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

Seung-Hwan Do,Andrew D Christianson,V Ovidiu Garlea,Tae-Hwan Jang,J A Rodriguez-Rivera,Hao Zhang,Travis J Williams,Sang-Wook Cheong,Jae-Hoon Park,Cristian D Batista,Tao Hong

doi:10.1038/s41467-021-25591-7

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 Ba2FeSi2O7 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.

Highlights

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
After demonstrating that the generalization of the well-known 1=S-expansion of the SU(2) spin-wave theory[35,36,37,38,39,40,41] is a loop expansion[42] of the SU(3) spin-wave theory, we show that the one-loop correction is enough to account for the broadening of the longitudinal mode and the large renormalization of the gap and the dispersion of this mode
The five S 1⁄4 2 energy levels are split into a singlet Sz 1⁄4 0 ground state and two excited Sz 1⁄4 ±1 and Sz 1⁄4 ±2 doublets with energies D and 4D, respectively

Summary

Introduction

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. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. Energy modes, they are unable to reproduce the quasi-particle decay and renormalization generated by the interaction between these modes To capture these effects requires going beyond the linear level and an objective of this paper is to generalize the 1/S-expansion of the SU(2) treatment to SU(3) in order to account for the quasi-particle decay and renormalization produced by the interaction (nonlinear) terms using the quintessential example of interacting longitudinal and transverse modes for an S 1⁄4 1 easy-plane quantum magnet as a test case

Methods

Results

Conclusion