Strong-coupling expansion for the spin-1 Bose-Hubbard model
  In this study, we perform a strong-coupling expansion up to third order of the hopping parameter $t$ for the spin-1 Bose--Hubbard model with antiferromagnetic interaction. As expected from previous studies, the Mott insulator phase is considerably more stable against the superfluid phase when filling with an even number of bosons than when filling with an odd number of bosons. The phase-boundary curves are consistent with the perturbative mean-field theory in the limit of infinite dimensions. The critical value of the hopping parameter $t_{\rm C}$ at the peak of the Mott lobe depends on the antiferromagnetic interaction. This result indicates the reliability of the strong coupling expansion when $U_2$ possesses large (intermediate) values for Mott lobe with an even (odd) number of bosons. Moreover, in order to improve our results, we apply a few extrapolation methods up to infinite order in $t$. The fitting results of the phase-boundary curves agree better with those of the perturbative mean-field approximation. In addition, the linear fit error of $t_{\rm C}$ is very small for the strong antiferromagnetic interaction. 
Takashi Kimura
cond-mat.quant-gas