E. H. Hartford

Epitaxial Ba1−xKxBiO3 films on MgO: Nucleation, cracking, and critical currents

A two‐step growth technique is used to obtain (100) oriented Ba1−xKxBiO3 films with good superconducting properties by molecular beam epitaxy. The films are nucleated without potassium at higher temperatures to get the perovskite (100) orientation on (100)MgO. Ba1−xKxBiO3 is then grown at a lower temperature using potassium flux from the decomposition of K2O. The oxygen content of the films is adjusted in a post‐growth anneal in oxygen to reduce cracking which can result from oxidation‐induced shrinkage. The annealed films have superconducting transitions as narrow as 0.5 K at temperatures between 20 and 25 K. Critical currents in the range of 105 A/cm2 at 4.2 K have been measured by the magnetization hysteresis loop method.

All‐high Tc Josephson tunnel junction: Ba1−xKxBiO3/Ba1−xKxBiO3 junctions

We have fabricated all‐high Tc superconducting tunnel junctions in the material Ba1−xKxBiO3. The junctions are of very high quality and have a Josephson supercurrent. For the low‐resistance junctions, the IcR product approaches the theoretical limit.We have fabricated all-high {ital T}{sub {ital c}} superconducting tunnel junctions in the material Ba{sub 1{minus}{ital x}}K{sub {ital x}}BiO{sub 3}. The junctions are of very high quality and have a Josephson supercurrent. For the low-resistance junctions, the {ital I}{sub {ital c}R} product approaches the theoretical limit.

Molecular beam epitaxy of superconducting (Rb,Ba)BiO3

The simple cubic perovskite (Rb,Ba)BiO3 can be grown at temperatures below 350 °C by molecular beam epitaxy using a rf plasma atomic oxygen source. Films with superconducting onsets in resistivity as high as 27 K are obtained without annealing. The epitaxy proceeds in the normal (100) orientation on {100} SrTiO3, despite a 10% lattice mismatch. (110) epitaxy and spotty reflection high‐energy electron diffraction (RHEED) patterns are obtained on {100} MgO substrates, despite the good lattice match for (100) growth. Streaked and spotty RHEED patterns have been obtained on either substrate. Sticking coefficients for bismuth depend on the growth conditions, indicating that the epitaxy is partially controlled by desorption kinetics.

Superconductor‐insulator‐superconductor tunneling in Ba1−xKxBiO3 grain boundaries

We have prepared single grain boundaries of the superconductor Ba1−xKxBiO3 by growing epitaxial thin films of this compound on SrTiO3 bicrystal substrates. The four‐terminal current‐voltage characteristics of the grain boundaries showed clear superconductor‐ insulator‐superconductor (SIS) tunneling behavior. The leakage at zero bias was smaller than 0.3% at 4.2 K. The differential conductance displayed sharp symmetric peaks at 2Δ close to 6.5 mV. At higher bias, an increase in conductance proportional to V2 was observed. The temperature dependence of the conductance was found to be in qualitative accord with conventional SIS theory.

Penetration depth, microwave surface resistance, and gap ratio in NbN and Ba1−xKxBiO3 thin films

We report values of the zero temperature magnetic penetration depth λ(0), microwave surface resistance Rs, and gap ratio 2Δ(0)/kBTc in technologically useful thin films of NbN and Ba1−xKxBiO3. A novel analysis technique was used to extract the absolute magnitude of λ(0) and 2Δ(0)/kBTc from shifts in resonant frequency of a parallel‐plate resonator. For NbN and Ba1−xKxBiO3 values of λ(0)=3900±200 A and 3300±200 A were obtained, respectively. The gap ratios were found to be 2Δ(0)/kBTc=4.1±0.1 and 3.8±0.5, respectively, for Tc=16.3 K in NbN and Tc=17.2 K in Ba1−xKxBiO3. The surface resistance measurements on Ba1−xKxBiO3 represent the lowest values ever reported at microwave frequencies in this material.

Cu valence and the formation of high Tc superconductor oxides studied by x‐ray photoemission spectroscopy on 200 Å Bi‐Sr‐Ca‐Cu oxide thin films

We have used x‐ray photoemission spectroscopy to study the core level spectra of 200 A Bi‐Sr‐Ca‐Cu oxide thin films, in the hope that the importance of the surface sensitivity of photoemission is minimized for these samples, whose thickness is only 6–7 unit cells. The samples were annealed in oxygen at temperatures ranging from 600 to 870 °C, over which temperature range the thin films are converted from an amorphous insulating phase to an ordered superconducting phase with Tc (R=0)=80 K. Regardless of the annealing temperature, the Cu 2p spectra of all films show satellite structure indicative of Cu2+ states. Even the spectrum of an as‐deposited film has a satellite similar to those observed for the annealed films, including the superconducting film. The similarity of the satellite structure in the core level spectra for all films suggests that the Cu‐O interactions, such as the Cu d‐O p orbital charge transfer energies and the hopping integrals, remain similar when the oxides undergo the insulator‐super...

Ion beam patterning of spin‐on metalorganic precursors and formation of high Tc superconductors

A new technique using ion beams to produce patterned thin films of the high‐temperature perovskite superconductor YBa2Cu3O7−x from spin‐on metalorganic precursors is described. Spin‐coated precursor films are irradiated through a stencil mask with 2.5 MeV He+ ions, developed in solvent to remove unexposed material, and the remaining patterns are pyrolyzed. Black films of YBa2Cu3O7−x with pattern dimensions on the millimeter scale exhibit orientation with the c axis perpendicular to the film after heat treatments of 990 °C for 3 min. The conductivity of a highly oriented, ∼0.4‐μm‐thick patterned film is characterized by onset of the superconducting transition at 84 K and zero resistivity at 68 K, similar to those of an unirradiated film given the same thermal treatment. Studies on the nature of the ion beam exposed material are discussed.

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.

Synthesis of new oxide materials by molecular beam epitaxy: the DyBaCuO and BaKBiO systems

The usefulness of molecular beam epitaxy (MBE) as a synthetic tool for oxide systems is demonstrated using examples of new materials in the Dy Ba Cu O and Ba K(Rb) Bi O systems. We concentrate on those regimes of temperature and pressure for which bulk synthetic techniques are impractical or impossible.

Tunneling spectroscopy in Ba1−xKxBiO3

Abstract Ba 1−x K x BiO 3 is to date the highest Tc member of a class of superconductors which, it could be argued, belong in the same class of materials as the copper oxides. While there are striking differences between the bismuthates and the cuprates, there are some similarities. In this paper, we will report the results of our tunneling measurements on BKBO. The determination f the energy gap in this material suggests that it is a relativity weakly coupled BCS superconductor - a similar conclusion is reached in the sister compound BaPb 1−x Bi x O 3 . These results are substantailly different from similar measurements in the cuprates. In addition, we have studied the background tunneling conductance in four of the compounds in this class (BaPbO 3 , BaPb 1−x Sb x O 3 , BaPb 1−x Bi x O 3 and Ba 1−x K x BiO 3 ) and find a remarkable correlation between the strength of the linear conductance and the superconducting transition temperature. Finally, the Josephson tunneling in these compounds will be discussed.