Abstract
Abstract In this chapter we discuss approximations to the dynamic susceptibility used to evaluate χ (Q, ω) in liquid 3He. In the previous chapter we obtained exact equations for χ (Q, ω)- We discuss, firstly, the mean field or random phase approximation (RPA) to these equations. The RPA is obtained from the full equation by approximating the full interaction by a local one and is widely used. In §15.2 we consider the conserving approximations of Baym and Kadanoff (1961). The low Q limit is considered in §15.3 which allows us to relate the interaction to the Landau parameters. Finally, approximations valid at high momentum transfer, Q ≫kF and ω ≃ ωR ≫ ∈ F are presented in §15.4.
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