Thesis--Institut fiziki AN Ėstonii and Tartuskiĭ universitet. Doctoral dissertation in Solid State Physics, 1994. / Includes bibliography of author's publications (p. 89-[90]).

As an electromagnetic response detection technique, non-resonant microwave absorption (NRMA) has been without doubt, one of the fundamental tools in characterizing high temperature superconductors (HTSC). The technique can explicitly give factual information on flux pinning, granularity, magnetization, and detection of iota superconducting phases among many more. The emergence of iron pnictides superconductors has brought an enormous impact on HTSC field due to their relatively high 𝑇c, high critical fields 𝐵c2, huge critical current density and low anisotropy. Accordingly, they look appealing candidates in industrial applications more especially in high magnetic field applications. As of yet, its electromagnetic response particularly the low field microwave absorption (LFMA) or the non-resonant microwave absorption (NRMA) is relatively unknown. Consequently, in this work, systematic studies have been done on SmFeAs(O,F), superconductors to determine the low field sweep microwave absorption. Furthermore, effect of varying temperature, microwave power and field modulation amplitude on NRMA line shape have been addressed and the results obtained are compared with NRMA results of cuprates superconductors. Interestingly, the NRMA line shape has been found to evolve as a function of temperature, microwave power and field modulation amplitude. A structure i.e a broad peak 1 and a narrow peak 2 have been identified. Furthermore, the line shape shows a phase reversal at moderately high microwave power. This dissertation presents the theoretical background of superconductors, experimental techniques, working principles of the equipments, results, discussions and conclusions. As pertains to the future work, recommendations have been suggested in trying other forms of sample and also different sample materials of iron based superconductors to fully understand the NRMA and ensure a progressive and continuous work in this field. Also, we will carry out extensive studies on critical current density, fluxon dynamics and irreversibility fields on iron-based superconductors by means of NRMA technique. / Physics / M. Sc. (Physics)

The effect of oxygen partial pressure and temperature on the solid state sintering kinetics of the superconducting ceramic YBa₂Cu₃O₇₋₈ has been investigated. The isothermal contraction behavior of YBa₂Cu₃O₇₋₈ in the temperature range 930 - 960°C with oxygen partial pressures varying from 0.001 to 1.0 atmospheres was studied. The change in the unit cell lattice parameters of YBa₂Cu₃O₇₋₈ over this oxygen partial pressure range was also determined in order to evaluate its effect on the sintering behavior. The sintering kinetics of YBa₂Cu₃O₇₋₈ were investigated through the variation of the isothermal steady state contraction rate with respect to oxygen partial pressure at different temperatures. Isothermal contraction rates between 930 and 955°C reached a maximum at some critical oxygen partial pressure, PO₂CRIT, with decreasing contraction rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O₇₋₈ sintered at 960°C. SEM micrographs of the fracture surface of these samples suggest that a liquid phase was formed at this temperature. The unit cell volume and a-lattice parameter of YBa₂Cu₃O₇₋₈ decreased with decreasing oxygen partial pressure, while the c-lattice parameter increased. The activation energy for sintering at oxygen partial pressures above PO₂CRIT has been estimated to be approximately 191 kJ/mole, which is similar to oxygen ion diffusion in this compound, indicating that for oxygen partial pressures above PO₂CRIT, oxygen diffusion is the rate controlling mechanism for mass transport. The activation energy for sintering below PO₂CRIT was found to be ∽130 kJ/mole, which could also be due to oxygen ion diffusion. Several possible factors have been considered for the sintering behavior of YBa₂Cu₃O₇₋₈ for oxygen partial pressures below PO₂CRIT. These are formation of copper vacancies, positive hole creation, and lattice strain effect. However, although the oxygen partial pressure dependence on the sintering rate can be correlated with either the formation of Cu²⁺ vacancies in the lattice or hole formation, neither of these mechanisms are considered to be probable. Mathematically, the contraction rate was found to depend exponentially on the lattice strain (Δɛlat), ie. έ∞еΔɛlat. The significance of this relationship is not clearly understood. Intuitively, it appears that as the vacancies are created due to the removal of oxygen ions from the lattice, the lattice is not contracting to the extent needed to accommodate the vacancies. This elastic strain hinders the migration of the oxygen vacancies through the lattice, thus affecting the sintering rate of the YBa₂Cu₃Ox compound below PO₂CRIT. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Graduation date: 1994

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

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Superconductivity in high magnetic fields has attracted considerable atten- tion in recent years. The topic is important both for our fundamental un- derstanding of superfluids and for numerous practical applications. In this thesis, we consider several effects originating from the interplay between the Landau level structure of the normal state quasiparticle spectrum, and the tendency of the quasiparticles to form Cooper pairs below the critical tem- perature. A formalism designed to describe extreme type-II superconductors close to the upper critical field H_c2 is developed. The theory which utilizes the selection rules coming from the symmetry properties of the vortex lattice, simplifies the algebra describing a superconductor in the mixed state signifi- cantly. We are, on the mean field level, able to include the quantizing effects of the magnetic field on the electron motion exactly. A main achievement is the exact calculation of the expansion coefficients giving the grand canonical potential of a superconductor in terms of a power series in the size of the or- der parameter. The result is an expression for the grand canonical potential in terms of a polynomial in a finite set of variables close to H_c2. Using this formalism, a theory for the experimentally observed damped de Haas-van Alphen (dHvA) oscillations in the mixed state of a 2-dimensional (2D) superconductor is presented. The theory is compared with numerical results and the agreement is found to be good. A simple physical interpreta- tion of the damping is provided. The dependence of the damping on a finite Zeeman term, temperature, and the magnetic field is considered. A compar- ison of the theory with experimental data for the quasi-2D superconductor K-(ET)₂Cu(NCS)₂ yields good agreement. The attenuation of a longitudinal sound wave in the mixed state is then calculated. In analogy with the dHvA effect, we predict that there should be damped oscillations in the sound attenuation in the mixed state as the exter- nal magnetic field is varied. Furthermore, the dependence of the oscillations on the sound frequency and temperature is shown to yield information on the low lying quasiparticle spectrum. Especially, the presense of gapless excita- tions due to the magnetic field makes the attenuation qualitatively different as compared to the attenuation in the Meissner state. Some formal convergence properties of the Gor'kov theory for type-II su- perconductors close to H_c2 are derived. We show that the theory is essentially a high temperature expansion; the convergence radius of the Gor'kov series is proportional to k_BT when there is a Landau level at the chemical potential.

Thesis (Master)--İzmir Institute of Technology,İzmir, 2004 / Includes bibliographical references (leaves. 75).

Thesis (Ph. D.)--Ohio State University, 2007.