Nature of the insulating phases in the half-filled ionic Hubbard model

TL;DR

This paper explores the phase diagram of the 1D ionic Hubbard model at half-filling, revealing an insulator-insulator transition characterized by changes in charge, bond-charge order, and excitation gaps, with potential for a second transition.

Contribution

It provides a detailed numerical analysis of phase transitions in the ionic Hubbard model, identifying a transition from band to correlated insulator with specific order parameters.

Findings

Insulator-insulator phase transition identified

Transition characterized by vanishing optical gap

Possible second transition into Mott insulator discussed

Abstract

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulator-insulator phase transition from a band to a correlated insulator with simultaneous charge and bond-charge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.