arXiv:cond-mat/0301376 · Physical review. B, Condensed matter

Minimal charge gap in the ionic Hubbard model

Krunoslav Požgajčić, Claudius Gros

Published Aug 14, 2003• 10 citations

Abstract

We study the ionic Hubbard model at temperature $T=0$ within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions $U>{U}_{c}$ for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.

DOI: 10.1103/physrevb.68.085106

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arXiv:cond-mat/0301376 · Physical review. B, Condensed matter

Minimal charge gap in the ionic Hubbard model

Krunoslav Požgajčić, Claudius Gros

Published Aug 14, 2003• 10 citations

arXiv HTML PDF

Abstract

We study the ionic Hubbard model at temperature

T=0

within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions

U>{U}_{c}

for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.

DOI: 10.1103/physrevb.68.085106

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We study the ionic Hubbard model at temperature

T=0

U>{U}_{c}

for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.

We investigate the ground-state phase diagram of the Hubbard model for the ${\mathrm{AB}}_{N\ensuremath{-}1}$ chain with filling

1∕N

, where

N

is the number of atoms per unit cell. In the strong-coupling limit, a charge transition takes place from a band insulator (BI) to a correlated insulator for increasing on-site repulsion

U

and positive on-site energy difference $\ensuremath{\Delta}$ (energy at A sites lower than at B sites). In the weak-coupling limit, a bosonization analysis suggests that for

N>2

the physics is qualitatively similar to the case

N=2

which has already been studied: an intermediate phase emerges, which corresponds to a bond-ordered ferroelectric insulator (FI) with spontaneously broken inversion symmetry. We have determined the quantum phase diagram for the cases

N=3

and

N=4

from the crossings of energy levels of appropriate excited states, which correspond to jumps in the charge and spin Berry phases, and from the change of sign of the localization parameter

{z}_{L}^{c}

. From these techniques we find that, quantitatively, the BI and FI phases are broader for

N>2

than when

N=2

, in agreement with the bosonization analysis. Calculations of the Drude weight and

{z}_{L}^{c}

indicate that the system is insulating for all parameters, with the possible exception of the boundary between the BI and FI phases.

U

. We obtain a critical region in which the metallic phase disappears and a coexistence phase between the band and the Mott insulating state sets in. Our results are consistent with those obtained at low dimensions, thus they provide a concrete description for the charge degrees of freedom of the ionic Hubbard model.