PHASE-DIAGRAMS, CRITICAL, AND MULTICRITICAL BEHAVIOR OF HARD-CORE BOSE-HUBBARD MODELS

We determine the zero-temperature phase diagram of the hard-core Bose- Hubbard model on a square lattice by mean-field theory supplemented by a linear spin-wave analysis. Due to the interplay between nearest-and next-nearest-neighbor interaction and cubic anisotropy several supers olid phases with checkerboard, stripe-domain, or intermediate symmetry are stabilized. The phase diagrams show three different topologies de pending on the relative strength of nearest- and next-nearest-neighbor interaction. We also find a rich variety of new quantum critical beha vior and multicritical points and discuss the corresponding effective actions and universality classes.