ABSTRACT
The properties of weak solutions of impurities in superfluid helium may be discussed on the basis of extremely general considerations. By impurities we will mean atoms of the isotope He3, electrons, ions of He4, etc. In weak solutions the solute particles will practically not interact with one another. Since they have a large wavelength they will not be localized in one spot in the liquid. The interaction of the impurities with the atoms of He4 leads to the appearance of additional energy levels. The states of the impurities may be classified by the values of the momentum, which is a continuous variable. Thus to each impurity there corresponds an elementary excitation characterized by an energy which is a function of the momentum. In principle various types of spectra are possible. However, in practice, we shall always deal with the simplest case in which the energy ϵ is just a quadratic function of the momentum p with an effective mass m* () ϵ = ϵ 0 + p 2 2 m * https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/20d72e0b-9d39-4175-acc6-a943547edf8a/content/eq726.tif"/>
