Understanding the effects of disorder in magnesium diboride films and development of novel magnesium diboride based Josephson junctions

Magnesium diboride, with a critical temperature of 39 Kelvin, is an unusual superconductor with two conduction bands (pi and sigma) in which carriers couple with lattice vibrations resulting in two superconducting gaps. The properties of magnesium diboride films and related multi-layers were respectively explored on two fronts: (i) to understand the role that disorder in magnesium diboride plays in making it a practicable material for magnet applications; and, (ii) to design and fabricate magnesium diboride-based Josephson junctions for fast-switching circuits.;Magnesium diboride films required for Josephson junctions were grown in a vacuum chamber using evaporative sources. The potential of native and thermal oxides formed on magnesium diboride, for use as tunnel barriers, was explored. The barrier height and thickness of these oxides were obtained by studying conductance-voltage characteristics of junctions with lead as the top electrode. Junctions with both electrodes as magnesium diboride were obtained after in situ growth and patterning using semiconductor fabrication techniques. At optimal growth conditions, the junctions exhibited tunneling characteristics reproducibly and only the smaller of the two gaps was observed. Finite values for this gap were obtained for temperatures up to the critical temperature of the electrodes.;The role that the two bands play in establishing upper critical field and resistivity is quantitatively shown by modifying disorder in magnesium diboride through controlled introduction of point defects in films with different levels of as-grown disorder, followed by annealing. Point defects were introduced iteratively by irradiation using a helium ion beam, and their effects on critical temperature, resistivity, and upper critical field of magnesium diboride films were studied. Results suggest that resistivity and upper critical field are determined by scattering in pi and sigma bands, respectively, and they can be uncorrelated when the scattering rates in the two bands are altered independently. A common correlation was found between the upper critical field (at zero Kelvin) and critical temperature of magnesium diboride samples made in several forms and disordered by different means. As an alternate means of introducing disorder, oxygen was incorporated in magnesium diboride films and the effects on the normal and superconducting properties were analyzed.