Tunneling states in vitreousGeO2s

Abstract

Ultrasonic measurements of the attenuation and the velocity variation have been carried out in amorphous GeO 2 at low temperature ͑0.3-10 K͒ and high frequencies ͑80-210 MHz͒. From numerical fits to the tunneling model, the typical parameters of the tunneling states ͑TS͒ were determined and compared to those found for vitreous SiO 2 . The study reveals that in a-GeO 2 , which is considered as a close structural analog to a-SiO 2 , although the density of states is found to be very similar in both materials, the coupling between the TS and the phonons is significantly smaller. In the model of coupled tetrahedra as the origin of the TS, this difference can be understood in view of the fact that numerical calculations about the vibrational characteristics of network amorphous solids indicate that the tetrahedra are more decoupled in vitreous GeO 2 than in vitreous silica.

Sound absorption in glasses

“…The plateau of the 6.3 kHz data [50] in Fig. 5(a) agrees with the tunneling prediction, but the plateau of longitudinal ultrasonic data [51,61] is markedly higher, in agreement with the soft potential calculation. [55], and at 87 kHz [54] in polystyrene with the soft potential calculation (parameters see Table 1).…”

“…Again, the 𝐶 𝑙 -value determined from the 80 MHz sound velocity change in Fig. 5(b), due to unsaturated resonant tunneling at low temperature is only compatible with the measured plateau height in the same sample [51] in the soft potential model, not in the tunneling model. The tunneling model fits yield the same density of states in both measurements, but a markedly stronger coupling constant in the absorption measurement, a result which according to the authors [51] has also been found in ultrasonic measurements of the sound absorption and the sound velocity change of neutron-irradiated quartz [62] and in vitreous silica [63].…”

Sound absorption in glasses

“…5. The plateau of the 6.3 kHz data [51] agrees with the tunneling prediction, but the plateau of longitudinal ultrasonic data [52,53] is markedly higher, in agreement with the soft potential calculation. Again, the C l -value determined from the 80 MHz sound velocity change at low temperature [52] is only compatible with the measured plateau height in the soft potential model, not in the tunneling model.…”

Acoustic behaviour of normal and densified vitreous GeO2

“…the region between the plateau and the loss peak, can arise from incoherent tunnelling effects within the two wells as a consequence of increasing thermal motion which prevents the phase coherence of the TLS tunnelling motion [1]. Limiting the present analysis to the temperature region above 20 K where only the relaxation process is dominant, the loss peak can be described by the asymmetric double-well potential (ADWP) model, following the procedure described by Gilroy and Phillips [14]. Barrier heights V varied randomly over the defect sites with Gaussian distribution,…”