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

We study the ground-state phase diagram of the frustrated spin-1/2 J(1)-J(2) antiferromagnet with J(2) = xJ(1) > 0 (J(1) > 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 x(c1) < x < x(c2), where x(c1) approximate to 0.207 +/- 0.003 and x(c2) approximate to 0.385 +/- 0.010. The transition at x(c1) to the Neel phase seems to be a continuous deconfined transition (although we cannot exclude a very narrow intermediate phase in the range 0.21 less than or similar to x less than or similar to 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 less than or similar to 1.