Slowly relaxing tunneling states in neutron-irradiated quartz: A study of the internal friction at audio-frequencies

The internal friction of two neutron-irradiated quartz samples was measured at audio-frequency as a function of temperature. This is an extension of acoustic measurements to much lower frequencies and thus to much longer time scales. A temperature independent behaviour has been observed and is discussed in the framework of the tunnelling model. One of the major assumptions of this model regarding the tunnelling states with long relaxation time could be verified for neutron-irradiated quartz.     

Saturation of the dielectric absorption of vitreous silica at low temperatures

The dielectric absorption of vitreous silica containing 1200 ppm of OH^?-impurities has been measured at 10 GHz in the temperature range between 0.4 K and 8 K. A strong intensity dependence of the absorption coefficient has been observed at temperatures below 3 K.     

Low-temperature elastic experiments on glasses at very low frequencies

The Grüneisen parameter and the low-frequency elastic loss of vitreous silica, PMMA, and of the metallic glasses PdSiCu, PdZr, and CuZr have been measured between 0.4 K and 6 K using an elasto-caloric technique. The absorption measurements between 5·10^–3 Hz and 30 Hz give further support for the tunneling model in a hitherto poorly investigated domain of relaxation times. New features of the low-temperature behaviour of glasses are a long-time creep and an absorption peak of the metallic glass PdZr nearT _c.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Anomalous low temperature dielectric behaviour of γ-alumina

The temperature- and frequency-dependence of the dielectric constant of γ-alumina — with and without adsorbed water — has been studied at low temperatures and at microwave frequencies. The temperature dependence of the dielectric constant demonstrates the existence of the two-level- systems (2LS), characteristic also for other glassy materials. The 2LS are located inside the γ-alumina and not in the adsorbed water. 2LS of an energy splitting of less than 36 K have an exceedingly long lifetime in the excited state probably because of the lack of low energy phonons in this porous material. The size of 2LS seems to be less than 120 Å.     

“Glassy” properties of crystalline Li3N at low temperatures

We have studied the acoustic and dielectric behaviour of the superionic conductor Li₃N at temperatures between 100 K and 20 mK. At low temperatures its properties are determined by low-energy excitations similar to those observed in glasses. In Li₃N these excitations are ascribed to the tunneling of hydrogen impurities, whereas thermally activated hopping of lithium ions accounts for the behaviour at higher temperatures.