FRUSTRATED INTERACTIONS AND TUNNELING - 2-LEVEL SYSTEMS IN GLASSES

It is argued that the standard model of weakly interacting two-level systems (TLS) is compatible with the proposal that low-temperature properties of glasses are determined by strongly interacting defects. Typical strongly interacting defects have their tunneling suppressed, and observable tunneling occurs only for TLS which appear to be weakly coupled because their interactions are frustrated. Localized TLS are generated rather than collective modes because tunneling lowers the energy of the system. This explains both the success and the universality of the TLS model.