Strong-coupling expansion for the two-species Bose-Hubbard model

TL;DR

This paper develops a strong-coupling expansion method to analytically determine the phase boundaries of a two-species Bose-Hubbard model, revealing re-entrant quantum phase transitions in various dimensions.

Contribution

It introduces a third-order strong-coupling analytical approach for the two-species Bose-Hubbard model, including attractive interactions, and predicts re-entrant phase transitions.

Findings

Re-entrant quantum phase transition from paired superfluid to Mott insulator and back.

Analytical phase boundary expressions up to third order in hopping.

Applicability to ultracold atomic systems for experimental testing.

Abstract

To analyze the ground-state phase diagram of Bose-Bose mixtures loaded into $d$-dimensional hypercubic optical lattices, we perform a strong-coupling power-series expansion in the kinetic energy term (plus a scaling analysis) for the two-species Bose-Hubbard model with onsite boson-boson interactions. We consider both repulsive and attractive interspecies interaction, and obtain an analytical expression for the phase boundary between the incompressible Mott insulator and the compressible superfluid phase up to third order in the hoppings. In particular, we find a re-entrant quantum phase transition from paired superfluid (superfluidity of composite bosons, i.e. Bose-Bose pairs) to Mott insulator and again to a paired superfluid in all one, two and three dimensions, when the interspecies interaction is sufficiently large and attractive. We hope that some of our results could be tested with ultracold atomic systems.