Decay and renormalization of a longitudinal mode in a quasi-two-dimensional antiferromagnet

TL;DR

This study combines experimental neutron scattering and advanced theoretical modeling to investigate the decay and renormalization of longitudinal spin modes in a quasi-two-dimensional antiferromagnet near a quantum phase transition.

Contribution

It introduces a generalized spin-wave theory with one-loop corrections to explain many-body effects in interacting low-energy modes of anisotropic quantum magnets.

Findings

Observation of the decay and renormalization of the longitudinal mode across the Brillouin zone.

The decay is most pronounced at the zone center.

Theoretical predictions match experimental results when including all one-loop corrections.

Abstract

An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba$_{2}$FeSi$_{2}O$_{7}$ reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. The measured mode decay and renormalization is reproduced by including all one-loop corrections. The theoretical framework developed here is broadly applicable to quantum magnets with more than one type of low energy mode.