  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.
