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The high-barrier quantum tunneling regime of a Bose-Einstein condensateconfined in a ring-shaped optical lattice is investigated. By means of a changeof basis transformation, connecting the set of `vortex' Bloch states and aWannier-like set of localized wave functions, we derive a generalizedBose-Hubbard Hamiltonian. In addition to the usual hopping rate terms, such aHamiltonian takes into account interaction-driven tunneling processes, whichare shown to play a principal role at high filling factors, when the standardhopping rate parameter turns out to be negative. By calculating the energy andatomic current of a Bloch state, we show that such a hopping rate must bereplaced by an effective hopping rate parameter containing the additionalcontribution an interaction-driven hopping rate. Such a contribution turns outto be crucial at high filling factors, since it preserves the positivity of theeffective hopping rate parameter. Level crossings between the energies perparticle of a Wannier-like state and the superfluid ground state areinterpreted as a signature of the transition to configurations withmacroscopically occupied states at each lattice site.

Bose-Hubbard model in a ring-shaped optical lattice with high filling factors