Low-frequency Raman scattering from disordered or heterogeneous materials

Abstract

Raman scattering occurs from localized modes of vibration. In heterogeneous materials, it is observed from localized modes at the surface of heterogeneities; in disordered materials, it can be observed from fracton modes. In the latter case the Ramas intensity depends on the fractal dimensionality D and spectral dimensionality ᵭ, and on a superlocalization exponent d_Φ. From existing models for superlocalization, the Raman intensity can be expressed in terms of ᵭ only. Experimental results are given for three different materials: silica aerogels, Vycor and epoxy resin. The Alexander-Orbach conjecture is in agreement with experiment for Vycor and epoxy resin.