E.S. Hellman

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.

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.

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.

Compositionally modulated nucleation and growth of barium bismuth oxide thin films on MgO

Abstract The nucleation and growth of barium bismuth oxide thin films on MgO has been examined using transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and reflection high-energy electron diffraction (RHEED). Analysis of the deposits by RBS showed that the thinnest films were barium rich. The data suggest that the bismuth starts to stick only after BaO has formed on the MgO substrate. The RHEED patterns indicated that the initial BaO layer was disordered. Further film growth resulted in the formation of epitaxial islands of BaBi 2 O y which were observed by both TEM and electron diffraction.

Ba1−xKxBiO3/BaBi2OY thin film heterostructures

Abstract We report the synthesis by molecular beam epitaxy of a new insulating phase in the barium bismuth oxide system, BaBi 2 O y , and its use as a building block for bismuth oxide heterostructures.

SUPERCONDUCTING (Rb,Ba)BiO3 THIN FILMS GROWN BY MOLECULAR BEAM EPITAXY

Abstract The simple cubic perovskite (Rb,Ba)BiO 3 can be grown by molecular beam epitaxy using an RF plasma oxygen source. Films with superconducting onsets in resistivity as high as 27K are obtained without annealing. The epitaxy proceeds in the normal (1 0 0) orientation on (1 0 0) SrTiO 3 , despite a 10% lattice mis-match. (1 1 0) epitaxy is obtained on (1 0 0) MgO substrates, despite the good lattice match for (1 0 0) growth. The films with the lowest normal state resistivity are those grown on SrTiO 3 at 300±10°C and are Bi deficient despite excess Bi flux during growth. The temperature dependence of the resistivity for these films is metallic down to T c . H c2 for a T c =20K film is 10±T.