Quantum and classical spin dynamics across temperature scales in the S = 1/2 Heisenberg antiferromagnet

TL;DR

This study combines neutron scattering experiments and semi-classical Landau-Lifshitz dynamics simulations to explore the temperature-dependent spin dynamics in a S=1/2 Heisenberg antiferromagnet, revealing quantum effects and their crossover to classical behavior.

Contribution

It demonstrates the effectiveness of Landau-Lifshitz dynamics in modeling quantum spin systems and elucidates the temperature evolution of quantum effects in the excitation spectrum.

Findings

Excellent agreement between experiment and LLD in the paramagnetic phase.

Identification of quantum renormalization of magnon energies.

Correlation of quantum effects with three-dimensional long-range order.

Abstract

Using the framework of semi-classical Landau-Lifshitz dynamics (LLD), we conduct a systematic investigation of the temperature-dependent spin dynamics in the S = 1/2 Heisenberg square-lattice antiferromagnet (SqAF). By performing inelastic neutron scattering measurements on Zn2VO(PO4)2 (ZVPO) and corresponding finite-temperature spin dynamics simulations based on LLD, we present a comprehensive analysis that bridges quantum and classical spin dynamics over a broad temperature range. First, a remarkable agreement between experimental data and LLD simulations is found in the paramagnetic phase of ZVPO, demonstrating the capability of LLD in accurately determining the spin Hamiltonian of S = 1/2 systems and capturing the quantum-to-classical crossover of their spin dynamics. Second, by analyzing the discrepancies between the experimental data and the LLD simulations at lower temperatures, we determine the experimental temperature dependence of the quantum effects in the excitation spectrum of the S = 1/2 SqAF: the quantum renormalization factor for the magnon energies and the quantum continuum above the one-magnon bands. Notably, the emergence of each quantum effect is found to correlate with the formation of three-dimensional long-range order. This work demonstrates the utility of LLD in gaining experimental insights into the temperature-induced modifications of quantum spin dynamics and their convergence towards classical expectations at higher temperatures. This motivates further applications to more challenging quantum antiferromagnets dominated by stronger quantum fluctuations.