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|Title:||An Ultrasonic Study of Trapped Hydrogen Tunneling Systems in Niobium and Tantalum|
|Author(s):||Maschhoff, Kevin Robert|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Condensed Matter|
|Abstract:||The properties of trapped hydrogen tunneling systems in Nb and the analogous systems, H in Ta, and D in Nb have been studied with ultrasonic techniques at low temperature. Attenuation due to H in Ta was measured between 0.5 K and 10 K for frequencies between 10 MHz and 55 MHz. The sound velocity change due to H in Nb was measured in both the normal and superconducting state at 10.8 MHz and 33 MHz at temperatures between 0.5 K and 18 K. The attenuation and velocity change due to trapped D in Nb was measured in both the superconducting and normal states for two concentrations of oxygen trapping impurity.
It is concluded that nearly all of the experimentally observed properties of this system can be understood in terms of a two-level model. All parameters of the model can be derived from the velocity data.
Several features of the response predicted by the model have been compared to the ultrasonic data on H and D tunneling systems. The observed relaxation rate of H in Ta was found to be consistent with an electronic mechanism for the relaxation. The most significant result found was that driving the Nb host into the normal state reduces the energy splitting of the tunneling systems as predicted for Nb-O-H. Qualitative differences between the velocity data at high and low OH concentrations have been attributed to a distribution of internal strain created by the O impurities. The width of the distribution was derived from the velocity data. It was found that the tunneling parameter for D is much smaller than it is for H. This enhances the effects of internal strain on D tunneling systems and leads to relaxation spectra that are much broader for D than they are for H.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.
|Date Available in IDEALS:||2015-05-13|