Correlated electrons and generalized statistics

Several important generalizations of Fermi-Dirac distribution are compared to numerical and experimental results for correlated electron systems. It is found that the quantum distributions based on incomplete information hypothesis can be useful for describing this kind of systems. We show that the additive incomplete fermion distribution gives very good description of weakly correlated electrons and that the nonadditive one is suitable to very strong correlated cases.Comment: 13 pages, RevTex file, 4 ps figures. The European Physical Journal B (2002), in pres

Charge on the quantum dot in the presence of tunneling current

The calculation of the charge present in central region of the double barrier structure at non-equilibrium conditions is discussed. We propose here a simple method to calculate non equilibrium Green's functions which allows consistent calculations of retarded and distribution functions. To illustrate the approach we calculate the charge on the quantum dot coupled {\it via} tunnel barriers to two external leads having different chemical potentials $\mu_L$ and $\mu_R$. The obtained results have been compared with other approaches existing in the literature. They all agree in the equilibrium situation and the departures grow with increasing the difference $\mu_L-\mu_R$.Comment: 9 pages, 2 (.eps) figures, to be published in Solid State Commu

Self-Consistent Strong-Coupling-Perturbation Theory for the Anderson Model, Based on Wicks Theorem

A strong-coupling-perturbation theory around the Atomic Limit of the Anderson model with large $U$ for a localized $f$-orbital coupled to a conduction-electron band is presented. Although an auxiliary-particle representation is {\em not} used, application of the canonical Wick's theorem is possible and yields an expansion in the hybridization $V$ via dressed skeleton-Feynman diagrams. The Self-Consistent T-Approximation is constructed as a $\Phi$-derivable approximation. From a numerical solution of self-consistency equations the $f$-electron-excitation spectrum is investigated. Comparison to the Non-Crossing Approximation is made in virtue of exact formal relations and numerical results. An extension of this Feynman-diagram approach to the Anderson-lattice model is indicated, and application within the Local-Approximation scheme (limit of infinite spatial dimension) is given.Comment: 19 pages, revtex3.0, epsf, 11 figures included as .eps file

Classification and Stability of Phases of the Multicomponent One-Dimensional Electron Gas

The classification of the ground-state phases of complex one-dimensional electronic systems is considered in the context of a fixed-point strategy. Examples are multichain Hubbard models, the Kondo-Heisenberg model, and the one-dimensional electron gas in an active environment. It is shown that, in order to characterize the low-energy physics, it is necessary to analyze the perturbative stability of the possible fixed points, to identify all discrete broken symmetries, and to specify the quantum numbers and elementary wave vectors of the gapless excitations. Many previously-proposed exotic phases of multichain Hubbard models are shown to be unstable because of the ``spin-gap proximity effect.'' A useful tool in this analysis is a new generalization of Luttinger's theorem, which shows that there is a gapless even-charge mode in any incommensurate N-component system.Comment: 15 pages revtex. Final version as publishe

Charge Condensation in QED3_3 with a Chern-Simons Term

Introducing a chemical potential in the functional method, we construct the effective action of QED$_3$ with a Chern-Simons term. We examine a possibility that charge condensation $\langle\psi^\dagger\psi \rangle$ remains nonzero at the limit of the zero chemical potential. If it happens, spontaneous magnetization occurs due to the Gauss' law constraint which connects the charge condensation to the background magnetic field. It is found that the stable vacuum with nonzero charge condensation is realized only when fermion masses are sent to zero, keeping it lower than the chemical potential. This result suggests that the spontaneous magnetization is closely related to the fermion mass.Comment: 13 pages, phyzzx, 2 figure

Impurity correlations in dilute Kondo alloys

The single impurity Kondo model is often used to describe metals with dilute concentrations (n_i) of magnetic impurities. Here we examine how dilute the impurities must be for this to be valid by developing a virial expansion in impurity density. The O(n_i^2) term is determined from results on the 2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial fixed point of the 2-impurity problem could produce novel singularities in the heat capacity of dilute alloys at O(n_i^2).Comment: 6 pages, no figure

The Cumulant Expansion for the Anderson Lattice with Finite U: The Completeness Problem

``Completeness'' (i.e. probability conservation) is not usually satisfied in the cumulant expansion of the Anderson lattice when a reduced state space is employed for $U\to \infty$. To understand this result, the well known ``Chain'' approximation is first calculated for finite $U$, followed by taking $U\to \infty$. Completeness is recovered by this procedure, but this result hides a serious inconsistency that causes completeness failure in the reduced space calculation. Completeness is satisfied and the inconsistency is removed by choosing an adequate family of diagrams. The main result of this work is that using a reduced space of relevant states is as good as using the whole space.Comment: Latex 22 pages, 6 figures with postscript files attached, accepted for publication in the Int. J. of Mod. Phys. B (1998). Subject field : Strongly Correlated System