Abstract
We study the ground-state properties of ultracold bosons in an optical lattice in the regime of strong interactions. The system is described by a non-standard Bose–Hubbard model with both occupation-dependent tunneling and on-site interaction. We find that for sufficiently strong coupling, the system features a phase transition from a Mott insulator with one particle per site to a superfluid of spatially extended particle pairs living on top of the Mott background—instead of the usual transition to a superfluid of single particles/holes. Increasing the interaction further, a superfluid of particle pairs localized on a single site (rather than being extended) on top of the Mott background appears. This happens at the same interaction strength where the Mott-insulator phase with two particles per site is destroyed completely by particle–hole fluctuations for arbitrarily small tunneling. In another regime, characterized by weak interaction but high occupation numbers, we observe a dynamical instability in the superfluid excitation spectrum. The new ground state is a superfluid, forming a two-dimensional (2D) slab, localized along one spatial direction that is chosen spontaneously.
Highlights
Systems of ultracold atoms in optical lattices provide a unique playground for controlled realizations of many-body physics [1, 2]
After writing down the effective single-band Hamiltonian including the effect of the site occupation, we find that for strong enough interaction, there is a transition from a Mott state with one particle localized at each lattice site to a superfluid of pairs extended over neighboring sites, rather than to a superfluid of single atoms
Having written down a suitable model Hamiltonian describing the regime of strong interaction, we study the transition from the Mott insulator having on average n particles per site to a superfluid of single particles/holes
Summary
Systems of ultracold atoms in optical lattices provide a unique playground for controlled realizations of many-body physics [1, 2]. After writing down the effective single-band Hamiltonian including the effect of the site occupation, we find that for strong enough interaction (characterized by the s-wave scattering length as), there is a transition from a Mott state with one particle localized at each lattice site to a superfluid of pairs extended over neighboring sites, rather than to a superfluid of single atoms This feature is novel, considering the fact that the extended pairs emerge in the singlespecies repulsive bosonic system without the presence of any long-range interaction. We show that this mechanism will eventually become relevant when the s-wave scattering length is increased, and that one finds a phase transition to a superfluid of extended pairs. With the increasing superfluid density, the condensate may become dynamically unstable
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