  Using the density-matrix renormalization group, we determine the phase
diagram of the spin 1/2 Heisenberg antiferromagnet on a honeycomb lattice with
a nearest neighbor interaction J1 and a frustrating, next-neighbor exchange J2.
As frustration increases, the ground state exhibits Neel, plaquette and dimer
orders, with critical points at J2/J1 = 0.22 and 0.35. We observe that both the
spin gap and the corresponding order parameters vanish continuously at both the
critical points, indicating the presence of deconfined quantum criticality.
