The well known Landau theory of the low-temperature properties of superfluid 4He starts from a weakly interacting gas of phonons and rotons. This theory is very successful but it is essentially phenomenological since it makes no reference to the Bose condensate. The core of this book is a discussion of the modern microscopic theory of Bose-condensed systems based on finite-temperature Green's function techniques (the dielectric formalism). My emphasis is on developing the language and concepts of this formalism in a way that brings out the essential physics. This book is the first general account of the progress made in the last two decades toward understanding the excitations in superfluid 4He specifically within the framework of a Bose-condensed liquid. I hope it will be a guide and stimulus to a new generation of experimentalists and theorists studying superfluid 4He. The book should also be of interest to a much wider audience, since the phenomenon of Bose condensation, with its associated macroscopic quantum effects, plays a central role in modern condensed matter physics (Anderson, 1984).

The goal of this book is two-fold: (a) to summarize the field-theoretic analysis of a Bose-condensed fluid and (b) to use this formalism to understand the nature of the excitations in superfluid 4He. I emphasize how a Bose broken symmetry inevitably leads to certain characteristic features in the structure of various correlation functions, the most spectacular being the phenomenon of superfluidity. A major theme is the way in which a Bose condensate mixes the elementary excitation and density fluctuation spectra.