paper

Collinear antiferromagnetic phases of a frustrated spin- ---- Heisenberg model on an -stacked bilayer honeycomb lattice

arXiv:1805.01272 · doi:10.1016/j.jmmm.2019.03.033

Abstract

The zero-temperature quantum phase diagram of the spin- ---- model on an -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 and . The two layers are coupled via a comparable Heisenberg exchange interaction between NN interlayer pairs, with a strength . The complete phase boundaries of two quasiclassical collinear AFM phases, namely the Néel and Néel-II phases, are calculated in the half-plane with . Whereas on each monolayer in the Néel state all NN pairs of spins are antiparallel, in the Néel-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 and the excitation energy from the ground state to the lowest-lying excited state (where is the total component of spin for the system as a whole, and where the collinear ordering lies along the 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 () in a systematic series of approximations involving -spin clusters. The sole approximation made is then to extrapolate the sequences of th-order results for and to the exact limit, .

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