Modeling a ‘‘tunneling’’ state in amorphous silicon dioxide

A computer simulation of amorphous silicon dioxide has produced an example of a realistic model in which there exist two configurations of locally minimum energy differing by only 0.6×{}10^-15 erg/atom, separated by a maximum of 10^-15 erg/atom. The configurations can be interconverted almost entirely by rigid rotations of about seven SiO₄ tetrahedra which are connected by sharing corners. The inelastic neutron scattering by the lowest vibrational modes agrees well with a recent experiment. It is suggested that the model is typical of the states responsible for the unusual properties of glasses at very low temperatures.