A FRACTAL MODEL FOR SILICA AEROGELS

We have studied the density of states (DOS) of the fracton regime, whi ch is localized in the very-low-frequency region (less than 1/100 of t he spectrum), of a very large (more than 10(6) atoms) realistic model for silica aerogels. Our model is based on a percolating fractal struc ture, with an homogeneous particle represented by a sample of vitreous silica (V-SiO2), given by a molecular dynamics simulation. Interactio ns are represented by the Born-Mayer-Higgins potential. To evaluate th e DOS We have developed the linear-frequency moments method, which all ows us to compute the Dos of a very large system with great efficiency in the low-frequency region. A comparison with the experimental resul ts for silica aerogels is reported.