Roton-phonon interaction: from superfluid helium to quantum magnets

High-energy gapped quasiparticles (rotons), which interact with low-energy acoustic excitations (phonons), are ubiquitous in condensed matter physics. I discuss two recent examples which exhibit novel features. The high-precision neutron spin-echo measurements of rotons in the superfluid helium reveal a non-monotonous temperature dependence of the roton gap [1]. We explain this new phenomenon by competition between the standard roton-roton scattering effective above ~1K, and the roton-phonon three-particle processes, which appear due to the presence of the BoseEinstein condensate in the superfluid helium and dominate in the sub-Kelvin region.