Browsing by Subject "Superconductors."
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Item Open Access Design and implementation of A 06 GHz - 09 GHz RF-SQUID Read-out system and investigation of rf-SQUID signal characteristics(2005) Eker, TaylanDesign and implementation of a transceiver system for rf-SQUID (Superconducting Quantum Interference Device) operation is investigated in this work. Besides, experiments to characterize the rf-SQUID have been performed using the implemented system. The steps in system design and implementation are presented. The difficulties and drawbacks of the system are reported and alternative techniques required to overcome these problems are determined. Also, for the operation of the rf-SQUID at much higher frequencies, different transceiver architecture is proposed and possible drawbacks are stated. Using implemented system, several experiments were performed on two high Tc rfSQUID gradiometers with a tank-circuit resonating at 720 MHz. In these experiments, the frequency and amplitude of the applied rf signal were swept and output flux to voltage transfer signal (modulation added by rf-SQUID), Vspp, and incoming rf signal spectrum are reported and analyzed.Item Open Access Fabrication and optimization of rf-SQUID and SQUID integration assembly for high resolution magnetic imaging systems(2005) Akram, RizwanSuperconducting QUantum Interference Devices or SQUIDs are by far the most sensitive devices known for sensing magnetic flux down to a resolution of about 10−21Wb and finds innumerous applications in various fields. In order to apply these sensors to sense nano structures or to use in antibody or antigen ammunoassay experiments, especially in unshielded environments at Liquid Nitrogen temperatures, one needs to optimize the SQUIDs for high SNR and low background magnetic field sensitivity. The chief motivation for this research work and this thesis is to optimize and characterize HTc rf-SQUIDs so that high field sensitivity and high spatial resolution could be obtained. During the course of this study, rf-SQUIDs were fabricated using SEJ and Bicrystal technology and optimized for the above mentioned characteristics. In the next stage, these optimized sensors were integrated into an imaging system after a thorough investigation on problems related to the front-end assembly of such a system. Different SQUID microscope systems have been designed, fabricated, and tested in order to achieve high field sensitivity and high spatial resolution magnetic imaging under the constraint of keeping the sample at room temperature, while the SQUID is in Liquid Nitrogen Temperature. Using the developed systems, magnetic imaging of room temperature samples with sensitivities in the range of about 100fT/√ (Hz) and spatial resolution of about 100 µm were achievedItem Open Access HTSC superconducting edge-transition infrared detectors; principles, fabrication, and characterization(2000) Akram, RizwanSince the discovery of High temperature superconductors, superconductivity became one of the fast emerging technologies being used in numerous applications ranging from large-scale to small-scale applications such as IR,-detectors. High temperature superconductors such as YBü'zCuíOj due to their properties are the prime candidates for the fabrication of detectors. Their responsivity and detectivity can be improved by changing the design and SC characteristics. In this thesis, one-dimensional thermal model has been considered to improve the characteristic parameters of the SC. Based on the model, the annealing profile and manufacturing process is modified to further enhance the responsivity and detectivity of the detectors. A characterization setup is established to perform dc & ac measurements at low temperatures. In order to improve the cryogenic setu]), a thermal equivalent model for the system is also proposed.Item Open Access Impurity effects and symmetry of the order parameter in high-temperature oxide superconductors(1997) Bayındır, MehmetIdentification of the symmetry of the order parameter (OP) of high-l'c cuprates is important because it helps to understand possible mechanism that govern the physics of these materials. However, up to now, there is no consensus on the symmetry of the OP. On the other hand, nonmagnetic impurity substitutions would test the symmetry of OP. Present theoretical calculations overestimate suppression of the critical temperature (by a factor of 2 or more) in comparison to the experimental data. So far, differences between zinc [Zn) and nickel {Ni) substitutions have not been well understood. Considering the above arguments, effects of nonmagnetic impurities on the high-temperature cuprates are investigated by solving the Bogoliulpv-de Gennes (BdG) equations on two-dimensional square lattice. The critical impurity concentration is found to be very close to the experimental values. Possibility of extracting the symmetry of OP from the obtained results is discussed. Finiteranged impurity potentials and different potential strengths for different impurities are proposed to explain differences between Zn and Ni substitutions. Finally, it is concluded that our results are in favor of d-wave symmetry for tetragonal and s+d-wave symmetry for orthorhombic phase, and explain quite well the effects of nonmagnetic impurity substitutions in the high-7'c oxide superconductors. Beside these, the physical properties of the high-temperature oxide superconductors, the BdG equations and effects of nonmagnetic impurities on isotropic and anisotropic superconductors are reviewed.Item Open Access Superconducting systems of low dimensionality(1992) Gedik, M. ZaferIt is possible to call the last five years as the golden age of superconductivity. The two most important developments in the field are the discovery of copper oxide and fullerene superconductors. In this work, some possible pairing mechanisms for these materials ai’e examined by giving emphasis on the reduced dimensionality. First, an older problem, spatially separated electron-hole system, is investigated to identify the possible phases in coupled double quantum well structures in electric field. Secondly, the superconducting transition temperature and response to external magnetic fields of layered systems with varying number of layers are studied by means of a microscopic model and its GinzburgLandau version. It is also shown that an interlayer pairing mechanism, phonon assisted tunneling, can induce superconductivity. Finally, effects of the spherical structure of fullerenes are examined by solving a two fermion problem on an isolated molecule where the particles interact via a short range attractive potential. As a possible mechanism of superconductivity in alkali metal doped fullerenes, coupling between electrons and the radial vibrations of the molecule is investigated.