Density and quasiparticle excitations in liquid 4He.

The recent interpretation of the phonon-roton excitations in superfluid He as a density mode at low wave vector and a quasiparticle excitation at higher Q proposed by Glyde and Griffin is developed. When there is a condensate, no( T ), the quasiparticle response of liquid He, becomes a component of the observed density dynamic structure factor, S(g, to). The quasiparticle and density response functions also share a common denominator, due to a coupling via the condensate. At low Q, the observed S(g,cv) is confined predominantly to a single peak. The peak broadens with temperature but remains