Orientational bond and N\'eel order in the two-dimensional ionic Hubbard model

TL;DR

This paper investigates the phase transitions in the two-dimensional ionic Hubbard model, revealing a sequence of phases including bond order with orientational symmetry breaking, coexistence of antiferromagnetic and bond order, and a Ne9el Mott insulator, using effective models and mean-field analysis.

Contribution

It introduces a mean-field analysis of phase transitions in the 2D ionic Hubbard model, highlighting the emergence of d-wave bond order and its coexistence with antiferromagnetism, which is a novel insight.

Findings

Identification of a bond-ordered phase with orientational symmetry breaking.

Coexistence of antiferromagnetic order with d-wave bond order.

Persistence of Ne9el Mott insulator after bond order vanishes.

Abstract

Unconventional phases often occur where two competing mechanisms compensate. An excellent example is the ionic Hubbard model where the alternating local potential $\delta$, favoring a band insulator (BI), competes with the local repulsion $U$, favoring a Mott insulator (MI). By continuous unitary transformations we derive effective models in which we study the softening of various excitons. The softening signals the instability towards new phases that we describe on the mean-field level. On increasing $U$ from the BI in two dimensions, we find a bond-ordered phase breaking orientational symmetry due to a d-wave component. Then, antiferromagnetic order appears coexisting with the d-wave bond order. Finally, the d-wave order vanishes and a N\'eel-type MI persists.