ABSTRACT
We have considered thus fax the nature of the elementary excitation spectrum in an arbitrary Bose liquid. We now consider what this spectrum is like for the only Bose liquid found in nature, He II. As we have remarked in Chapter 1, indirect information on the excitation spectrum of He II can be obtained via a measurement of ρ n and, as well, through measurements of the specific heat. If one assumes the Landau theory to be correct (and we shall see later that it is to within minor modifications), one can work backwards from the experimental data to a form of the quasi-particle spectrum consistent with that data. At the very lowest temperatures (T ≲ 0.5°K) only long wave-length phonons are thermally excited. The experimental results for ρ n and C v are consistent with a phonon velocity equal to that of the measured macroscopic sound velocity, 237 m/sec. As the temperature increases, roton excitations become important; above 1°K they represent the dominant excitation mode. The roton contribution to p n and C v is consistent with the Landau excitation spectrum, () ε roton ( p ) = Δ + ( p − p o ) 2 2 μ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429495717/2cc6e3e4-f702-48c8-b407-59783ef3d6a6/content/eq59_v2.tif"/>
