Kedar Damle, Fabien Alet, Sumiran Pujari
We study a spin-1/2 Heisenberg model on the honeycomb lattice with nearest-neighbor antiferromagnetic exchange J that favors N\'eel order, and competing 6-spin interactions Q which favor a valence bond solid state in which the bond-energies order at the "columnar" wavevector K = (2\pi/3;-2\pi/3). Using projector Quantum Monte Carlo techniques, we find evidence for a direct continuous quantum phase transition between the N\'eel and VBS states, analogous to the deconfined critical point between these states on the square lattice. This implies that such deconfined critical points can exist on the honeycomb lattice although Berry phase effects allow tripled hedgehog defects of the N\'eel order parameter to play a role in the critical theory, unlike the square lattice case in which only quadrupled hedgehog defects are allowed.
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http://arxiv.org/abs/1302.1408
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