Aspects of Condensation in ⁴He II

Abstract

The primary aim of the present article is to review certain aspects of the theoretical problem of ⁴He II, liquid helium below the lambda transition T=T _λ≅2.2°K. We shall not be concenred in what follows with the low temperature thermodynamic properties which make virtually no reference to superfluidity; these aspects seem now to be adequately understood [1]. Superfluidity has a profound effect on few of the thermally excited modes, and does not affect the correlation energy appreciably. Instead we give our attention here to certain aspects related to the appearance of a macroscopically occupied “coherent” mode, which is generally believed to be the most relevant property of the superfluid or superconducting state. In particular, a method for the experimental determination of ϱ_C(T), the density of the condensate as a function of the temperature, will be discussed, as well as a connection between ϱ_C(T) and the superfluid density ϱ_S(T) of the two fluid model.