Weak plaquette valence bond order in the $S=1/2$ honeycomb $J_1-J_2$   Heisenberg model
  Using the density matrix renormalization group, we investigate the $S=1/2$ Heisenberg model on the honeycomb lattice with first- ($J_1$) and second-neighbor ($J_2$) interactions. We are able to study long open cylinders with widths up to 12 lattice spacings. For $J_2/J_1$ near 0.3, we find an apparently paramagnetic phase, bordered by an antiferromagnetic phase for $J_2\lesssim 0.26$ and by a valence bond crystal for $J_2\gtrsim 0.36$. The longest correlation length that we find in this intermediate phase is for plaquette valence bond (PVB) order. This correlation length grows strongly with cylinder circumference, indicating either quantum criticality or weak PVB order. 
Zhenyue Zhu, David A. Huse, Steven R. White
cond-mat.str-el