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arXiv:0705.0755 · Physical Review B

Metallic phase in the two-dimensional ionic Hubbard model

Karim Bouadim, Norman Paris, F. Hébert, G. G. Batrouni, Richard T. Scalettar

Published Aug 9, 2007• 70 citations
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Abstract

We investigate the phases of the ionic Hubbard model in a two-dimensional square lattice using determinant quantum Monte Carlo. At half-filling, when the interaction strength or the staggered potential dominates, we find Mott and band insulators, respectively. When these two energies are of the same order, we find a metallic region. Charge and magnetic structure factors demonstrate the presence of antiferromagnetism only in the Mott region, although the externally imposed density modulation is present everywhere in the phase diagram. Away from half-filling, other insulating phases are found. Kinetic energy correlations do not give clear signals for the existence of a bond-ordered phase.

DOI
10.1103/physrevb.76.085112

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We investigate the phases of the ionic Hubbard model in a two-dimensional square lattice using determinant quantum Monte Carlo. At half-filling, when the interaction strength or the staggered potential dominates, we find Mott and band insulators, respectively. When these two energies are of the same order, we find a metallic region. Charge and magnetic structure factors demonstrate the presence of antiferromagnetism only in the Mott region, although the externally imposed density modulation is present everywhere in the phase diagram. Away from half-filling, other insulating phases are found. Kinetic energy correlations do not give clear signals for the existence of a bond-ordered phase.
The Quantum Monte Carlo simulations of the ionic Hubbard model on a two-dimensional square lattice at half filling were performed. The method based on the direct-space, proposed by Suzuki and al., Hirsch and al., was used. Cycles of increasing and decreasing values of the Coulomb interaction $ U $ were performed for fixed temperature ($ kT=0.01 $). Results indicate that, at low temperature, the two insulator phases are separated by a metallic phase for weak to intermediate values of the staggered potential $ Δ$. For large Coulomb repulsion the system is in a Mott insulator with an antiferromagnetism order. On increasing and decreasing the Coulomb interaction $ U $ the metal-Mott insulator transition shows an hysteresis phenomenon while the metal-band insulator transition is continue. For large $ Δ$ it seems that the metallic region shrinks to a single metallic point. However, the band insulator to the Mott insulator transition is not direct for the studied model. A phase diagram is drawn for the temperature $ kT=0.01 $. For $ Δ=0.5 $ cycles of increasing and decreasing temperature were programmed for different values of the Coulomb interaction $ U $ . A behaviour change appears for $ U\simeq 1.75 $. This suggests that a crossover line divides the metallic region of the phase diagram.
We investigate the quantum phase transitions in the half-filled ionic Hubbard model on a two-dimensional (2D) square lattice using the variational cluster approach (VCA). We present explicit evidence for the tendency toward a novel intermediate phase in this model. This phase is characterized by bond-located magnetization. For weak Coulomb repulsion U , the system is a band insulator, and then undergoes a transition to the intermediate phase at the first-phase boundary U = U c1 . As U is increased beyond the second transition point U c2 , there occurs a Mott insulator accompanied by a long-range antiferromagnetic (AF) order. The bond-located spin density wave competes with the antiferromagnetism while the charge-density modulation exists all the way due to the staggered potential Δ.
We study the transitions from band insulator to metal to Mott insulator in the ionic Hubbard model on a two-dimensional square lattice using determinant quantum Monte Carlo. Evaluation of the temperature dependence of the conductivity demonstrates that the metallic region extends for a finite range of interaction values. The Mott phase at strong coupling is accompanied by antiferromagnetic order. Inclusion of these intersite correlations changes the phase diagram qualitatively compared to dynamical mean field theory.
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We use a density-matrix renormalization group method to study quantitatively the phase diagram of the half-filled one-dimensional (1D) extended Hubbard model in the presence of a staggered ionic potential Δ. An extensive finite-size scaling analysis is carried out on the relevant structure factors and localization operator to characterize the Mott-insulator (MI)-bond-ordered insulator (BOI)-band-insulator (BI) transitions. The intermediate BOI phase occupies a small region of the phase diagram, and this region is enlarged in the presence of Δ. In addition, the phase diagram of ionic Hubbard (the nearest-neighbor electron-electron interaction V=0) is also given.
The ionic Hubbard model in a two-dimensional square lattice is revisited using determinant quantum Monte Carlo (DQMC) and numerical analytic continuation. From the dc conductivity extracted by stochastic maximum entropy of the DQMC data, we find that the band insulator induced by a staggered potential [Formula: see text] goes through a metallic phase before entering a Mott insulator as the on-site Hubbard interaction U is increased. Moreover, we map out the phase diagram in the [Formula: see text] plane, and reveal a smaller metallic region than that found in the previous studies. Our results provide a more exact location of the phase boundaries, and are an important complement to those obtained by an approximate approach.