Transitions between metastable states in silica clusters

Relaxation phenomena in glasses can be related to jump processes between different minima of the potential energy in the configuration space. These transitions play a key role in the low temperature regime, giving rise to tunnelling systems responsible for the anomalous specific heat and thermal conductivity in disordered solids with respect to crystals. A recently developed numerical algorithm is used to study the potential energy landscape of silica clusters, taking as a starting point the location of first-order saddle points. This makes it possible to find a great number of adjacent minima. An analysis is made of the degree of cooperativity of these transitions and the connection between physical properties and the topology of the configuration space. Two-level systems (pairs of minima constituting a tunnelling system) are identified, and the quantum mechanical ground state splitting is calculated by means of the Wentzel-Kramers-Brillouin approximation.