| We investigate the scenario of the breakdown of a valence bond solid (VBS) in an S = 1/2 quantum antiferromagnet with non-frustrating four-spin interaction on the single and bilayer hexagonal lattice. By means of quantum Monte Carlo in the valence bond basis we find a strong first-order VBS-Neel phase transition in the single layer. In the bilayer the Neel regime separates the VBS and a disordered (zero tilted) phase by a continuous quantum phase transition. A quantum critical point marking a direct VBS-VBS or VBS-disorder transition is absent. This behavior is attributed to the Z3 ground state degeneracy imposed by the lattice and allows rigid valence bond configurations to survive while spin-fluctuations are enhanced. |
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