published_or_final_version / Physics / Master / Master of Philosophy

Thesis (M.S.)--Ohio University, August, 1989. / Title from PDF t.p.

Thesis (M.S.)--Ohio University, March, 1990. / Title from PDF t.p.

Thesis (M.S.)--Ohio University, August, 1992. / Title from PDF t.p.

Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references.

Thesis (Ph. D)--Ohio State University, 2003. / Title from first page of PDF file. Document formatted into pages; contains xiv, 124 p.: ill. Includes abstract and vita. Advisors: Edward W. Colllings, Robert L. Snyder, Dept. of Materials Science and Engineering. Includes bibliographical references (p. 114-124).

Some properties of thin film aluminum superconductors (80-130 Å) were investigated for use as a high frequency second sound detector. The films' transition temperatures, Tc, were raised to near the lambda point of He by a surface enhancement of the BCS coupling constant. The films were useful as sensitive thermometers over the temperature range of their transition, Δ Tc= 0.06 K, with a sensitivity given by the transition slope, 2-3 x 10³ ohms/K. The films mechanically and electrically withstood the thermal shock of five thermal cycles from room temperature to 2 K. The instability of Tc after cycling was at most 0.05 K. By increasing the bias current, Tc (the He bath temperature at the transition) could be decreased by at most 0.025 K. The slope of the transition curve, dR/dT, increased by at most 30% upon thermal cycling and within experimental accuracy the bias current had no effect on dR/dT, for the range 2-60μA. From measurements of one film's time response to thermal excitation, the time constant was found to be less than 45.5 n sec, which means that the film is capable of fully responding to second sound of frequencies of at least 3.5 MHz. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The generalized Langevin equation, which is derived by H. Mori,as a general, microscopic formulation of the stochastic theory of Brownian motions, is applied to evaluate the longitudinal attenuation rate of sound in weak-coupling, pure superconductors. In order to qualify the ultrasonic absorption as stochastic processes, the relaxation time of the random interactions on the phonon must be much shorter than the relaxation time of the phonon. We show that this is indeed the case by showing that the former is of the order of the relaxation time of the distribution function of the excitations from the BGS condensate. As compared, with the earlier result derived by first order perturbation theory, the inclusion of the life-time effect has improved the agreement with experimental data. When Ƭ≿Δ,ω, it is shown that Cooper pairs participate in the absorption processes even when ω <2 Δ. A sum rule for the longitudinal ultrasonic absorption rate is also derived. We also employ the Lamrevin approach to evaluate the spectra, variances and covariances of the magnetic flux, the currents and the magnetisation in a lohec, thin, hollow superconducting cylinder. When the wall thickness of a cylinder is arbitrary, the langevin approach becomes inconvenient. Modifying slightly Kubo's formulation of perturbation theory to take care of the fact that generalized susceptibility may not be zero at infinite frequency, we establish in the classical limit, a general relation between generalized susceptibility αBA(ω) and the cross-spectrum of two quantities A and B, where ω is the frequency. Putting ω to zero, a particularly simple relation between covariance and generalized susceptibility is obtained. This relation and fluctuation dissipation theorem are then applied to evaluate the spectra, variances and covariances of magnetic flux, currents and magnetisation for the case of a long, hollow, superconducting cylinder of arbitrary thickness. Setting the inner radius of the cylinder to be zero, we obtain the particular case of a solid cylinder and our theory is compared with an experiment by Vant-Hull et. al., which measures the fluctuations of the magnetic flux in solid, metal cylinders. According to our theory, the thermal magnetic noise does not disappear when the metal becomes superconducting. With the dimensions of the tin rod used in the experiment, we expect theoretically, however, the standard deviation of the magnetic flux to drop more than an order of magnitude from its normal state value when becomes greater than 10⁻⁵. Experiment agrees with our theoretical expectation. More accurate data, however, are needed to decide whether our theory is quantitatively adequate. In the above two approaches, we have assumed the fluctuation of the electron density ns in the BCS-condensate to be negligible. To take care of the fluctuation of pair density, we make use of the fact that the probability density of ns and ϕ is proportional to exp {-βGs(ns, ϕ)} where ϕ is the magnetic flux and Gs is the Gibbs free energy. To avoid, mathematical complications, we keep to the case of a long, thin, hollow cylinder in the Landau-Ginzburg temperature domain. We also discuss the responses of magnetic flux and currents to an external magnetic field which changes stepwise at time t=0. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 174-175). Available also in a digital version from Dissertation Abstracts.