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Modern Physics Letters B

EXACT MEAN FIELD THEORY FOR CORRELATED LATTICE FERMIONS IN HIGH DIMENSIONS

P. G. J. van Dongen

Published Jun 10, 1991• 14 citations
Publisher

Abstract

We review the recently introduced concept of exact mean-field theories for fermionic lattice models. The method is applied to the Extended Simplified Hubbard Model, which is an Extended Hubbard model (including nearest-neighbor interaction) where only the ↓-electrons can hop. The construction of an exact mean-field Hamiltonian is discussed and a summary is given of the results obtained for the critical temperature, the order parameter and the density of states.

DOI
10.1142/s0217984991001064

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We review the recently introduced concept of exact mean-field theories for fermionic lattice models. The method is applied to the Extended Simplified Hubbard Model, which is an Extended Hubbard model (including nearest-neighbor interaction) where only the ↓-electrons can hop. The construction of an exact mean-field Hamiltonian is discussed and a summary is given of the results obtained for the critical temperature, the order parameter and the density of states.
The recently introduced concept of exact mean-field theories is applied to the extended simplified Hubbard model, which is a one-band extended Hubbard model (including nearest-neighbor interaction) where only the \ensuremath{\downarrow} electrons can hop. An exact mean-field Hamiltonian is derived for the model on a Bethe lattice in the limit of a large coordination number (Z\ensuremath{\rightarrow}\ensuremath{\infty}). The mean-field Hamiltonian is used to calculate the Green functions and the free energy at half-filling. This exact solution is analyzed analytically at small and large values of the on-site (U) and nearest-neighbor (V) interaction strengths. One finds that a phase transition occurs at sufficiently low temperatures for all U,V>0. The low-temperature phase is a charge-density wave for V>1/2U and a spin-density wave for V1/2U. Special attention is paid to (i) the critical temperature as a function of U and V, (ii) the order parameter as a function of temperature, and (iii) the density of states. The density of states shows that the low-temperature phase is an insulator for all U,V provided that V/U\ensuremath{\ne}1/2, while the high-temperature phase is a paramagnetic metal for all V if U2, and a paramagnetic insulator if U>2. Many detailed results are given.
We show that even for fermionic lattice models with on-site interaction a mean-field Hamiltonian can be constructed, which--in analogy with spin-lattice models--becomes exact in the limit of high spatial dimensions d. Here the mean fields are fermion operators, rather than numbers. We use this method to obtain the exact solution of a simplified Hubbard model on a Bethe lattice for all temperatures. The order parameter is found to have the conventional mean-field form, but exhibits unusually rich behavior as a function of the interaction.
Dynamical mean-field theory (DMFT) is a successful method to investigate interacting lattice fermions. In these lecture notes we present an introduction into the DMFT for lattice fermions with interaction, disorder and external inhomogeneous potentials. This formulation is applicable to electrons in solids and to cold fermionic atoms in optical lattices. We review here our investigations of the Mott-Hubbard and Anderson metal-insulator transitions in correlated, disordered systems by presenting selected surprising results. 1.
We investigate the problem of constructing solvable models of nonrelativistic spin-1/2 interacting fermions on an infinite lattice. The extended Hubbard model, which includes the direct and exchange intersite interaction, is considered. The hopping term in the Hamiltonian is not restricted to nearest-neighbor sites (the ``overjumping'' model), but the short-range character of the standard Hubbard model is retained. The solutions in one- and two-particle sectors are obtained in an explicit form by using the theory of Weierstrass elliptic functions. The possibilities of further development of models of that type are also considered.
We review the dynamical mean-field theory of strongly correlated electron systems which is based on a mapping of lattice models onto quantum impurity models subject to a self-consistency condition. This mapping is exact for models of correlated electrons in the limit of large lattice coordination (or infinite spatial dimensions). It extends the standard mean-field construction from classical statistical mechanics to quantum problems. We discuss the physical ideas underlying this theory and its mathematical derivation. Various analytic and numerical techniques that have been developed recently in order to analyze and solve the dynamical mean-field equations are reviewed and compared to each other. The method can be used for the determination of phase diagrams (by comparing the stability of various types of long-range order), and the calculation of thermodynamic properties, one-particle Green's functions, and response functions. We review in detail the recent progress in understanding the Hubbard model and the Mott metal-insulator transition within this approach, including some comparison to experiments on three-dimensional transition-metal oxides. We present an overview of the rapidly developing field of applications of this method to other systems. The present limitations of the approach, and possible extensions of the formalism are finally discussed. Computer programs for the numerical implementation of this method are also provided with this article.