title: Ground states of the spin-1 Bose-Hubbard model.
abstract: We prove basic theorems about the ground states of the S=1 Bose-Hubbard model. The results are quite universal and depend only on the coefficient U2 of the spin-dependent interaction. We show that the ground state exhibits saturated ferromagnetism if U2<0, is spin-singlet if U2>0, and exhibits "SU(3)-ferromagnetism" if U2=0, and completely determine the degeneracy in each region.
authors.affiliation: Department of Physics, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan.
authors.forename: Hosho
authors.initials: H
authors.lastname: Katsura
authors.forename: Hal
authors.initials: H
authors.lastname: Tasaki