Ultrasonic attenuation measurements in neutron-irradiated silicon

At low temperatures, amorphous and partly disordered solids exhibit properties which are different from those in crystals. These anomalies can 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 low average coordination of the individual atoms, and also in partly disordered, low-coordinated solids such as neutron-irradiated SiO₂, TS have been observed. In order to explore the possibility of the presence of TS in topological more constrained disordered solids, ultrasonic attenuation measurements were started on the fourfold coordinated silicon which was irradiated with fast neutrons to create large regions of lattice disorder. In this paper we will present and discuss the first results of these measurements and we will compare them with results of similar measurements on unirradiated, single-crystalline silicon.