BEYOND THE EFFECTIVE-MEDIUM - QUASI-MODES IN DISORDERED MEDIA

For wave propagation in disordered media, the intermediate frequency r egime is defined as the frequency range where the wavelength is compar able to the scale of the inhomogeneities. While difficult to treat the oretically, the regime is rich in physical phenomena. The focus of thi s paper is the transport characteristics of the coherent part of the w ave energy in the intermediate frequency range. Through a re-formulati on of the well-known coherent potential approximation, a new scheme is developed which is capable of identifying the quasi wave modes as wel l as yielding their dispersion relations. With no adjustable parameter , this scheme is demonstrated to give an excellent quantitative accoun t of the experimental data that showed two branches of the acoustic wa ve in colloidal suspensions. The new approach is used to predict the q uasi modes in a random collection of metallic spheres.