On the CPA treatment of Magnetic Disordered Hubbard model (II)

In this contribution we continue the treatment of the model based on the Hubbard hamiltonian in which the stochastic fields acting at the sites of the atoms can assume arbitrary directions and values due to accepted distribution. The averaging procedure introduced by Brandt and Gross to study the behaviour of the magnetically disordered system is used for a model with stochastically localized magnetic fields, resulting from magnetic moments, at the atomic sites, as previously in I. The coherent potential approximation-like method is employed. The selfconsistency problem is regarded as well as the thermodynamic consequences like density of states, localization function, magnetization, high temperature susceptibility and dc conductivity as functions of temperature are discussed.Supported in part by Polish Academy of Science, Problem MR I. 9. Partly done at Institute of Physics and Chemistry of Metals, Silesian University, Katowice, Poland     

On the integrability of the SU(N) Hubbard model

We exhibit explicitly the intertwiner operator for the monodromy matrices of the SU(N) Hubbard model recently proposed by Maassarani [Phys. Lett. A 239 (1998) 187]. This produces a new family of non-additive R-matrices and generalizes an earlier result by Shastry [Phys. Rev. Lett. 56 (1986) 2453; J. Stat. Phys. 30 (1988) 57].     

Ferromagnetism and metal-insulator transition in the disordered Hubbard model

A detailed study of the paramagnetic to ferromagnetic phase transition in the one-band Hubbard model in the presence of binary-alloy disorder is presented. The influence of the disorder (with concentrations x and 1-x of the two alloy ions) on the Curie temperature T(c) is found to depend strongly on electron density n. While at high densities, n>x, the disorder always reduces T(c); at low densities, n相似文献   

Coupling of fluctuation modes in the disordered strongly attractive Hubbard model

The coupling of two different types of soft modes related to interaction-generated superconducting and disorder-generated (pseudo)diffusive fluctuations in the large negative-U Hubbard model with random local energies is studied in the framework of a Grassmann field theoretic approach with modified semionic Matsubara frequencies. In leading loop order it is found that the sound-like order parameter phase fluctuation mode retains its massless nature with only its velocity being modified, as expected from the generalized Ward identity. The disorder-induced fluctuations which are originally completely local due to the locality of disorder correlations acquire true diffusive behaviour by interaction effects in the superconducting phase.     

On ferromagnetism in the Hubbard model

We use a simple variational procedure for strictly excluding a totally ferromagnetic groundstate of the Hubbard model for the cubic lattices and a certain range of values of the electron concentrationn _e and the Coulomb repulsionV _o. The curve bounding this domain becomes exact in the limit ofn _e → 1, previously considered by Nagaoka, and leads to qualitatively correct results also in the opposite limiting cases ofn _e → 0, and 2, which were investigated by Kanamori.     

The su(n) Hubbard model

The one-dimensional Hubbard model is known to possess an extended su(2) symmetry and to be integrable. I introduce an integrable model with an extended su(n) symmetry. This model contains the usual su(2) Hubbard model and has a set of features that makes it the natural su(n) generalization of the Hubbard model. Complete integrability is shown by introducing the L-matrix and showing that the transfer matrix commutes with the Hamiltonian. While the model is integrable in one dimension, it provides a generalization of the Hubbard Hamiltonian in any dimension.     

Density of states of a disordered Hubbard model using the modified moments method

We apply the modified-moments method to compute the density of states of the impurity band of a doped semi-conductor in the intermediate region of impurity concentrations. This method is used to correct the density of states obtained by interpolating between the high and low concentration limit asymptotic expressions. The calculation is based on the Hubbard model in the atomic limit without spin ordering. The overlap integral is assumed to be a Gaussian function of the impurity separation. Use is made of the first seven moments of the exact distribution and of the low and high concentration limit approximations previously calculated. The first six moments are employed to determine the orthogonal polynomial expansion of the density of states while the seventh moment is used as a check on the accuracy of the distribution obtained. The results are similar to the previous ones using a truncated Edgeworth series for the correction term but the present method has the advantage of being a more systematic approach.     

On high-temperature expansions of the Hubbard model

We discuss the application of the high-temperature expansion method to the Hubbard model. We recalculate the expansion series of the susceptibility up to the sixth order in the transfer matrix element,t, in the strong correlation limit, and up to the fourth order int in case that the repulsive potential,U, is finite, butt/U 1. It is seen that the convergence of the series is very poor.     

Extended Hubbard model in the presence of a magnetic field

Within the Green’s function and equations of motion formalism it is possible to exactly solve a large class of models useful for the study of strongly correlated systems. Here, we present the exact solution of the one-dimensional extended Hubbard model with on-site U and first nearest neighbor repulsive V interactions in the presence of an external magnetic field h, in the narrow band limit. At zero temperature our results establish the existence of four phases in the three-dimensional space (U, n, h) – n is the filling – with relative phase transitions, as well as different types of charge ordering. The magnetic field may dramatically affect the behavior of thermodynamic quantities, inducing, for instance, magnetization plateaus in the magnetization curves, and a change from a single to a double-peak tructure in the specific heat. According to the value of the particle density, we find one or two critical fields, marking the beginning of full or partial polarization. A detailed study of several thermodynamic quantities is also presented at finite temperature.     

Ultracold fermions and the SU(N) Hubbard model

We investigate the fermionic SU(N) Hubbard model on the two-dimensional square lattice for weak to moderate interactions using renormalization group and mean-field methods. For the repulsive case U>0 at half filling and small N the dominant tendency is towards breaking of the SU(N) symmetry. For N>6 staggered flux order takes over as the dominant instability, in agreement with the large-N limit. Away from half filling for N=3 two flavors remain half filled by cannibalizing the third flavor. For U<0 and odd N a full Fermi surface coexists with a superconductor. These results may be relevant to future experiments with cold fermionic atoms in optical lattices.     

Dynamical spin magnetic susceptibility in the 2D Hubbard model

Magnetic properties of the two-dimensional Hubbard model are investigated by studying the imaginary part of the dynamical spin magnetic susceptibility as a function of momentum and doping. The calculations are performed by means of the composite operator method in the static approximation. It is shown that the results are in good qualitative agreement with the experimental data for LaSrCuO compounds.     

On cyclic region solutions of the Hubbard model

Exact many-electron solutions of the extended Hubbard Hamiltonian are found for four electrons in s-states on four atoms. These are discussed in relation to similar finite system solutions and in relation to the problem of characterizing electron correlation effects in infinite lattices.     

Metal-insulator transition in the Hubbard model with incommensurate magnetic structures

The metal-insulator transition for the square, simple cubic, and body centered cubic lattices has been studied within the Hubbard model at half-filling taking into account nearest- and next-nearest-neighbor electron hopping. Both staggered antiferromagnetic and incommensurate magnetic states (spin-spiral wave) have been considered. The inclusion of the latter states for the three-dimensional lattices does not change the general pattern of the metal-insulator transition, but opens the fundamentally new possibility of the metal-insulator transition of the first order between the magnetically ordered states for the square lattice.     

Another approach to localized magnetic moments in the Hubbard model

By using the band part of the Hubbard Hamiltonian as our starting point, we find that for one electron/atom, with Coulomb repulsion U and bandwidth Δ, there is a single band with sharp Fermi surface and enhanced Pauli paramagnetism for U < Δ, and a splitting into two symmetric bands with local moment formation for U > Δ.