Physics Departmentm Ben Gurion University
Amorphous solids, polymers, and disordered solids show striking universal characteristics at low temperatures, despite enormous differences in their microscopic structure. Discovered 40 years ago, this universality has emerged as one of the outstanding unsolved questions in condensed matter physics. The phenomenological model of tunneling two level systems (TLS) (Anderson, Halperin, and Varma, and Philips) accounts well for much of the observed phenomena. However, the model can not account for the smallness and universality of the phonon attenuation nor for the energy scale (3K) below which universality is observed, and the nature of the TLS is not known. Here we show that two kinds of TLS exist, differing by their symmetry under inversion. The symmetric TLS interact weakly with the phonon field, but gap the strongly interacting TLS below 3K, and therefore dictate weak and universal phonon attenuation at low temperatures.