Using a modified spin-wave theory which artificially restores zero sublattice magnetization on finite lattices, we investigate the entanglement properties of the Néel ordered $J_1-J_2$ Heisenberg antiferromagnet on the square lattice. Different kinds of subsystem geometries are studied, either corner-free (line, strip) or with sharp corners (square). Contributions from the $n_G=2$ Nambu-Goldstone modes give additive logarithmic corrections with a prefactor $n_G/2$ independent of the Rényi index. On the other hand, $\pi /2$ corners lead to additional (negative) logarithmic corrections with a prefactor $l_q^c$ which does depend on both $n_G$ and the Rényi index $q$, in good agreement with scalar field theory predictions. By varying the second neighbor coupling $J_2$ we also explore universality across the Néel ordered side of the phase diagram of the $J_1-J_2$ antiferromagnet, from the frustrated side $0<J_2/J_1<1/2$ where the area law term is maximal, to the strongly ferromagnetic regime $-J_2/J_1\gg 1$ with a purely logarithmic growth $S_q=\frac{n_G}{2}lnN$, thus recovering the mean-field limit for a subsystem of $N$ sites. Finally, a universal subleading constant term ${\gamma }_q^{\mathrm{ord}}$ is extracted in the case of strip subsystems, and a direct relation is found (in the large-$S$ limit) with the same constant extracted from free lattice systems. The singular limit of vanishing aspect ratios is also explored, where we identify for ${\gamma }_q^{\text{ord}}$ a regular part and a singular component, explaining the discrepancy of the linear scaling term for fixed width vs fixed aspect ratio subsystems.

• Received 14 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.115126