Influence of heat treatment on the relaxation ultrasonic attenuation due to two-level systems in neutron-irradiated quartz.

Abstract

This paper provides new data to be taken into account in the discussion of the microscopic origin of the two-level systems (TLS) in neutron-irradiated quartz. Grasse et al. [Phys. Rev. Lett 46, 261 (1981)] argued that the TLS in neutron-irradiated quartz are contained in 20-A\r{} ``amorphous'' clusters. This was contradicted by Gardner and Anderson [Phys. Rev. B 23, 474 (1981)] on the basis of thermal-conductivity measurements performed before and after a heat treatment of 840 \ifmmode^\circ\else\textdegree\fi{}C. We measured the longitudinal ultrasonic attenuation in neutron-irradiated quartz before and after a similar heat treatment at 840 \ifmmode^\circ\else\textdegree\fi{}C for a frequency of 640 MHz. The data indicate a drastic reduction of the density of states of TLS after annealing. This observation is discussed in view of the experimental results and the conclusions of both previous studies. It is shown that we can partly agree and partly disagree with both studies.