Low temperature glassy properties of disordered crystals

Low temperature measurements of specific heat, thermal conductivity, dielectric dispersion, ultrasonic dispersion and other properties have disclosed that a variety of disordered crystals exhibit the same anomalous behavior as found in amorphous solids. The anomalies arise from localized excitations having broad spectra in both energy and equilibration time. In some disordered crystals, these spectra may be changed systematically by varying the disorder present. The physical origin of the excitations, in crystals or glasses, lacks a satisfactory theoretical explanation. It is expected that the crystalline systems will be more amenable to theoretical study. The crystalline solids found to harbor the excitations include, at this time, certain fast-ion conductors, ferroelectrics, metallic alloys, and rotationally disordered solids.