High-order correlation effects in the two-dimensional Hubbard model

TL;DR

This paper investigates high-order correlation effects in the 2D Hubbard model using a 4-pole approximation, revealing complex band structures and physical features consistent with numerical simulations.

Contribution

It introduces a 4-pole approximation with additional operators to capture high-order correlations and energy scales beyond previous 2-pole models.

Findings

Discovery of a four-band structure with shadow bands

Identification of a quasi-particle peak at the Fermi level

Pinning of the Fermi level at the (pi,0) point across doping levels

Abstract

The electronic states of the two-dimensional Hubbard model are investigated by means of a 4-pole approximation within the Composite Operator Method. In addition to the conventional Hubbard operators, we consider other two operators which come from the hierarchy of the equations of motion and carry information regarding nearest-neighbor spin and charge configurations. By means of this operatorial basis, we can study the physics related to the energy scale of J=4t^2/U in addition to the one of U. Present results show relevant physical features, well beyond those previously obtained by means of a 2-pole approximation, such as a four-band structure with shadow bands and a quasi-particle peak at the Fermi level. The Fermi level stays pinned to the band flatness located at (pi,0)-point within a wide range of hole-doping (0 <= delta <= 0.15). A comprehensive analysis of double occupancy, internal energy, specific heat and entropy features have been also performed. All reported results are in excellent agreement with the data of numerical simulations.