Two-centre defects and low-energy excitations in vitreous silica

Abstract

A first-principle chemical model for IVAP-like defects in vitreous silica is proposed. It is shown that these two-centre defects are diamagnetic and, forT<10 K, display a two-level-like energy spectrum. They also exhibit a dipolar nature in agreement with recent experimental data. A close analogy with the basic idea underlying the tunnelling-atom theory is shown, supporting the connection between low-energy excitations in amorphous materials and twofold degenerate equilibrium positions of certain lattice sites. In contrast to previous theories, the present model predicts a large contribution of fast relaxing two-level systems at very low temperatures (T<0.1 K), while slowly relaxing excitations achieve thermal activation only at higher temperature. This result is indirectly confirmed by deviations from linearity of the low-temperature specific heat and seems to bridge in a satisfactory way experimental results which appeared contradictory in the framework of the tunnelling-atom theory as it is usually formulated.