Семинар международной лаборатории физики конденсированного состояния: Mikhail Zhitomirsky, (CEA, Grenoble, France) "Interacting quasiparticles in quantum magnets: spin-wave decay and magnon pairing"

Quantum solids, liquids, and magnets typically exhibit collective excitations - phonons, magnons and so on.These involve motion of many constituent particles, yet act like individual weakly interacting entities —quasiparticles. Recently, two new phenomena have been predicted in the field of Quantum Magnetism, where residual quasiparticle interactions play a paramount role. The first example is spontaneous magnon decay, which occurs at zero temperature and is inherent to noncollinear magnetic structures and spin liquids. The second effect corresponds to formation of bound magnon pairs in spin systems with competing ferro- and antiferromagnetic interactions. Their subsequent condensation in an applied magnetic field produces novel spin nematic states. I shall present an overview of main theoretical approaches to both phenomena and also discuss possible experimental observations of such effects.