Phase Diagram and Magnetic Excitations of Anisotropic Spin-One Magnets

TL;DR

This study combines analytical spin wave theory and quantum Monte Carlo simulations to map the phase diagram and excitations of anisotropic S=1 magnets, emphasizing the importance of local constraint enforcement for accurate modeling.

Contribution

It introduces a modified spin wave approach that enforces local constraints, achieving better quantitative agreement with QMC results for S=1 Heisenberg models with anisotropy.

Findings

Modified approach aligns well with QMC data

Standard approximation less accurate quantitatively

Insights applicable to low-temperature quantum paramagnets

Abstract

We use a generalized spin wave approach and large scale quantum Monte Carlo (QMC) simulations to study the quantum phase diagram and quasiparticle excitations of the S=1 Heisenberg model with an easy-plane single-ion anisotropy in dimensions d=2 and 3. We consider two alternative approximations for describing the quantum paramagnetic state: the standard Holstein-Primakoff approximation and a modified treatment in which the local constraint (finite dimension of the local Hilbert space) is enforced by introducing a Lagrange multiplier. While both approximations produce qualitatively similar results, the latter approach is the only one that is in good quantitative agreement with the quantum phase diagram and the quasiparticle dispersions obtained with QMC. This result is very important for low-temperature studies of quantum paramagnets in magnetic fields because it shows that a simple modification of the standard analytical approach should produce much better quantitative agreement between theory and experiment.