Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet

TL;DR

This paper investigates how local magnetic disorder affects the relaxation rates of gapped magnons in collinear and noncollinear antiferromagnets, combining theoretical models with neutron spin-echo experiments on BaNi2(PO4)2.

Contribution

It provides a theoretical framework for disorder-induced magnon relaxation and experimentally confirms predictions in a 2D XY antiferromagnet.

Findings

Disorder significantly increases magnon relaxation rates at finite temperatures.

Experimental data on BaNi2(PO4)2 matches the theoretical model.

Strong effects of disorder are also discussed for 3D and noncollinear antiferromagnets.

Abstract

We demonstrate that local modulations of magnetic couplings have a profound effect on the temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode, Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping, Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a high-resolution neutron-resonance spin-echo technique and find experimental data in close accord with the theoretical prediction. Similarly strong effects of disorder in the three-dimensional case and in noncollinear antiferromagnets are discussed.