Extremely Correlated Fermi Liquids: The Formalism

TL;DR

This paper develops a detailed formalism for extremely correlated Fermi liquids, specifically applied to the t-J model, introducing a systematic expansion and effective Hamiltonian to better understand strongly correlated electron systems.

Contribution

It introduces a systematic expansion in a parameter related to double occupancy and formulates an effective Hamiltonian for auxiliary electrons in the t-J model.

Findings

A new Green's function formalism with adaptive spectral weight.

Identification of a superconducting instability at the simplest level.

Derivation of expressions for Green's functions up to second order in the expansion parameter.

Abstract

We present the detailed formalism of the extremely correlated Fermi liquid theory, developed for treating the physics of the t-J model. We start from the exact Schwinger equation of motion for the Greens function for projected electrons, and develop a systematic expansion in a parameter \lambda, relating to the double occupancy. The resulting Greens function has a canonical part arising from an effective Hamiltonian of the auxiliary electrons, and a caparison part, playing the role of a frequency dependent adaptive spectral weight. This adaptive weight balances the requirement at low \omega, of the invariance of the Fermi volume, and at high \omega, of decaying as c_0/(i \omega), with a correlation depleted c_0 <1. The effective Hamiltonian H_{eff} describing the auxiliary Fermions is given a natural interpretation with an effective interaction V_{eff} containing both the exchange J(ij), and the hopping parameters t(ij). It is made Hermitian by adding suitable terms that ultimately vanish, in the symmetrized theory developed in this paper. Simple but important shift invariances of the t-J model are noted with respect to translating its parameters uniformly. These play a crucial role in constraining the form of V_{eff} and also provide checks for further approximations. The auxiliary and physical Greens function satisfy two sum rules, and the Lagrange multipliers for these are identified. A complete set of expressions for the Greens functions to second order in \lambda is given, satisfying various invariances. A systematic iterative procedure for higher order approximations is detailed. A superconducting instability of the theory is noted at the simplest level with a high transition temperature.