 Solution

Use the connected correlation function $G(x)=⟨\varphi (x)\varphi (0)⟩-⟨\varphi {⟩}^2$, so that the constant ordered contribution is removed. Away from a thermodynamic critical point its long-distance decay defines the correlation length: $-{\lim }_{R\to \infty }{R}^{-1}\log |G(R)|={\xi }^{-1}$ when this limit exists. In a massive rotation-invariant phase the exponential decay is typically multiplied by an algebraic factor.

Expand the Landau-Ginzburg theory free energy about a stable uniform minimum $M$, writing $\varphi =M+\delta \varphi$. To quadratic order,integration of the Gaussian field theory gives the Fourier-space connected correlation function $\tilde{G}(q)=k_{B}T/(c+\kappa q^2)$. Thus the Ornstein--Zernike correlation function hasFor an ordinary quartic transition, $M=0$ above the transition and $M^2=-r/u$ below, so $c=r$ and $c=-2r$, respectively. The correlation length diverges as $|T-T_c{|}^{-1/2}$ on both sides, with different amplitudes. At criticality the mass vanishes and the Gaussian correlation function is proportional to $R^{-(D-2)}$ for $D>2$. Interactions can change this power to $R^{-(D-2+\eta )}$.