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 $U\ensuremath{\sim}V$. We study this model on square and cubic lattices in the limit of large $U$ and $V$, with $V\ensuremath{\sim}U$. 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 ${T}_{c}\ensuremath{\sim}t$, 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.