Crossover temperature from tunneling to thermal activation in neutron-irradiated quartz

Abstract

The low-temperature internal friction data in neutron-irradiated quartz and amorphous ${\mathrm{SiO}}_{2}$ are compared. The region of the tunneling plateau in irradiated quartz extends to much higher temperatures in comparison with amorphous ${\mathrm{SiO}}_{2}.$ It is interpreted within the soft-potential model that two-level systems in irradiated quartz have a 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.