ULTRASONIC-ATTENUATION MEASUREMENTS IN NEUTRON-IRRADIATED SILICON

At low temperatures, amorphous and partly disordered solids exhibit pr operties which are different from those in crystals. These anomalies c an phenomenologically be described by low-energy excitations which are characterized by a broad distribution of energy and relaxation times : the so-called tunneling states(TS). It was first believed that these TS can only occur in typical glass-forming amorphous solids with a lo w average coordination of the individual atoms, and also in partly dis ordered, low-coordinated solids such as neutron-irradiated SiO2, TS ha ve been observed. In order to explore the possibility of the presence of TS in topological more constrained disordered solids, ultrasonic at tenuation measurements were started on the fourfold coordinated silico n which was irradiated with fast neutrons to create large regions of l attice disorder. In this paper we will present and discuss the first r esults of these measurements and we will compare them with results of similar measurements on unirradiated, single-crystalline silicon.