P.F. Miceli

On the the crystal growth and chemistry of the new electron-type superconducting oxides

Abstract The effect on transport and superconducting properties produced by changes in x and y in the Nd2−xCexCuOy compound were studied in both polycrystalline ceramics and single crystals. Thermogravimetric analysis shows that the total oxygen content y for the as-prepared samples is always greater than 4 (i.e. presence of interstitial oxygen) whereas for the reduced sample y becomes equal to or smaller than 4 only when x is 0.15 or greater. This is the range of Ce content for which the material superconducts. For a material with Ce x=0.15 the superconducting properties can be varied reversibly by changing the oxygen content. In addition, we propose that the oxygen in these materials can be either ordered or disordered, thereby affecting the transport properties. Platelet-like crystals of Nd2−xCexCuOy with x=0 to 0.18, have been grown via a flux technique. Those having a Ce content between 0.14 and 0.17 are superconducting with the sharpest transitions (Tc = 21 ± 1K) for x = 0.14. Metallic-like behavior above Tc, with a linear temperature dependence above 150K was observed on all the crystals. Below 30K, the in-plane resistivity is independent of T. The Hall coefficient is sensitive to processing conditions but is usually negative above 100K. The upper critical field with field along the c-axis is 6T at 4.2K.

Studies of proximity-effect and tunneling in YBCO/metal layered films

Abstract The short coherence length of the high-T c superconductors, coupled with their tendency to form non-superconducting surface layers, accounts for the difficulty in achieving good tunnel junctions. A proximity layer of a longer coherence length normal metal (N) is expected to “draw out” Cooper pairs. Our goal is to fabricate reproducible, planar tunnel junctions of SNIS layered structures for proximity tunneling spectroscopy. Such structures of YBCO/N/I/Pb and SNS structures of YBCO/N/Pb indicate that the normal metal produces a low resistance contact to the YBCO surface with a supercurrent observed in the SNS. The insulating barrier in the SNIS is reproducible, insulating and continuous: A sharp Pb gap and phonons from the counter-electrode are routinely observed.

Magnetic ordering in YBa2Cu3−xMxO6+y

Abstract The results of neutron scattering studies of magnetic ordering in YBa 2 Cu 3−x M x O 6+y are presented. Two antiferromagnetic transitions are observed as a consequence of the two distinct Cu sites present in this system. We discuss the evolution of these ordered states and the dependence on composition.

Tunneling attempts in single-phase Bi4Sr3Ca3Cu4O16+y and chemical doping on the Cu-sites in 90 and 40K superconductors

Planar tunneling junctions on Bi 4 Sr 3 Ca 3 Cu 4 O 16+y (4334) which clearly exhibit the gap of the top electrode (In or Pb) have been prepared. Junction resistances range from 1Ω to 6kΩ over a 1 – 2mm 2 area, the lower resistances being obtained only after plasma oxidation of the surface. We observe a linear conductance at varying biases above the gap region. No evidence of a gap in the 4334 material and no Josephson current has been observed even in the lowest resistance junctions. A short summary of the Bellcore work on the Cu-doped YBa 2 Cu 3 O 7−y and La 1.85 Sr .15 CuO 4−y is also given.