# Frustrated spin-$\frac{1}{2}$ Heisenberg magnet on an $AA$-stacked honeycomb bilayer: High-order study of the collinear magnetic phases of the $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model

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## Abstract

The zero-temperature phase diagram of the frustrated spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg magnet on an $AA$-stacked honeycomb bilayer lattice is studied using the coupled cluster method implemented to very high orders. On each monolayer the spins interact via nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic Heisenberg interactions with respective strength parameters $J_{1}>0$ and $J_{2}\equiv\kappa J_{1}>0$. The two layers are coupled such that NN interlayer pairs of spins also interact via a similar isotropic Heisenberg interaction of strength $J_{1}^{\perp}\equiv \delta J_{1}$, which may be of either sign. In particular, we locate with high accuracy the complete phase boundaries in the $\kappa$-$\delta$ half-plane with $\kappa>0$ of the two quasiclassical collinear antiferromagnetic phases with N\'{e}el or N\'{e}el-II magnetic order in each monolayer, and the interlayer NN pairs of spins either aligned (for $\delta 0$) to one another. Compared to the two-sublattice N\'{e}el order, in which all NN intralayer pairs of spins are antiparallel to one another, the four-sublattice N\'{e}el-II order is characterized by NN intralayer pairs of spins on the honeycomb lattice being antiparallel to one another along zigzag (or sawtooth) chains in a specified direction from among the three equivalent honeycomb-lattice directions, and parallel to one another for the corresponding interchain pairs.