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We describe the transition from extended to localized modes in a disordered elastic medium in 2+ dimensions as a phase transition in an appropriate nonlinear model. The latter is derived by considering fluctuations about the mean-field approximation to the replica field Lagrangian for the system. Within this framework, we calculate the averaged one- and two-particle phonon Green's functions obtaining the phonon density of states and frequency-dependent, zero-temperature thermal diffusivity, respectively. Momentum-shell integration of the nonlinear model reveals how this diffusivity renormalizes at longer length scales and hence the nature of normal modes at a given frequency. We demonstrate that all finite-frequency phonons in one and two dimensions are localized with low-frequency localization lengths diverging as 1{^2} and e^1{{^2}}, respectively, and that a mobility edge, *, separating low-frequency extended states from high-frequency localized states exists above d=2. The phonon localization length at this mobility edge is shown to diverge as |-{*|}^-1{}.
John et al. (1983) studied this question.