Statistical Physics

136 9 Second-Order Phase Transitions√Ψ = ±− a 2b . (9.7)Therefore, this situation describes the ordered phase below the critical temperature,and only one of the two possible choices of the order parameter isrealized. It is natural to assume that the coefficient a has the following Tdependence near the critical temperature T c :a(T )=a 0 (T − T c ) . (9.8)The order parameter then has the following temperature dependence:Fig. 9.1. Dependence of F L(T,V,N,Ψ) − F 0(T,V,N)onΨ at TT c (dashed line)Fig. 9.2. Temperature dependence of the order parameter around T c. The orderparameter is proportional to √ T c − T for T