A description of the Hubbard model on a square lattice consistent with its global $SO(3)\times SO(3)\times U(1)$ symmetry

TL;DR

This paper presents a symmetry-consistent description of the Hubbard model on a square lattice, identifying key quasiparticles and their configurations that span the relevant excitation subspace, applicable to both insulating and doped regimes.

Contribution

It introduces a novel framework involving charge fermions, spinons, and $ ext{eta}$-spinons, consistent with the model's global symmetry, to describe the Hubbard model's eigenstates and excitations.

Findings

The description captures nearly all spectral weight of low-energy excitations.

It is consistent with antiferromagnetic order at half filling.

It applies to finite hole doping with short-range spin order.

Abstract

In this paper a description of the Hubbard model on the square lattice with nearest-neighbor transfer integral $t$, on-site repulsion $U$, and $N_a^2\gg 1$ sites consistent with its exact global $SO(3)\times SO(3)\times U(1)$ symmetry is constructed. Our studies profit from the interplay of that recently found global symmetry of the model on any bipartite lattice with the transformation laws under a suitable electron - rotated-electron unitary transformation of a well-defined set of operators and quantum objects. For $U/4t>0$ the occupancy configurations of these objects generate the energy eigenstates that span the one- and two-electron subspace. Such a subspace as defined in this paper contains nearly the whole spectral weight of the excitations generated by application onto the zero-spin-density ground state of one- and two-electron operators. Our description involves three basic objects: charge $c$ fermions, spin-1/2 spinons, and $\eta$-spin-1/2 $\eta$-spinons. Alike in chromodynamics the quarks have color but all quark-composite physical particles are color-neutral, the $\eta$-spinon (and spinons) that are not invariant under that transformation have $\eta$ spin 1/2 (and spin 1/2) but are part of $\eta$-spin-neutral (and spin-neutral) $2\nu$-$\eta$-spinon (and $2\nu$-spinon) composite $\eta\nu$ fermions (and $s\nu$ fermions) where $\nu=1,2,...$ is the number of $\eta$-spinon (and spinon) pairs.The description introduced here is consistent with a Mott-Hubbard insulating ground state with antiferromagnetic long-range order for half filling at $x=0$ hole concentration and a ground state with short-range spin order for a well-defined range of finite hole concentrations $x>0$.