The zero-temperature quantum phase diagram of the spin-
21 J1--
J2--
J1⊥ model on an
AA-stacked bilayer honeycomb lattice is investigated using the coupled cluster method (CCM). The model comprises two monolayers in each of which the spins, residing on honeycomb-lattice sites, interact via both nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic (AFM) Heisenberg exchange iteractions, with respective strengths
J1>0 and
J2≡κJ1>0. The two layers are coupled via a comparable Heisenberg exchange interaction between NN interlayer pairs, with a strength
J1⊥≡δJ1. The complete phase boundaries of two quasiclassical collinear AFM phases, namely the Neel and Neel-II phases, are calculated in the
κδ half-plane with
κ>0. Whereas on each monolayer in the Neel state all NN pairs of spins are antiparallel, in the Neel-II state NN pairs of spins on zigzag chains along one of the three equivalent honeycomb-lattice directions are antiparallel, while NN interchain spins are parallel. We calculate directly in the thermodynamic (infinite-lattice) limit both the magnetic order parameter
M and the excitation energy
Δ from the
sTz=0 ground state to the lowest-lying
∣sTz∣=1 excited state (where
sTz is the total
z component of spin for the system as a whole, and where the collinear ordering lies along the
z direction) for both quasiclassical states used (separately) as the CCM model state, on top of which the multispin quantum correlations are then calculated to high orders (
n≤10) in a systematic series of approximations involving
n-spin clusters. The sole approximation made is then to extrapolate the sequences of
nth-order results for
M and
Δ to the exact limit,
n→∞.
We use the coupled cluster method (CCM) to study a frustrated spin-12J1−J2−J1⊥ Heisenberg antiferromagnet on a bilayer honeycomb lattice with AA stacking. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor antiferromagnetic (AFM) exchange interactions are present in each layer, with respective exchange coupling constants J1 > 0 and J2 ≡ κJ1 > 0. The two layers are coupled with NN AFM exchanges with coupling strength J1⊥≡δJ1>0. We calculate to high orders of approximation within the CCM the zero-field transverse magnetic susceptibility χ in the Néel phase. We thus obtain an accurate estimate of the full boundary of the Néel phase in the κδ plane for the zero-temperature quantum phase diagram. We demonstrate explicitly that the phase boundary derived from χ is fully consistent with that obtained from the vanishing of the Néel magnetic order parameter. We thus conclude that at all points along the Néel phase boundary quasiclassical magnetic order gives way to a nonclassical paramagnetic phase with a nonzero energy gap. The Néel phase boundary exhibits a marked reentrant behavior, which we discuss in detail.