Tunneling – thermal activation crossover in neutron irradiated quartz

Abstract

The low-temperature internal friction data in neutron-irradiated and amorphous quartz are compared. The region of the tunneling plateau in irradiated quartz extends too much higher temperatures in comparison with amorphous SiO 2. It is interpreted within the soft-potential model that two-level systems in irradiated quartz have higher crossover temperature, T c, from tunneling to thermal activation than in the glassy state. The characteristic energies, W, of the particles in the double-well potentials are estimated. Their difference is ascribed to higher effective masses of the tunneling centers inside the amorphous network than in the distorted crystalline regions of irradiated quartz. The high value of T c in neutron irradiated quartz explains also why the onset of the tunneling plateau can still be seen in the ultrasound data at high frequencies where one does not expect to see the tunneling states anymore.