  We study a generalized honeycomb lattice spin-1/2 Heisenberg model with
nearest-neighbor antiferromagnetic 2-spin exchange, and competing 4-spin
interactions which serve to stabilize a staggered dimer state which breaks
lattice rotational symmetry. Using a combination of quantum Monte Carlo
numerics, spin wave theory, and bond operator theory, we show that this model
undergoes a strong first-order transition between a Neel state and a staggered
dimer state upon increasing the strength of the 4-spin interactions. We
attribute the strong first order character of this transition to the spinless
nature of the core of point-like Z(3) vortices obtained in the staggered dimer
state. Unlike in the case of a columnar dimer state, disordering such vortices
in the staggered dimer state does not naturally lead to magnetic order,
suggesting that, in this model, the dimer and Neel order parameters should be
thought of as independent fields as in conventional Landau theory.
