volnag - The vortex transport properties of the...

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Unformatted text preview: The vortex transport properties of the superconducting films Volodymyr Nahirnyak Department of Physics University of Cincinnati Cincinnati, Ohio 45221 Abstract Superconductors, materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery. Not only have the limits of su- perconductivity not yet been reached, but the theories that explain superconductor behavior seem to be constantly under review. The explanations of the energy losses in the different practical applications of high temperature superconductors take sig- nificant part in modern investigation of transportation phenomena in the supercon- ducting films. This work considers the movement of the vortices in superconductors which produces a pseudo-resistance and the undesirable energy losses. 1 1 History of superconductivity In 1911, superconductivity was first observed in mercury by Dutch physicist Heike Kamer- lingh Onnes of Leiden University. When he cooled it to the temperature of liquid helium, 4 degrees Kelvin, its resistance suddenly disappeared. The Kelvin scale represents an abso- lute scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of super- conductivity. The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. Walter Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by a con- ductor induces currents in the conductor. This is the principle upon which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as diamagnetism and is today often referred to as the Meissner effect . The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material. In subsequent decades other superconducting metals, alloys and compounds were discovered. In 1941 Niobium-Nitride was found to su- perconduct at 16 K. In 1953 Vanadium-Silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of Niobium and Titanium. The first use of this wire in high-energy, particle- accelerator electromagnets, however, did not come until 1987 when it was employed at the Fermilab Tevatron. 2 Type I and Type II superconductors The Type I category of superconductors is mainly comprised of pure metals that normally show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in Cooper pairs in order to help each other overcome molecular obstacles - much like race cars on...
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