• Description of long-range ordered spin configurations near a phase transition T N: S=0, T>T N and S∫0, T TT, is characterised by Q=O, P"'O [27]. Landau Theory of the Fermi Liquid 5.1 Adiabatic Continuity The results of the previous lectures, which are based on the physics of non-interacting systems plus lowest orders in perturbation theory, motivates the following description of the behavior of a Fermi liquid at very low tempera-tures. The fundamental idea of Landau theory is to define an order parameter, Q (or sometimes ζ), i.e. If the free energy is expanded to sixth order in the order parameter, the system will undergo a first order phase transition if $\alpha_0 > 0$, $\beta 0$, and $\gamma > 0$. system is always in a single homogeneous phase. On the other hand, when interfaces are The manifest differences in the properties of the phases must then appear as discontinuities in some derivatives of G. Many different physical properties can be used as an order parameter for different kinds of transition, and usually there are different properties which could equally well be used for a given transition. As one author