Lattice vibrations of disordered solids

The recent discovery of an amorphous solid without low-energy tunneling excitations (Two-Level-Systems) has shown that the amorphous structure is not their primary cause, as had previously been widely accepted. It has also been shown that these excitations arise as the crystalline order is gradually disturbed, and that their number densities saturate at their characteristic level regardless of whether the crystal amorphizes or not. It is concluded that the nature of the defects, and the cause for their saturation density, should be explored in disordered crystals, rather than in amorphous solids. (C) 1999 Elsevier Science Ltd. All rights reserved.