title: Lifetime of gapped excitations in a collinear quantum antiferromagnet.
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, Γ(imp)≈Γ(0)+A(TlnT)2, greatly exceeds the magnon-magnon damping, Γ(m-m)≈BT5, 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.
authors.affiliation: Department of Physics, University of California, Irvine, California 92697, USA.
authors.forename: A L
authors.initials: AL
authors.lastname: Chernyshev
authors.forename: M E
authors.initials: ME
authors.lastname: Zhitomirsky
authors.forename: N
authors.initials: N
authors.lastname: Martin
authors.forename: L-P
authors.initials: LP
authors.lastname: Regnault