Low-energy parameters and spin gap of a frustrated spin-s Heisenberg antiferromagnet with s ≤ ³/₂ on the honeycomb lattice

The coupled cluster method is implemented at high orders of approximation to investigate the zero-temperature (T = 0) phase diagram of the frustrated spin-s J₁–J₂–J₃ antiferromagnet on the honeycomb lattice.  The system has isotropic Heisenberg interactions of strength J₁ > 0, J₂ > 0 and J₃ > 0 between nearest-neighbour, next-nearest-neighbour and next-next-nearest-neighbour pairs of spins, respectively.  We study it in the case J₃ = J₂ ≡ κ J₁, in the window 0 ≤ κ ≤ 1 that contains the classical tricritical point (at κ_cl = ½) of maximal frustration, appropriate to the limiting value s → ∞ of the spin quantum number.  We present results for the magnetic order parameter M, the triplet spin gap Δ, the spin stiffness ρ_s and the zero-field transverse magnetic susceptibility χ  for the two collinear quasiclassical antiferromagnetic (AFM) phases with Néel and striped order, respectively.  Results for M and Δ are given for the three cases s = ½ , s = 1 and s = ³/₂, while those for ρ_s and χ are given for the two cases s = ½ and s = 1.  On the basis of all these results we find that the spin-½ and spin-1 models both have an intermediate paramagnetic phase, with no discernible magnetic long-range order, between the two AFM phases in their T = 0 phase diagrams, while for s > 1 there is a direct transition between them.  Accurate values are found for all of the associated quantum critical points. While the results also provide strong evidence for the intermediate phase being gapped for the case s = ½, they are less conclusive for the case s = 1.  On balance however, at least the transition in the latter case at the striped phase boundary seems to be to a gapped intermediate state.