title: Rotons in a hybrid Bose-Fermi system.
abstract: We calculate the spectrum of elementary excitations in a two-dimensional exciton condensate in the vicinity of a two-dimensional electron gas. We show that attraction of excitons due to their scattering with free electrons may lead to formation of a roton minimum. The energy of this minimum may go below the ground state energy which manifests breaking of the superfluidity. The Berezinsky-Kosterlitz-Thouless phase transition temperature decreases due to the exciton-exciton attraction mediated by electrons.
authors.affiliation: Science Institute, University of Iceland, Dunhagi-3, IS-107, Reykjavik, Iceland.
authors.forename: Ivan A
authors.initials: IA
authors.lastname: Shelykh
authors.forename: Thomas
authors.initials: T
authors.lastname: Taylor
authors.forename: Alexey V
authors.initials: AV
authors.lastname: Kavokin