Shao-Han Liou

Evidence for weak link and anisotropy limitations on the transport critical current in bulk polycrystalline Y1Ba2Cu3Ox

Measurements of the transport critical‐current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3Ox from several different laboratories indicate that the transport Jc is limited by weak‐link regions between high Jc regions. The weak‐link Jc has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain‐boundary region in Y1Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 A). The effect of intrinsic conduction anisotropy is discussed. A current‐transfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors.

Low magnetic field superconducting phase diagram of the high-Tc YBa2Cu3O9-δ

Abstract The low magnetic field phase diagram of the high-Tc superconductor YBa2Cu3O9-δ with δ∼2.1 has been determined in fields to 5000 0e. The lower critical field is about 300 0e near T=0 and less than 100 0e at liquid nitrogen temperature. The mixed phase shows significant relaxation and field cooling effects.

Single Crystal Superconducting Y1Ba2Cu3O7−x Oxide Films by Molecular Beam Epitaxy

Recent breakthrough of superconductivity over 90 K in the Cu oxide based perovskite-type materials is of immense importance in both science and technology.1,2 The superconducting oxide crystals prepared in thin film form are vital for fundamental physical studies as well as for device applications.3,4 We report here the structural and superconducting properties of single crystal Y1Ba2Cu3O7−x films5 prepared by molecular beam epitaxy (MBE) developed for thin film semiconductor technology. The films are epitaxial crystals grown on SrTiO3 (100) face with an orientation of a axis perpendicular to the film plane. Sharp resistive superconducting transitions with Tc (R=0) above 77 K have been reproducibly achieved. The critical current densities are 103 A/cm2 at 77 K, 5 × 104 A/cm2 at 70 K, and ~ 106 A/cm2 at 4.2 K.

Thin film research on high Tc superconductors

We have used metal molecular beam epitaxy, sputter deposition with single composite target, and spin‐on/pyrolysis with metallo‐organic precursors to prepare thin films of high Tc oxide superconductors mostly in the Y‐Ba‐Cu‐O system. We report the superconducting properties and the structural characteristics of the films. The processing parameters of these thin‐film techniques are also discussed.

High Tc superconducting Y‐Ba‐Cu‐O oxide films by sputtering and molecular beam epitaxy: Morphology, structural characterization and superconducting properties

We have studied the morphology, structure and superconducting properties high Tc oxide films, mostly in the Y‐Ba‐Cu‐O sytem, prepared by various sputtering techniques (diode and magnetron sputtering) and molecular beam epitaxy (MBE). In the as‐deposited condition the chemical homogeneity both across the film and through the thickness is excellent as measured by Rutherford backscattering spectrometry (RBS). Post oxygen annealing at temperature above 800°C yields the orthorhombic superconducting phase as revealed by the x‐ray diffraction. High‐quality epitaxially grown Y1Ba2Cu3O7−x films deposited on single‐crystal SrTiO3 substrates with various orientations were obtained with a narrow rocking curve (<0.3°) in the direction perpendicularly to the film plane. Also, for the first time we observed the electron channeling in this class of materials. Sharp superconducting resistive transitions with Tc(R=0) above 89K and critical current density at 77K have been reproducibly achieved.