Tunneling-thermal activation crossover in neutron irradiated quartz

The low-temperature internal friction data in neutron-irradiated and amorph ous quartz are compared. The region of the tunneling plateau in irradiated quartz extends too much higher temperatures in comparison with amorphous Si O2. It is interpreted within the soft-potential model that two-level system s in irradiated quartz have higher crossover temperature, T-c, from tunneli ng to thermal activation than in the glassy state, The characteristic energ ies, 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 expla ins also why the onset of the tunneling plateau can still be seen in the ul trasound data at high frequencies where one does not expect to see the tunn eling states anymore. (C) 1999 Elsevier Science B.V. All rights reserved.