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Physics World

A paper a day for a well posed problem

Thomas Maurice Rice

Published Feb 1, 1992• 0 citations
Publisher

Abstract

The Hubbard model is the simplest interacting fermion model that describes the competition between the kinetic energy, which favours placing electrons in extended states, and the Coulomb energy, which suppresses charge fluctuations and favours strict localisation of electrons. Understanding this competition is at the heart of many key questions in solid state physics.

DOI
10.1088/2058-7058/5/2/34

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The Hubbard model is the simplest interacting fermion model that describes the competition between the kinetic energy, which favours placing electrons in extended states, and the Coulomb energy, which suppresses charge fluctuations and favours strict localisation of electrons. Understanding this competition is at the heart of many key questions in solid state physics.
This chapter provides an introduction to the theory of strongly correlated electrons and discusses some of the ideas that presently are being pursued to try to resolve this dilemma. The Hubbard Model is by now surely the most infamous or notorious model problem in solid state physics because the ratio of number of papers to understanding of this model must be an all time record. Yet it is a model which describes the competition between the kinetic energy and the Coulomb interaction in the simplest way. Its origin lies in work of Van Vleck. One tried and true method when have to deal with strong interactions is to look for the right mean-field or Hartree-Fock method. This method has of course been widely applied to the Hubbard Model. The one-dimesional Hubbard model with t not equal to zero for nearest neighbours was solved by Lieb and Wu using Bethe-Ansatz methods.
Electronic correlations arise from the competition between the electrons' kinetic and Coulomb interaction energy and give rise to a rich phase diagram and many emergent quasiparticles. The binding of doubly-occupied and empty sites into a doublon-holon exciton is an example of this in the Hubbard model. Unlike traditional excitons in semiconductors, in the Hubbard model it is the kinetic energy which provides the binding energy. Upon doping, we find the emergence of exciton complexes, such as a holon-doublon-holon trion. The appearance of these low-lying collective excitations make screening more effective in the doped system. As a result, Hubbard-based modelling of correlated materials should use different values of UUU for the doped system and the insulating parent compound, which we illustrate using the cuprates as an example.
The Hubbard model is one of the most widely studied many-body models for interacting electrons in solids. Here we present a study of the Extended Hubbard model, which provides a more realistic description of real materials by including some longer-range Coulomb interactions. We discuss the derivation of the model, numerical methods for many-body systems, the ground state of the one-dimensional model, and an application of the model to organic charge-transfer solids. Our discussion of numerical methods includes details of using the Constrained Path Monte Carlo procedure to treat extended interactions and possible extension of the method for imaginary times. The chapter on the one-dimensional model focuses on the region V>U, where previous work had suggested the possibility of superconductivity. Our results show that instead phase separation is prevalent in that parameter regime. The second application is to model the organic charge-transfer salts, which are intermediate between one and two dimensions. Here we find a coexistence of charge and spin ordering that helps to explain the results of recent experiments on these materials.
The Hubbard model is a `highly oversimplified model' for electrons in a solid which interact with each other through extremely short-ranged repulsive (Coulomb) interaction. The Hamiltonian of the Hubbard model consists of two parts: which describes quantum mechanical hopping of electrons, and which describes non-linear repulsive interaction. Either or alone is easy to analyse, and does not favour any specific order. But their sum is believed to exhibit various non-trivial phenomena including metal-insulator transition, antiferromagnetism, ferrimagnetism, ferromagnetism, Tomonaga-Luttinger liquid, and superconductivity. It is believed that we can find various interesting `universality classes' of strongly interacting electron systems by studying the idealized Hubbard model. In the present article we review some mathematically rigorous results relating to the Hubbard model which shed light on the `physics' of this fascinating model. We mainly concentrate on the magnetic properties of the model in its ground states. We discuss the Lieb-Mattis theorem on the absence of ferromagnetism in one dimension, Koma-Tasaki bounds on the decay of correlations at finite temperatures in two dimensions, the Yamanaka-Oshikawa-Affleck theorem on low-lying excitations in one dimension, Lieb's important theorem for the half-filled model on a bipartite lattice, Kubo-Kishi bounds on the charge and superconducting susceptibilities of half-filled models at finite temperatures, and three rigorous examples of saturated ferromagnetism due to Nagaoka, Mielke, and Tasaki. We have tried to make the article accessible to non-experts by giving basic definitions and describing elementary materials in detail.
The Hubbard model is the simplest model of interacting fermions on a lattice and is of similar importance to correlated electron physics as the Ising model is to statistical mechanics or the fruit fly to biomedical science. Despite its simplicity, the model exhibits an incredible wealth of phases, phase transitions, and exotic correlation phenomena. Although analytical methods have provided a qualitative description of the model in certain limits, numerical tools have shown impressive progress in achieving quantitative accurate results over the past several years. This article gives an introduction to the model, motivates common questions, and illustrates the progress that has been achieved over recent years in revealing various aspects of the correlation physics of the model.
Introduction In this chapter we study metal–insulator transitions which occur due to correlations between the charge carriers. This quantum phase transitions results from the competition between kinetic energy and Coulomb repulsion where the latter has the tendency to localise the electrons. The simplest model which describes this competition is the one-band Hubbard model with its two energy scales, the bandwidth W and the Coulomb repulsion U between electrons of opposite spins on the same site. For a fixed density of electrons, n = 1; as the ratio U/W increases, we expect to find a zero-temperature phase transition from a metallic state to an insulator where the electrons are frozen at the lattice sites. The method employed here to investigate this transition is the Gutzwiller approach (Gutzwiller, 1965). This is a variational method which played an important role in providing a framework for the ideas of Mott on the problem of the metal–insulator transition (Mott, 1974). On the metallic side of the transition, this approach describes a metal with renormalised Fermi liquid parameters, essentially with enhanced effective mass and uniform susceptibility. This is the so-called highly correlated electron gas (Brinkman and Rice, 1970) for which the metal oxide V 2 O 3 is the prototype system. In the insulating side, Gutzwiller's approximation yields a noninteracting system of localised electrons. This insulating phase is incompressible , i.e. has zero compressibility and infinite zero temperature susceptibility. The latter is not a consequence of interactions but simply due to the divergence of the Curie susceptibility, χ ∝ 1 /T , of non-interacting moments at T = 0. Then, in Gutzwiller's approach the transition is from a paramagnetic metal to a paramagnetic insulator. We will continue using the designation Mott transition to describe this type of phase transition where the insulating phase has no long-range magnetic order. In real systems, most frequently, localisation due to interactions is accompanied by the appearance of antiferromagnetic order.