We present a novel interpretation of the traditional Landau-Feynman picture of excitations in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Detailed consideration of neutron-scattering data on the temperature dependence of S(Q,ω) in the region 0.4≲Q≲2 A${\mathrm{̊}}^{\mathrm{-}1}$ strongly suggests that while the photon is a collective zero-sound mode, the maxon roton is a strongly renormalized single-particle excitation. The condensate-induced hybridization of these two modes explains the smooth photon-maxon-roton dispersion curve as well as the vanishing intensity of the maxon-roton peak in S(Q,ω) above ${\mathit{T}}_{\lambda }$.

• Received 24 January 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1454