Phonons and magnons are elementary excitations that correspond to well-defined deformations of nuclear and magnetic lattices. These excitations are normally long-lived for harmonic potentials, but in some cases they spontaneously decay, resulting in unusually short lifetimes. Relaxors are anharmonic systems that are characterized by a broad, frequency-dependent peak in the temperature dependence of the dielectric permittivity. The perovskite PbMg1/3Nb2/3O3 (PMN) is one of the most studied relaxors. An international team of researchers using the Merlin neutron spectrometer has found that specific phonon lifetimes in PbMg1/3Nb2/3O3 are unusually short within well-defined ranges of energy and momentum. Their results suggest that the well-known relaxor “waterfall effect" is a form of quasiparticle decay analogous to those observed in quantum magnets and fluids.
Related publication: C. Stock et al. “Spontaneous decay of a soft optical phonon in the relaxor ferroelectric PbMg1/3Nb2/3O3" Phys. Rev. Materials 2(2018), 024404, DOI: 10.1103/PhysRevMaterials.2.024404