Theory of temperature-dependent energy levels: Thermodynamic properties of He II

The statistical mechanics of interacting bosons is considered for cases where the interactions are sufficiently strong that the observed energy levels are dependent upon temperature. An approximate method is developed to handle the theory when energy levels are known from experiment. The results are applied to the thermodynamic properties of He II using Landau's theory of elementary excitations. This theory is assessed in several levels of approximation, and the results are compared extensively to measured thermodynamic properties. The relationship of neutron inelastic scattering measurements to these problems is discussed