  We generalize the mean-field theory for the spinless Bose-Hubbard model to
account for the different types of superfluid phases that can arise in the
spin-1 case. In particular, our mean-field theory can distinguish polar and
ferromagnetic superfluids, Mott insulators which arise at integer fillings at
zero temperature, and normal Bose liquids into which the Mott insulators evolve
at finite temperatures. We find, in contrast to the spinless case, that several
of the superfluid-Mott insulator transitions are first-order at finite
temperatures. Our systematic study yields rich phase diagrams that include,
first-order and second-order transitions, and a variety of tricritical points.
We discuss the possibility of realizing such phase diagrams in experimental
systems.
