Superconductivity from doublon condensation in the ionic Hubbard model

Abstract

In the ionic Hubbard model, the on-site repulsion U, which drives a Mott insulator, and the ionic potential V, which drives a band insulator, compete with each other to open up a window of charge fluctuations when U∼V. We study this model on square and cubic lattices in the limit of large U and V, with V∼U. Using an effective Hamiltonian and a slave-boson approach with both doublons and holons, we find that the system undergoes a phase transition as a function of V from an antiferromagnetic Mott insulator to a paramagnetic insulator with strong singlet correlations, which is driven by a condensate of “neutral” doublon-holon pairs. On further increasing V, the system undergoes another phase transition to a superconducting phase driven by condensate of “charged” doublons and holons. The superfluid phase, characterized by the presence of a coherent (but gapped) fermionic quasiparticle and hc/e flux quantization, has a high Tc∼t, which shows a dome-shaped behavior as a function of V. The paramagnetic insulator phase has a deconfined U(1) gauge field and associated gapless photon excitations. We also discuss how these phases can be detected in the ultracold-atom context.

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