Entanglement perturbation theory for the elementary excitation in one dimension

TL;DR

This paper introduces entanglement perturbation theory to compute excitation spectra in one-dimensional quantum spin systems, successfully reproducing known results and providing new insights into spin-1 magnon spectra including the Haldane gap.

Contribution

The paper develops a novel entanglement perturbation approach for excitation spectra, accurately reproducing Bethe ansatz results and first-time calculating spin-1 magnon spectra across the Brillouin zone.

Findings

Reproduces des Cloiseaux-Pearson Bethe ansatz results for spin-1/2 systems.

Determines spin-triplet magnon spectrum for spin-1 systems.

Identifies the Haldane gap at $k= ext{pi}$ for the first time in this context.

Abstract

The entanglement perturbation theory is developed to calculate the excitation spectrum in one dimension. Applied to the spin-$\frac{1}{2}$ antiferromagnetic Heisenberg model, it reproduces the des Cloiseaux-Pearson Bethe ansatz result. As for spin-1, the spin-triplet magnon spectrum has been determined for the first time for the entire Brillouin zone, including the Haldane gap at $k=\pi$.