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

Current-sensitive single-gun color cathode ray tube

Nonlinear phosphors for production of current sensitive single gun color cathode ray tube

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

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.Comment: 7 pages, 3 embedded Postscript figures, EuroTeX, submitted to EuroPhysics Letter

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-32\frac32 Hubbard-Kondo lattice model by means of the Composite Operator Method

We study the spin-$\frac32$ 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 $\frac32$ spins, together with usual Hund coupling between electrons and spins

Non-ergodic dynamics of the extended anisotropic Heisenberg chain

The issue of ergodicity is often underestimated. The presence of zero-frequency excitations in bosonic Green's functions determine the appearance of zero-frequency momentum-dependent quantities in correlation functions. The implicit dependence of matrix elements make such quantities also relevant in the computation of susceptibilities. Consequently, the correct determination of these quantities is of great relevance and the well-established practice of fixing them by assuming the ergodicity of the dynamics is quite questionable as it is not justifiable a priori by no means. In this manuscript, we have investigated the ergodicity of the dynamics of the $z$-component of the spin in the 1D Heisenberg model with anisotropic nearest-neighbor and isotropic next-nearest-neighbor interactions. We have obtained the zero-temperature phase diagram in the thermodynamic limit by extrapolating Exact and Lanczos diagonalization results computed on chains with sizes $L = 6 \div 26$. Two distinct non-ergodic regions have been found: one for $J^\prime/J_z \lesssim 0.3$ and $|J_\perp|/J_z < 1$ and another for $J^\prime/J_z \lesssim 0.25$ and $|J_\perp|/J_z = 1$. On the contrary, finite-size scaling of $T \neq 0$ results, obtained by means of Exact diagonalization on chains with sizes $L = 4 \div 18$, indicates an ergodic behavior of dynamics in the whole range of parameters.Comment: 6 pages, 7 figure