Bose-Hubbard model with occupation dependent parameters

TL;DR

This paper investigates a modified Bose-Hubbard model with occupation-dependent parameters, revealing novel phase transitions including pair superfluidity and a spontaneously localized superfluid slab in strongly and weakly interacting regimes.

Contribution

It introduces a non-standard Bose-Hubbard model with occupation-dependent tunneling and interactions, uncovering new quantum phases and transition mechanisms.

Findings

Transition from Mott insulator to pair superfluid at strong coupling

Emergence of localized pair superfluid state

Dynamical instability and spontaneous slab formation in weak interaction regime

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 2 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 2D slab, localized along one spatial direction that is spontaneously chosen.