ABSTRACT
We begin our study of the superfluid Bose liquid with a brief review of the experimental and theoretical background of the liquid 4He, the only Bose liquid found naturally in the laboratory. We then consider, in Chapter 2, the quasi-particle excitations and show how at T = 0 the macroscopic occupation of the zero momentum state leads, in the long wavelength limit, to an identical excitation spectrum for quasi-particles and density fluctuations. The spectrum consists in phonons with a velocity given by the macroscopic velocity of sound. We turn in Chapter 3 to a consideration of the observed elementary excitation spectrum of He II. We discuss both the experimental measurements and the theories that have thus far been proposed to explain them.
