Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by browse.metadata.advisor "De Kock, P. R."

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    Flux creep in pulsed laser deposited superconducting YBa₂Cu₃O₇ thin films
    (Stellenbosch : University of Stellenbosch, 2002-03) Maritz, E. J. (Erasmus Jacobus); Krylov, I. P.; Pretorius, R.; De Kock, P. R.; University of Stellenbosch. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT: High temperature superconductivity is an important topic in contemporary solid state physics, and an area of very active research. Due to it’s potential for application in low temperature electronic devices, the material has attracted the attention of researchers in the electronic engineering and material science fields alike. Moreover, from a fundamental point of view, several questions remain unanswered, related to the origin of superconductivity of this class of materials and the nature of quantised magnetic flux present in magnetised samples. In this work, flux creep phenomena in a thin superconducting YBa₂Cu₃O₇ film deposited by pulsed laser deposition, is investigated near the critical temperature 0 ≤ Tc – T ≤ 10 K. Creep activation energy U0 and critical current density jc were determined as a function of temperature close to Tc, providing important data to a problem of high-Tc superconductivity which is still a matter of debate. In particular it is still an open question whether restoring the temperature in a creep freezing experiment in fact restores the film to it's original state before the freezing. The most important novel results concern the regime of critical fluctuations in the vicinity Tc - T < 1 K. We studied the isothermal relaxation of trapped magnetic flux, and determined that the long time decay follows a power law, where the exponent is inversely proportional to the creep activation energy. The temperature dependence of the critical current density jc(T) of the YBa₂Cu₃O₇ film close to Tc was obtained during warming runs. It was determined that jc(T) follows a square root dependence on T to high accuracy in the range 0.2 ≤ Tc – T ≤ 1.5 K. During flux creep experiments an interesting phenomenon called creep freezing related to the strong temperature dependence of the relaxation rate was observed. A pronounced slowing of relaxation with only a small drop in temperature from a starting temperature close to Tc was detected. Experiments were conducted by initiating an isothermal flux decay run. At a certain point the temperature was slightly lowered, and the flux decay stopped within experimental accuracy. When the temperature was restored to the initial value, the flux decay resumed at the previous rate before cooling. An argument based on vortex drift velocity was employed to explain the phenomenon qualitatively. During the course of this investigation, a pulsed laser deposition (PLD) system was designed and built from scratch. PLD involves the interaction of a focussed laser pulse with a multielemental solid target material. Material ablated from the target forms a fast moving plume consisting of atomic and molecular particles, directed away from the target, and towards a usually heated substrate on which the particles condense layer by layer to form a thin film. The substrate temperature and background gas are carefully controlled to be conductive to the growth of a desired phase of the multi-elemental compound. The PLD system proved to be quite versatile in the range of materials that could be deposited. It was used to deposit thin films of different materials, most notable were good quality superconducting YBa₂Cu₃O₇, thermochromic VO2, and magnetoresistive LaxCa1-xMnO3. Metallic Au and Ag layers were also successfully deposited on YBa2Cu3O7 thin films, to serve as protective coatings. The critical temperatures of the best superconducting films were 90 K as determined by resistivity measurement. The optimal deposition conditions to deposit high quality superconducting YBa₂Cu₃O₇ thin films was found to be: deposition temperature 780°C, laser energy density 2-3 J/cm2, oxygen partial pressure 0.2 mbar, and target-substrate distance 35 mm. This yields film with Tc ~ 90 K. It was found that deposition temperature plays the predominant role in determining the quality of YBa₂Cu₃O₇ thin films deposited by PLD.
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    Relativistic descriptions of polarization transfer observables for quasielastic proton scattering
    (Stellenbosch : Stellenbosch University, 1999-03) Hillhouse, G. C. (Gregory Campbell); De Kock, P. R.; Stellenbosch University. Faculty of Science. Department of Physics.
    ENGLISH ABSTRACT: This thesis is devoted to the development of relativistic Dirac-based models for describing complete sets of quasielastic (p,p') and (p, ii) polarization transfer observables at medium energies. The original relativistic-plane-wave-impulse-approximation (RPWIA) model of Horowitz and Murdock is modified to include the phenomenological Horowitz-Love-Franey meson-exchange (HLF) model for the NN amplitudes, and new HLF parameter sets are generated between 80 and 200 MeV. Medium effects are incorporated by replacing free nucleon masses in the Dirac plane waves with more refined effective projectile and target nucleon masses. For a 4°Ca target at a fixed momentum transfer of 1.97 fm-1, and incident energies between 135 and 300 MeV, the sensitivity of complete sets of quasielastic (P,p') and (P, ii) polarization transfer observables is investigated with respect to nuclear medium effects, ambiguities in 1rNN coupling, exchange contributions to NN amplitudes, and spin-orbit distortions. It is seen that, (1) compared to (p,p 1 ) scattering, the (p, ii) polarization transfer observables are more sensitive to pseudoscalar versus pseudovector forms of the 1rNN coupling, (2) as the incident proton energy is lowered, nuclear medium effects and spin-orbit distortions become more important, (3) nuclear medium effects are extremely sensitive to the type of pion coupling, (4) contrary to the original RPWIA, exchange contributions cannot be neglected at energies as high as 500 MeV. For an optimal study of nuclear medium effects, this investigation stresses the urgent need for measurements of complete sets of quasielastic polarization transfer observables for both (p,p') and (p, n) reactions at energies lower than 200 MeV. Comparison of RPWIA predictions with the small amount of available data yields an inconsistent picture: The (P, p') data favour a pseudoscalar coupling for the pion, whereas the limited (P, n) data suggest a pseudovector form. Our poor treatment of distortions is considered to be the main source for this inconsistency. The issue of distortion effects on polarization transfer observables is addressed by developing the theoretical framework for the relativistic distorted wave impulse approximation. As an additional improvement over the RPWIA, models of nuclear structure (relativistic Fermi-gas model, relativistic mean-field approximation, and local-density-approximation) are developed, whereby the nuclear structure information is contained in a large set of nuclear respon