Physical review b vol:53 issue:22 pages:14849-14861
We report the results of a systematic study of the influence of heat treatment on the tunneling states (TS) in neutron-irradiated quartz. Measurements of the ultrasonic attenuation and velocity change are carried out before and after heat treatment, as a function of temperature (0.3-300 K) and in the ultrasonic frequency range (50-650 MHz), on well-characterized specimens. Two doses have been studied, irradiated, and heat treated under similar conditions. One dose is lying well below the so-called threshold dose, the other is situated near the threshold. From numerical fits of the data with the tunneling model the density of states (P) over bar and the coupling parameter gamma(l) were determined independently. This allows us to investigate the influence of the heat treatments on the number and nature of the TS. Attention is paid to both the influence of the annealing temperature and the length of the heat treatment. All samples still show the typical TS behavior after heat treatment, but the density of states is drastically reduced. Regarding the coupling, the two doses show a different behavior: while there is no influence on the coupling for the higher dose, a significant decrease in gamma(l) is observed for the lowest dose. This decrease can be interpreted in terms of changes in the tunneling entity itself and is in agreement with the changes in the distribution of the TS.