Abstract
It is shown that two-centre defects, closely related to intimate valence-alteration pairs (IVAPs), display the same statistical features as those suggested in the tunnelling-atom theory for two-level, low-energy excitations. Comparisons are made with measured short- and long-time relaxation effects in vitreous silica. The possibility that two-level systems originating from IVAPs and from tunnelling can co-exist in a-SiO2 is discussed, with particular reference to very-long-time relaxation effects.
