Spin-charge-density wave in a squircle-like Fermi surface for ultracold atoms

TL;DR

This paper models ultracold atoms in an optical lattice with a staggered Zeeman field, revealing an exotic Fermi surface and the potential for coupled spin-charge-density wave order, which is absent in traditional Hubbard models.

Contribution

It introduces a realistic model with a unique band structure and develops a generalized theory showing the emergence of coupled spin-charge-density waves.

Findings

Fermions exhibit a rounded squircle Fermi surface with imperfect nesting.

The system can develop coupled spin-charge-density wave order.

This order is not present in standard Hubbard model studies.

Abstract

We derive and discuss an experimentally realistic model describing ultracold atoms in an optical lattice including a commensurate, but staggered, Zeeman field. The resulting band structure is quite exotic; fermions in the third band have an unusual rounded picture-frame Fermi surface (essentially two concentric squircles), leading to imperfect nesting. We develop a generalized SO(3,1)xSO(3,1) theory describing the spin and charge degrees of freedom simultaneously, and show that the system can develop a coupled spin-charge-density wave order. This ordering is absent in studies of the Hubbard model that treat spin and charge density separately.