magnetization $ $ and square magnetic moment $ $, susceptibility and specific heat are reported as functions of temperature and external magnetic field both for ferromagnetic and antiferromagnetic couplings. It is worth noticing the use we made of composite operators describing occupation transitions among the 3 species of localized particles and the related study of single, double and triple occupancy per site.

cond-mat.str-el↗
arXiv2005

The Hubbard model within the equations of motion approach

The Hubbard model has a special role in Condensed Matter Theory as it is considered as the simplest Hamiltonian model one can write in order to describe anomalous physical properties of some class of real materials. Unfortunately, this model is not exactly solved except for some limits and therefore one should resort to analytical methods, like the Equations of Motion Approach, or to numerical techniques in order to attain a description of its relevant features in the whole range of physical parameters (interaction, filling and temperature). In this manuscript, the Composite Operator Method, which exploits the above mentioned analytical technique, is presented and systematically applied in order to get information about the behavior of all relevant properties of the model (local, thermodynamic, single- and two- particle ones) in comparison with many other analytical techniques, the above cited known limits and numerical simulations. Within this approach, the Hubbard model is shown to be also capable to describe some anomalous behaviors of the cuprate superconductors.

cond-mat.str-el↗
arXiv2004

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.

cond-mat.str-el↗
arXiv2002

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.

cond-mat.str-el↗
arXiv2002

The 2D Mott-Hubbard transition in presence of a parallel magnetic field

The half-filled two-dimensional Hubbard model in presence of a uniform and static parallel magnetic field has been studied by means of the Composite Operator Method. A fully self-consistent solution, fulfilling all the constrains coming from the Pauli principle, has been found. The relevant features of a metal-insulator transition in presence of a magnetic field have been analyzed. The results qualitatively agree with the ones recently obtained by means of experimental investigations.

cond-mat.str-el↗
arXiv2002

Non-Ergodicity of the 1D Heisenberg Model

The relevance of zero-energy functions, coming from zero-energy modes and present in the structure of bosonic Green's functions, is often underestimated. Usually, their values are fixed by assuming the ergodicity of the dynamics, but it can be shown that this is not always correct. As the zero-energy functions are connected to fundamental response properties of the system under analysis (specific heat, compressibility, susceptibility, etc.), their correct determination is not an irrelevant issue. In this paper we present some results regarding the zero-energy functions for the Heisenberg chain of spin-1/2 with periodic boundary conditions as functions of the number of sites, temperature and magnetic field. Calculations are pursued for finite chains, using equations of motion, exact diagonalization and Lanczos technique, and the extrapolation to thermodynamic limit is studied.

cond-mat.str-el↗
arXiv1999

Two-scale analysis of the SU(N) Kondo Model

We show how to resolve coherent low-energy features embedded in a broad high-energy background by use of a fully self-consistent calculation for composite particle operators. The method generalizes the formulation of Roth, which linearizes the dynamics of composite operators at any energy scale. Self-consistent equations are derived and analyzed in the case of the single-impurity SU(N) Kondo model.

cond-mat.str-el↗
arXiv1998

Comment on "Symmetry properties of magnetization in the Hubbard model at finite temperature"

The results of G. Su and M. Suzuki [Phys. Rev. B 54, 8291 (1996); ibidem 57, 13367 (1998)] for the spin and pseudo-spin symmetry properties of the Hubbard model are reexamined. We point out that the exact relations they have found are valid down to zero temperature and that their solutions for both spin and pseudo-spin correlation functions are incorrect.

cond-mat.str-el↗
arXiv1998

The half-filled Hubbard chain in the Composite Operator Method: A comparison with Bethe Ansatz

The one-dimensional Hubbard model at half-filling is studied in the framework of the Composite Operator Method using a static approximation. A solution characterized by strong antiferromagnetic correlations and a gap for any nonzero on-site interaction U is found. The corresponding ground-state energy, double occupancy and specific heat are in excellent agreement with those obtained within the Bethe Ansatz. These results show that the Composite Operator Method is an appropriate framework for the half-filled Hubbard chain and can be applied to evaluate properties, like the correlation functions, which cannot be obtained by means of the Bethe Ansatz, except for some limiting cases.

cond-mat.str-el↗
arXiv1998

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.

cond-mat.str-el↗
arXiv1998

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.

cond-mat.str-el↗
arXiv1998

The overdoped regime in LSCO

Recent experimental data for the overdoped LSCO have firmly established the properties which characterize the transition between the superconducting and the Fermi liquid metallic phases. The thermodynamic response functions show a pronounced feature at this point, while the Fermi surface undergoes a dramatic change. By use of the Composite Operator Method for the two-dimensional Hubbard model, it is found that the presence of a van Hove singularity in the lower Hubbard band can explain these behaviors.

cond-mat.supr-con↗
arXiv1997

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.

cond-mat.str-el↗