Superconductivity from Doublon Condensation in the Ionic Hubbard Model

In the ionic Hubbard model, the onsite 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 $U \sim V$. We study this model on square and cubic lattices in the limit of large $U$ and $V$, with $V\sim U$. Using an effective Hamiltonian and a slave boson approach with both doublons and holes, 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-hole pairs. On further increasing $V$, the system undergoes another phase transition to a superconducting phase driven by condensate of "charged" doublons and holes. The superfluid phase, characterized by presence of coherent (but gapped) fermionic quasiparticle, and $hc/e$ flux quantization, has a high $T_c \sim t$ which shows a dome shaped behaviour 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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