arXiv:1506.03703 · Physical Review B
Spin-wave approach for entanglement entropies of the J1−J2 Heisenberg antiferromagnet on the square lattice
Nicolas Laflorencie, David J. Luitz, Fabien Alet
Abstract
Using a modified spin-wave theory which artificially restores zero sublattice magnetization on finite lattices, we investigate the entanglement properties of the N\'eel ordered ${J}_{1}\ensuremath{-}{J}_{2}$ 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 R\'enyi index. On the other hand, $\ensuremath{\pi}/2$ corners lead to additional (negative) logarithmic corrections with a prefactor ${l}_{q}^{c}$ which does depend on both ${n}_{G}$ and the R\'enyi index $q$, in good agreement with scalar field theory predictions. By varying the second neighbor coupling ${J}_{2}$ we also explore universality across the N\'eel ordered side of the phase diagram of the ${J}_{1}\ensuremath{-}{J}_{2}$ antiferromagnet, from the frustrated side $0<{J}_{2}/{J}_{1}<1/2$ where the area law term is maximal, to the strongly ferromagnetic regime $\ensuremath{-}{J}_{2}/{J}_{1}\ensuremath{\gg}1$ 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 ${\ensuremath{\gamma}}_{q}^{\mathrm{ord}}$ 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 ${\ensuremath{\gamma}}_{q}^{\text{ord}}$ a regular part and a singular component, explaining the discrepancy of the linear scaling term for fixed width vs fixed aspect ratio subsystems.