Orbital currents in insulating and doped antiferromagnets

TL;DR

This paper explores spin liquid states on square lattices that break time-reversal symmetry and exhibit spontaneous orbital currents, with implications for understanding cuprate superconductors and novel topological phases.

Contribution

It introduces new spin liquid states with broken time-reversal symmetry and orbital currents, analyzed through bosonic and fermionic approaches, including states with topological order and applications to cuprates.

Findings

Spin liquids with non-zero scalar spin chirality and spontaneous orbital currents.

States with Z2 and U(1) topological order, including chiral spin liquids.

Orbital currents depend on the orbital formulation, appearing in three-band models.

Abstract

We describe square lattice spin liquids which break time-reversal symmetry, while preserving translational symmetry. The states are distinguished by the manner in which they transform under mirror symmetries. All the states have non-zero scalar spin chirality, which implies the appearance of spontaneous orbital charge currents in the bulk (even in the insulator); but in some cases, orbital currents are non-zero only in a formulation with three orbitals per unit cell. The states are formulated using both the bosonic and fermionic spinon approaches. We describe states with $\mathbb{Z}_2$ and U(1) bulk topological order, and the chiral spin liquid with semionic excitations. The chiral spin liquid has no orbital currents in the one-band formulation, but does have orbital currents in the three-band formulation. We discuss application to the cuprate superconductors, after postulating that the broken time-reversal and mirror symmetries persist into confining phases which may also break other symmetries. In particular, the broken symmetries of the chiral spin liquid could persist into the N\'eel state.