Enhancement of magnetic properties in magnesium diboride and characterization of Josephson and quasiparticle currents in magesium diboride contact junctions

Methods to produce MgB2 thin films and tunneling devices have been developed. Thin films have been optimized for high field applications by exploring a range of doping and growth conditions. Josephson junction devices have been produced using a novel technique to optimize the quality of the barrier and near-interface superconducting materials.The effects of impurities incorporation in MgB2 thin films in the physical properties were studied. First, rubidium and cesium were ex-situ introduced into thin films by annealing in quartz ampoules containing elemental alkali metals. No significant change in transition temperature (Tc) was observed by resistivity measurement, in contrast to an earlier report of enhanced Tcs (>50 K) from susceptibility measurements. A significant drop in Jc and an increase in Deltarho (rho 300-rho40) arise from a decrease in inter-granular connectivity during annealing. Second, oxygen was incorporated using in-situ post-growth anneals in an oxygen environment. Analysis of the electrical data indicates that oxygen is distributed both within and between the grains. High values of Jc (&sim4x105 A/cm2 at 8 T and 4.2 K), extrapolated Hc2(0) > 45 T and |dHc2/dT| Tc| &ap 1.4 T/K are observed.A novel deposition approach allows films of magnesium plus boron to be deposited on unheated c-plane sapphire substrates by co-evaporation, and then subsequently annealed in a reducing atmosphere at temperatures below 600°C. The use of a combination of a magnesium rich stoichiometry (Mg/B > 1/2) in the as-deposited film, and a two-step annealing process, was found to be critical in obtaining unusual high values of Hc2(0) > 43 T and |dH c2/dT|Tc| &ap 2.5 T/K.Josephson junctions fabricated by pressing two oxidized MgB2 thin films together. This facilitates the production of Josephson Junctions with two MgB2 electrodes without exposure to the high second electrode deposition temperature. These junctions, with electrode Tcs of &sim32 K, have critical currents up to &sim25 K, broadened energy gaps at over 30 K, subharmonic gap structures at moderate voltages (&sim0.6 - 2.0 mV) and Fiske modes at low voltage (<0.6 mV). For comparison, lead junctions fabricated in the same way and had electrical characteristics similar to multiply connected junctions modulated by their self-field.