Quasi-one-dimensional magnetic properties of NiNb2−xVxO6 compounds synthesized at high pressure in a nonstandard columbite-type structure

We report on the low-dimensional magnetic behavior of the series of compounds NiNb2−xVxO6 (0 x 2), with a columbite-type crystal structure stabilized at high pressure and temperature. Based on susceptibility, magnetization, and specific-heat measurements, the system is characterized as presenting quasi-one-dimensional magnetism, with Ni2+ magnetic moments that can be modeled as Ising spins, placed along zigzag chains in the crystal structure. The low-temperature phase is found to consist of an antiferromagnetic arrangement of ferromagnetic chains, and a metamagnetic transition to uniform ferromagnetic order is observed under magnetic fields slightly above μ0H = 1 T. We discuss the effects of substituting vanadium for niobium, maintaining the same crystal structure along the whole series of samples. In particular, the long-range magnetic order, most clearly seen for x = 0, tends to be suppressed as the vanadium content is increased. The exchange interactions are quantified, revealing that the ferromagnetic intrachain interactions vary from about 7 K for NiNb2O6 to 2 K for NiV2O6, remaining one order of magnitude larger than the mean antiferromagnetic interchain coupling

Phase transitions in asymmetric potts models : breakdown of classical mean-field picture

It is shown that mean-field theory fails to give a correct qualitative picture of the thermodynamic behavior of the q-state Potts model when the exchange interaction is anisotropic in spin space. The correct picture is recovered either by introducing a single-particle anisotropy or by taking correlations into account via a Bethe-Peierls approximation. This analysis helps the interpretation of previous renormalization-group results for asymmetric Potts models

Simple approximation for the bethe ansatz solution of the one-dimensional fermi gas

We present a simple approximation scheme for the solution of the integral equations resulting from the Bethe-ansatz diagonalization of the one-dimensional Fermi gas with ô-function attraction. These equations arise for the Hubbard model with attractive interaction in the limit of weak coupling and low density. We obtain the ground-state energy as a function of coupling and density in very good agreement with numerical solutions, as well as a value for the parameter determining the gap in the magnetic excitation spectrum which strongly supports a conjecture of Larkin and Sak

Simple approximation for the bethe ansatz solution of the one-dimensional fermi gas

We present a simple approximation scheme for the solution of the integral equations resulting from the Bethe-ansatz diagonalization of the one-dimensional Fermi gas with ô-function attraction. These equations arise for the Hubbard model with attractive interaction in the limit of weak coupling and low density. We obtain the ground-state energy as a function of coupling and density in very good agreement with numerical solutions, as well as a value for the parameter determining the gap in the magnetic excitation spectrum which strongly supports a conjecture of Larkin and Sak

Phase transitions in asymmetric potts models : breakdown of classical mean-field picture

It is shown that mean-field theory fails to give a correct qualitative picture of the thermodynamic behavior of the q-state Potts model when the exchange interaction is anisotropic in spin space. The correct picture is recovered either by introducing a single-particle anisotropy or by taking correlations into account via a Bethe-Peierls approximation. This analysis helps the interpretation of previous renormalization-group results for asymmetric Potts models

Simplified periodic anderson model : exact solution in infinite dimensions

We present a diagrammatic perturbative treatment of the hybridization for the periodic Anderson model that recovers the dynamical mean-field equations in the limit of infinite dimensions. The resulting effective singlesite problem is naturally addressed by perturbation theory on the dynamical mean field. We introduce a simplified version of the model in which only electrons with a given spin orientation hybridize. The perturbation series can be summed in this case, yielding an exact solution for the single-particle Green’s functions. Electronic and transport properties are analyzed, showing the existence of a metallic regime with non-Fermiliquid behavior

Tight-binding treatment of the Hubband model in infinite dimensions

We discuss the infinite dimension limit of the Hubbard model by means of a perturbative expansion of the one-particle Green’s function around the atomic limit. The diagrammatic structure is simplified in this limit, allowing a formal resummation that reproduces a previously proposed mapping to a single-site mean-field problem. The method provides a natural way of addressing this effective problem by means of a perturbative expansion in the local mean field. This gives the correct exact result for the Falicov-Kimball model, which is used as a starting point to study the Hubbard case

Nature of insulating state in the three-band hubbard model : a tight-binding approach

We study the three-band Hubbard model, commonly used to describe the copper-oxygen planes of high-Tc superconductors, from the point of view of electronic properties. Utilizing perturbation theory around the atomic limit to evaluate finite-temperature Green's functions, one-particle densities of states are calculated in some simple approximations. Both metallic paramagnetic and insulating antiferromagnetic solutions are obtained, depending on the hole concentration. We discuss the overall scenario obtained here in comparison with experimental results and other theoretical approaches

El Eco de Santiago : diario independiente: Año VI Número 1938 - 1901 Agosto 29

We discuss criteria for determining the optimal nesting vector from the maximum of the zero-field susceptibility for imperfectly nested Fermi surfaces. In particular, we present results for some of the bis(ethylenedithio) tetrathiafulvalene charge-transfer salts, a family of quasi-two-dimensional organic conductors. The criteria proposed gives better nesting than the previous criteria that has been applied to the tetramethyltetraselenafulvalene family, in which the nesting vector translates a point of inflection on the Fermi surface. We discuss the nesting vectors deduced, and consequences for Fermi-surface reconstruction and low-temperature behavior

From the atomic limit to a metal-insulator transition in the hubbard model

Starting from the exact solution of the Hubbard model in the atomic limit, and treating the hopping term as a perturbation by means of a diagram technique, we discuss the electronic structure of the model in some simple approximations. We show that a metal-insulator transition is obtained above a criticai value of the on-site Coulomb interaction in a self-consistent evaluation of the oneparticle Green's function to one-loop order. A relationship with results in infinite dimensions is briefly discussed