Phonon-Like Excitations in Li,:uid Helium

An attempt is made to formulate a theory of phonon-like _excitations in 'liquid helium by generalizing the theory of phonons in quantum crystals to quantum. disordered syst~s. By using experimental data for pair correlation functions, dispersion curves of longitudinal phonon modes are calculated, with a. result for their over-all behavior in fairly good agreement with phonon-raton dispersion curves obtained by neutron scattering measurements. It is shown that the temperature-dependence of the calculated raton-minimum is in good.a,greement with experiment for liquid helium I, but not for liquid helium II. A brief discussion is given on the nature of rotons in liquid helium. It is suggested that, within certain restrictions, the rotan spectrum in liquid helium is likely to be a general feature of the dispersion of elementary excitations in liquids, classical or quantum. Since the pioneering work of Landau/> many attempts have been made to study the physical properties of elementary excitations in liquid helium.2> On the other hand, several works, both experimental and theoretical, have implied that the over-all behavior of the dispersion of elementary excitations in classical simpl