Ultraschalluntersuchung der Tunnelsysteme im Quarzglas bei hohem Druck


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BARTELL, Ulrich, 1983. Ultraschalluntersuchung der Tunnelsysteme im Quarzglas bei hohem Druck

@phdthesis{Bartell1983Ultra-9462, title={Ultraschalluntersuchung der Tunnelsysteme im Quarzglas bei hohem Druck}, year={1983}, author={Bartell, Ulrich}, address={Konstanz}, school={Universität Konstanz} }

This thesis presents an experimental study of the influence on the acoustic and thermal low temperature properties caused by elastically strong coupling excitations of static network distortions in amorphous silica Suprasil W1.<br />With reference to the current literature an account is given of the Low Temperature Anomalies in network glasses and of recent results about the microstructural properties of vitreous silica.<br />Following an introduction to the quantum mechanical model of Tunneling Systems (TLS), which is central to explaining quantitatively the low temperature acoustic end thermal properties of amorphous substances, the one-particle energy of a single TLS is derived and, taking into account an external static strain field, it is demonstrated, that two different nonlinear branches of the energy exist for both positive and negative deformation potential of the TLS respectively.<br />Starting point of the experiment is the assumption that hydrostatic pressure on the quartz sample may lead to a shift of the energy levels of all the TLS.<br />Measuring the effect on the propagation of ultrasound of these energy shifts might allow the determination of the energy distribution of the TLS density of states. Since TLS in dynamic coupling to ultrasonic waves feature a high value of the deformation potential of about 1 eV, a striking effect on the ultrasonic properties influenced by TLS of external pressure is expected.<br />For the experiments, a pressure cell has been built for performing ultrasonic measurements under hydrostatic pressure of up to 10 kBar (1 GPascal) in a cylindrical sample. By fitting the cell into a He3/He4 dilution refrigerator, the temperature range between 60 mKelvin and 300 Kelvin was covered.<br />Measurements of both the ultrasonic attenuation and sound velocity change have been carried out in the frequency range between 45 MHz and 200 MHz for longitudinal polarization of the sound wave.<br />Contrary to expectation, the applied hydrostatic pressure affects the observed ultrasonic attenuation and velocity change of TLS below T = 4 Kelvin only little by about 20 percent, whereas between T = 4 Kelvin and T = 300 Kelvin the ultrasonic propagation changes strongly with pressure: the attenuation peak at 45 Kelvin well-known in vitreous silica is shifted to 97 Kelvin by a pressure of 7 kBar, while the peak attenuation increases by about a factor of 7 with respect to the zero-pressure value.<br />In oder to facilitate a comparison between the experimental results and the TLS model, the complex acoustic susceptibility of the TLS has been calculated taking into account the dynamics of elastically coupling multipoles.<br />The energy density of states of TLS has been calculated with the assumption of independent TLS with particular regard to the sign of the TLS-deformation potential in order to evaluate the influence of hydrostatic pressure on the thermal and elastic properties determined by TLS.<br />With the supposition, all TLS had positive deformation potential, the calculated density of states below E/kB = 4 Kelvin would double its value even at a pressure as low as 0.1 kBar resulting in a strong increase of ultrasonic attentuation. With all the TLS having negative deformation potential, a pressure of 0.1 kBar would lead to an energy gap below E/kB = 4 Kelvin leading to a complete vanishing of ultrasonic attenuation in this case.<br />Since the experimental results of utrasonic propagation under pressure exhibit neither of these extreme cases, concord with the TLS ultrasonic susceptibility developed here is reached only, if an unequal mixture of TLS with deformation potentials of either sign is assumed. The measured ultrasonic absorption change under pressure is indicative of a slight prevalence of TLS having positive deformation potential.<br />The observed decrease of ultrasonic attenuation only at above a pressure of about 4 kBar means that the energy distribution of TLS extends to an upper limit of about 200 Kelvin.<br />The same model developed here was used to calculate the Grüneisen-Parameter leading to good agreement with known measurement results of thermal expansion in quartz glass, provided, the additional assumption is made of a symmetric distribution of static strains on the order of 10^-5 , which is readily plausible to exist in real samples.<br />Based on the surprisingly strong pressure dependence of the observed ultrasonic attenuation in the temperature range T = 4 .. 300 Kelvin, the shift in temperature of the absorption peak caused by a thermally activated structural relaxation process is explained in terms of a change with pressure of the potential barrier separating two equilibrium positions of the oxygen atoms which amounts to 75 Kelvin/kBar. The strong increase of the maximum absorption value may be explained by an increase of the number of relaxation centres in proportion. deposit-license Bartell, Ulrich 2011-03-24T17:57:10Z Ultrasonic Investigation of Tunneling Systems in Vitreous Silica under High Pressure Bartell, Ulrich 2011-03-24T17:57:10Z Ultraschalluntersuchung der Tunnelsysteme im Quarzglas bei hohem Druck 1983 deu application/pdf

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