Thesis (Ph. D.)--University of Alberta, 2009. / Title from PDF file main screen (viewed on Oct. 20, 2009). In the title, [2], [3] and [7-delta] are subscripted on their elements and delta is represented by the Greek letter. "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Department of Physics, University of Alberta." Includes bibliographical references.

Thesis (MScIng)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

abstract: The study of high energy particle irradiation effect on Josephson junction tri-layers is relevant to applications in space and radioactive environments. It also allows us to investigate the influence of defects and interfacial intermixing on the junction electrical characteristics. In this work, we studied the influence of 2MeV Helium ion irradiation with doses up to 5.2×1016 ions/cm2 on the tunneling behavior of Nb/Al/AlOx/Nb Josephson junctions. Structural and analytical TEM characterization, combined with SRIM modeling, indicates that over 4nm of intermixing occurred at the interfaces. EDX analysis after irradiation, suggests that the Al and O compositions from the barrier are collectively distributed together over a few nanometers. Surprisingly, the IV characteristics were largely unchanged. The normal resistance, Rn, increased slightly (<20%) after the initial dose of 3.5×1015 ions/cm2 and remained constant after that. This suggests that tunnel barrier electrical properties were not affected much, despite the significant changes in the chemical distribution of the barrier's Al and O shown in SRIM modeling and TEM pictures. The onset of quasi-particle current, sum of energy gaps (2&#916;), dropped systematically from 2.8meV to 2.6meV with increasing dosage. Similarly, the temperature onset of the Josephson current dropped from 9.2K to 9.0K. This suggests that the order parameter at the barrier interface has decreased as a result of a reduced mean free path in the Al proximity layer and a reduction in the transition temperature of the Nb electrode near the barrier. The dependence of Josephson current on the magnetic field and temperature does not change significantly with irradiation, suggesting that intermixing into the Nb electrode is significantly less than the penetration depth. / Dissertation/Thesis / M.S. Materials Science and Engineering 2012

This thesis describes applications of the ac Josephson effect. Firstly, results are presented from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at 72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured capacitance was less than that expected from a calculation of the parallel plate shunt capacitance. The explanation was thermal noise suppression of the hysteresis and the junction saw the shunt capacitor as a distributed impedance rather than a lumped circuit element. It was found during these investigations that the influence of the SrTiO3 substrate on the intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at 4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The dispersion relation consisted of two branches, one at low frequencies with a high resonator capacitance per unit length and a high frequency branch with a low resonator capacitance per unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3, which drops above the soft optic phonon frequency. From the dispersion relation, the permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz. The ac Josephson effect was exploited to measure the permittivity of thin films of SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators. The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the 50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz. The grain boundary capacitance was used to probe grain boundary current transport. The capacitance per unit area scaled inversely with resistance area product and increased linearly with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24° bicrystals. This behaviour could not be explained by tunneling models of grain boundary current transport, and requires current flow over a fraction of the area of the grain boundary.

No description available.

Superconductivity in magnesium diboride (MgB2) was first discovered in 2001. It is unique in that it has two superconducting gaps. The transition temperature of 39 K exceeded the maximum transition temperature thought to be possible through phonon mediated superconductivity. Through the study of MgB2, a general paradigm is being formulated to describe multi-gap superconductors. The paradigm includes inter-band and intra-band scattering between the gaps which can cause a smearing of the gap parameter over a distribution instead of a single value. Although each gap is individually thought to be well described by the BCS theory, the interaction between the two gaps causes complications in describing the overall superconducting properties of MgB2. The focus of this work was to lay the groundwork for an MgB2-based Josephson junction technology. This includes improving on a previously established baseline for all-MgB2 Josephson junctions, utilizing the Josephson Effect to experimentally verify a model pertaining to the two-gap nature of MgB2, specifically the magnetic penetration depth, and designing, fabricating, and testing multi-junction devices and circuits. The experiments in this work included fabrication of Josephson Junctions, DC superconducting quantum interference devices (SQUIDs), Josephson junction arrays, and a rapid single flux quantum (RSFQ) circuit. The junctions were all made utilizing the hybrid physical-chemical vapor deposition method, with an MgO sputtered barrier. The current process consists of three superconducting layers which are patterned using standard UV photolithography and etched with Ar ion milling. There were SQUIDS made with sensitivity to magnetic fields parallel to the film surface, which were used to measure the inductance of MgB2 microstrips. This inductance was used in design of more complicated devices as well as in calculating the magnetic penetration depth of MgB2, found to be about 40 nm at low temperature, in good agreement with a previously published theoretical model. Planar-type DC SQUIDs were also made to present the feasibility of the technology for application purposes. The large voltage modulation of over 500 &#956;V at 15 K for these devices along with operation up to 37 K shows that MgB2 is a potential replacement for low temperature devices. The junction series arrays were fabricated with 100 junctions of equal size to present the ever-increasing robustness of the technology. The devices served well to measure the large property spread associated with these junctions and have been well established as a diagnostic tool for improving this spread. The culmination of this work was a basic RSFQ toggle flip flop circuit. A DC measurement of these circuits yielded digital operation up to 180 GHz at low temperature and about 63 GHz at 20 K. This is not yet near the potential limit of MgB2 established by the value of the superconducting gap parameters, but a huge success in showing that MgB2 is a viable option for pursuing superconducting digital electronics suitable for low power, cryogen-free operation. / Physics

Dynamic properties of Josephson junctions are interesting due to the emission of high frequency radiation (up to THz range) from Josephson junctions, closely related to fluxon dynamics. A better understanding of this dynamics can help to improve the Josephson devices used for applications. Josephson junctions can also be of great use as T-shaped multiple Josephson junctions in Josephson electronic circuits. In general, T-junctions consist of two attached Josephson transmission lines: a main Josephson transmission line (MJTL) along the -axis, and an additional Josephson transmission line (AJTL) along the -axis. These junctions can use to create fluxons (solitons) in junctions without applied magnetic field, (called flux cloning phenomenon). This work is devoted to contributing to a clarification of the dynamic behaviour of solitons (fluxons) in 2D extended conventional T-shaped Josephson junctions (extended means an AJTL is larger than MJTL). A conventional T-junction is a MJTL along the x-axis which divides into two Josephson transmission lines along the x- and y-axes. In addition, we also attempt to elucidate further the concept of flux cloning in rotated T-junctions, which are 90 degrees anticlockwise rotation of conventional T-junction. In rotated Tjunction, a MJTL along the x-axis divide into two Josephson transmission lines along the y-axis. We find the first evidence of moving semifluxon and observe for the first time new phenomena of semifluxons and anti-semifluxons in both extended conventional and rotated T-junctions. We numerically study the electrodynamics behaviour of solitons in the standard Tshaped Josephson junction (conventional T-junction) in a magnetic field. Therefore, we describe theoretically how flux cloning circuits exist and give an opportunity for use as flux flow oscillators operating without applied magnetic field. The results that emerge give further support to the flux cloning mechanism.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by David Bazeley Tuckerman. / M.S.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

In this thesis, investigations into the suitability of three 'weak-link' models, designed for the simulation of superconducting cluster systems, are reported. The focus of the investigation is on both the accuracy of the transport properties produced by these models, and the time taken to produce their results. The thesis develops the theory behind a previous approach which was exclusively used to model percolation systems for coverages below the critical coverage. The modifications made allow the simulations to extend to system coverages above the critical coverage. An additional 'current-ramping' algorithm, to simulate the systematic increase or decrease of current forced through the system, is described and explored. The results for the three models are compared, and their suitability for future investigations is discussed.