Mass-imbalanced ionic Hubbard chain
AbstractA repulsive Hubbard model with both spin-asymmetric hopping (t↑≠t↓) and a staggered potential (of strength Δ) is studied in one dimension. The model is a compound of the mass-imbalanced (t↑≠t↓,Δ=0) and ionic (t↑=t↓,Δ>0) Hubbard models, and may be realized by cold atoms in engineered optical lattices. We use mostly mean-field theory to determine the phases and phase transitions in the ground state for a half-filled band (one particle per site). We find that a period-two modulation of the particle (or charge) density and an alternating spin density coexist for arbitrary Hubbard interaction strength, U≥0. The amplitude of the charge modulation is largest at U=0, decreases with increasing U and tends to zero for U→∞. The amplitude for spin alternation increases with U and tends to saturation for U→∞. Charge order dominates below a value Uc, whereas magnetic order dominates above. The mean-field Hamiltonian has two gap parameters, Δ↑ and Δ↓, which have to be determined self-consistently. For U<Uc both parameters are positive, for U>Uc they have different signs, and for U=Uc one gap parameter jumps from a positive to a negative value. The weakly first-order phase transition at Uc can be interpreted in terms of an avoided criticality (or metallicity). The system is reluctant to restore a symmetry that has been broken explicitly.