Spontaneous magnon decays and beyond

by M. E. Zhitomirsky (CEA Grenoble)

Thursday 5 Mar 2015, 10:15 → 11:15 Europe/Zurich

Salle PH L1 503, EPFL (Salle PH L1 503, EPFL)

Magnons or spin-waves are elementary excitations in ordered quantum magnets. At zero temperature, magnons are stable quasiparticles in collinear antiferromagnets with only moderate energy renormalization due to magnon-magnon interaction. This conventional picture breaks down for noncollinear quantum antiferromagnets. The dynamics of noncollinear spin systems is dominated by three-magnon (cubic) processes, which may produce a finite magnon lifetime even at T=0. I discuss basic physical principles of spontaneous magnon decay using three examples (i) field-induced decays in the square-lattice Heisenberg antiferromagnet, (ii) zero-field decays in the triangular-lattice antiferromagnet, and (iii) triplon decays in spin liquids.
In the second part I discuss nontrivial physical effects that arise from interaction of high-energy gapped quasiparticles (rotons) with low-energy acoustic excitations (phonons). In the case of superfluid He-4 these lead to a non-monotonous temperature dependence of the roton gap observed recently in spin-echo experiments.
In the case of an easy-plane collinear antiferromagnet, I demonstrate presence of additional impurity assisted magnon-magnon scattering which yields the leading T-dependent contribution into the lifetime of an optical magnon.
This theoretical prediction is confirmed in the high-resolution neutron-scattering measurements on BaNi2(PO4)2.