Exciton condensation in bilayer spin-orbit insulator

TL;DR

This paper explores the quantum phase transition in a bilayer Hubbard model, revealing exciton condensation as the mechanism behind the transition from a paramagnetic to an antiferromagnetic insulator, with implications for materials like Sr3Ir2O7.

Contribution

It demonstrates that the quantum critical point is due to exciton condensation, providing insight into magnetic excitations in correlated electron systems.

Findings

Quantum phase transition is continuous in the band insulator regime.

Exciton condensation drives the transition between phases.

Longitudinal mode remains sharp in Sr3Ir2O7-like parameters.

Abstract

We investigate the nature of the magnetic excitations of a bilayer single-orbital Hubbard model in the intermediate-coupling regime. This model exhibits a quantum phase transition (QPT) between a paramagnetic (PM) and an insulating antiferromagnetic (AFM) phase at a critical value of the coupling strength. By using the random phase approximation, we show that the QPT is continuous when the PM state is a band insulator and that the corresponding quantum critical point (QCP) arises from the condensation of preformed excitons. These low-energy excitons reemerge on the other side of the QCP as the transverse and longitudinal modes of the AFM state. In particular, the longitudinal mode remains sharp for the model parameters relevant to Sr$_{3}$Ir$_{2}$O$_{7}$ because of the strong easy-axis anisotropy of this material.