S. Martin

Anisotropic critical current density in superconducting Bi2Sr2CaCu2O8 crystals

Critical current densities and resistivities were measured in a single crystal of the high Tc superconductor Bi2Sr2CaCu2O8 within the ab basal plane (∥) and along the c direction (⊥). A large anisotropy in critical current density is found, Jc∥/Jc⊥≊103, in quantitative agreement with the large normal‐state resistivity anisotropy near Tc. The data provide strong evidence for a two‐dimensional layered structure of metallic planes separated by semi‐insulating barriers.

Transport, tunneling X-ray, and penetration depth studies of superconducting Bi2+xSr2−yCuO6±δ crystals

Abstract In crystals of the single-layer cuprate Bi 2+x Sr 2−y CuO 6±δ , x ∼ 0.1, y ∼ 0.1 (BSCO), the ab-plane resistivity is linear in temperature and the ϱ c /ϱ ab resistivity anisotropy is 10 5 . The tunneling conductance is close to being a linear function of the bias voltage. Screening measurements show evidence of non-uniform superconductivity. X-ray diffraction indicates superlattice structure approaching commensurate periodicity.

Ba1−xKxBiO3 sandwich-type tunnel junctions grown by molecular beam epitaxy

Abstract We have grown Ba1−xKxBiO3/BaBi2Oy/Ba1−xKxBiO3 trilayers by molecular beam epitaxy. Junctions made from these trilayers exhibit transport ranging from hopping conduction to superconductor-insulator-normal metal tunneling and possibly to superconductor-insulator-supercon ductor Josephson tunneling. Critical currents have been measured for junctions grown on (110) NdGaO3 substrates.

Structural and physical properties in the Bi-Sr-Cu-O system

Abstract The structure/property relationships in the Bi-Sr-Cu-O system have been investigated. This system includes Bi2+xSr2-yCuO6±δ, the single Cu layer compound that is one member of the Bi-Sr-Ca-Cu-O family of superconductors. This family includes Bi2Sr2CaCu2O8, a 95K superconductor. We have determined that two structurally related phases exist near the composition Bi2Sr2CuO6. One phase is a solid solution, Bi2+xSr2-yCuO6±5, with a wave modulated structure. The second phase, Bi2Sr2CuO6, has a structure with a step dislocation. The former phase exhibits superconductivity while the latter phase is an insulator. The transition between structures is driven, atleast in part, by a change in cation ratios.

Nonstationary 1/f noise in InP/InGaAs heterojunction bipolar transistors

1/f noise for most electronic systems is stationary in that repeated measurements of the noise power will give the same result within statistical uncertainty. In contrast, we have observed highly nonstationary 1/f noise in the output current of a series of InP based heterojunction bipolar transistors (HBTs). We have applied higher order statistics to the non‐Gaussian fluctuations of these devices to probe the source of the intrinsic 1/f noise. We find signatures of superimposed Lorentzians making up an otherwise featureless 1/f noise spectrum consistent with generation‐recombination‐type noise in the base‐emitter region of the device. The nonstationarity of these devices scales inversely with the device size as would be expected for an intrinsic mechanism. We suggest that these techniques may be utilized to a greater extent in the future as device sizes are reduced.

Molecular Beam Epitaxy of Ba1-xKxBiO3 Films and Heterostructures

Ba1-xKxBiO3 with x≈0.4 is a very attractive material for superconducting tunnel junctions. We briefly describe the molecular beam epitaxy growth technique as applied to Ba1-XKXBiCO3. After a high temperature nucleation step, (1 0 0) oriented Ba1-xKxBiO3 films with good superconducting properties can be grown on MgO at quite low substrate temperatures (200–300°C). The films are oxygen deficient during growth, and the films must be annealed in oxygen to optimize superconducting properties. Oxidation induced shrinkage can result in cracking. We have used a new insulating phase, BaBi2Oy, along with Ba1-xKxBiO3 to build superlattices and sandwich junction heterostructures. We briefly discuss the electrical properties of these heterostructures.