The frustrated Heisenberg antiferromagnet on the honeycomb lattice: J1-J2 model

We study the ground-state phase diagram of the frustrated spin-1/2 J1-J2 antiferromagnet with J2 = xJ1 > 0 (J1 > 0) on the honeycomb lattice, using the coupled-cluster method. We present results for the ground-state energy, magnetic order parameter and plaquette valence-bond crystal (PVBC) susceptibility. We find a paramagnetic PVBC phase for xc1 < x < xc2, where xc1 ≈ 0.207 ± 0.003 and xc2 ≈ 0.385 ± 0.010. The transition at xc1 to the Néel phase seems to be a continuous deconfined transition (although we cannot exclude a very narrow intermediate phase in the range 0.21 ≲ x ≲ 0.24), while that at xc2 is of first-order type to another quasiclassical antiferromagnetic phase that occurs in the classical version of the model only at the isolated and highly degenerate critical point x= 1/2 . The spiral phases that are present classically for all values x> 1/6 are absent for all x ≲ 1. (paper)