Louis J. Bajuk

All high Tc edge‐geometry weak links utilizing Y‐Ba‐Cu‐O barrier layers

High quality YBa2Cu3O7−x/normal‐metal/YBa2Cu3O7−x edge‐geometry weak links have been fabricated using nonsuperconducting Y‐Ba‐Cu‐O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25–100 A thick barrier layers exhibit current‐voltage characteristics consistent with the resistively shunted junction model, with strong microwave and magnetic field response at temperatures up to 85 K. The critical currents vary exponentially with barrier thickness, and the resistances scale linearly with Y‐Ba‐Cu‐O interlayer thickness and device area, indicating good barrier uniformity, with an effective normal metal coherence length of 20 A.

Composition variations in pulsed-laser-deposited Y-Ba-Cu-O thin films as a function of deposition parameters

The composition of pulsed-ultraviolet-laser-deposited Y-Ba-Cu-O films was examined as a function of position across the substrate, laser fluence, laser spot size, substrate temperature, target conditioning, oxygen pressure and target-substrate distance. Laser fluence, laser spot size, and substrate temperature were found to have little effect on composition within the range investigated. Ablation from a fresh target surface results in films enriched in copper and barium, both of which decrease in concentration until a steady state condition is achieved. Oxygen pressure and target-substrate distance have a significant effect on film composition. In vacuum, copper and barium are slightly concentrated at the center of deposition. With the introduction of an oxygen background pressure, scattering results in copper and barium depletion in the deposition center, an effect which increases with increasing target-substrate distance. A balancing of these two effects results in stoichiometric deposition.

Design and performance of a high-T/sub c/ superconductor coplanar waveguide filter

The design of a coplanar waveguide low-pass filter made of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) on an LaAlO/sub 3/ substrate is described. Measurements were incorporated into simple models for microwave CAD analysis to develop a final design. The patterned and packaged coplanar waveguide low-pass filter of YBCO, with dimensions suited for integrated circuits, exhibited measured insertion losses when cooled in liquid nitrogen superior to those of a similarly cooled thin-film copper filter throughout the 0 to 9.5 GHz passband. Coplanar waveguide models for use with thin-film normal metal (with thickness either greater or less than the skin depth) and YBCO are discussed and used to compare the losses of the measured YBCO and copper circuits. >

Valence band and Ba core level study of chemically-etched YBa2Cu3O7-δ

Abstract Orthorhombic and tetragonal YBa 2 Cu 3 O 7-δ (o-YBCO and t-YBCO) thin films have been chemically etched with Br 2 in absolute ethanol and the surfaces characterized by X-ray photoelectron spectroscopy. The Ba4 d spectrum for o-YBCO is consistent with a single chemical state and does not exhibit the higher binding energy surface species reported in the literature for cleaved crystals. The Fermi edge for chemically-etched o-YBCO remains clearly detectable at room temperature in vacuum on a time scale of days, in contrast to the surface degradation reported for cleaved crystals on a time scale of hours. These differences are attributed to the surface termination of cleaved crystals with Ba-O planes, which have a different coordination than in the bulk and are reactive, while chemically-etched YBCO is known to be terminated with Cu-O planes. Comparison of the Ba4d core levels and Ba MNN Auger peaks for o-YBCO and t-YBCO shows that the observed 1.0 eV core level binding energy difference is not due to a difference in final state relaxation, and therefore must result from differences in the initial state, most likely electrostatic in origin. Cu3d-O2p π-bonding states in Cu-O chains in the o-YBCO valence band are identified by comparison with the valence bands of other high temperature superconductors and of t-YBCO.

Chemical nature of the barrier in Pb/YBa2Cu3O7−x tunneling structures

Several reports of reproducible tunneling measurements on YBa2Cu3O7−x thin films or single crystals with a Pb counterelectrode have recently appeared. The nature of the tunnel barrier, formed by air exposure, in these structures has been unknown. In the present work, the chemical nature of the tunnel barrier is studied with x‐ray photoelectron spectroscopy (XPS). Laser‐ablated films grown on LaAlO3 which have been chemically etched and heated in air are found to form nonsuperconducting surface Ba species, evident in an increase of the high binding energy Ba 3d and O 1s signals. A deposited Pb film ∼10 A thick is found to be oxidized, and Cu+2 is partially reduced to Cu+1. The tunneling barrier thus appears to consist of species resulting from a combination of the air exposure and a reaction between the superconductor and the deposited Pb counterelectrode.

X-ray photoelectron spectroscopy study of inequivalent oxygen sites in high temperature superconductors

Abstract Differentiation of O 1s spectra from variable angle X-ray photoemission measurements on high temperature superconductors is used to distinguish oxygen signals from inequivalent lattice sites. Features which are apparent only as a shoulder or as asymmetry on the main O 1s peak in the measured spectra can be clearly resolved in the second derivatives. In addition to the signals from CuO planes and chains, the O 1s spectrum from YBa 2 Cu 3 O 7− x exhibits an additional peak at higher binding energy which variable angle measurements show is surface-related and which is assigned as reconstruction of the surface-terminated CuO planes. The O 1s spectrum from Tl 2 Ba 2 CaCu 2 O 8+ x is resolvable into two components which variable angle measurements show are associated with CuO planes and with TlO bonding, with the surface being terminated in TlO bonds.

High-T/sub c/ superconducting coplanar waveguide filter

Coplanar waveguide (CPW) low-pass filters made of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) on LaAlO/sub 3/ substrates, with dimensions suited for integrated circuits, were fabricated and packaged. A complete filter gives a true idea of the advantages and difficulties in replacing thin-film metal with a high-temperature superconductor in a practical circuit. Measured insertion losses in liquid nitrogen were superior to the loss of a similar thin-film copper filter throughout the 0- to 9.5-GHz passband. These results demonstrate the performance of fully patterned YBCO in a practical CPW structure after sealing in a hermetic package.<<ETX>>

YBa/sub 2/Cu/sub 3/O/sub 7- delta //Au/Nb sandwich geometry SNS weak links on c-axis oriented YBa/sub 2/ Cu/sub 3/O/sub 7- delta /

Sandwich geometry superconductor/normal metal/superconductor structures have been fabricated on LaAlO/sub 3/ and cubic zirconia with laser-ablated, c-axis-oriented YBa/sub 2/Cu/sub 3/O/sub 7- delta / base electrodes, 100-600 AA of Au, and Nb counter electrodes, all formed in situ without breaking vacuum. Junctions range in size from 5 to 50 mu m on a side. Four probe I-V measurements at 4.2 K show R/sub n/A products as low as 6*10/sup -9/ Omega cm/sup 2/ and critical current densities up to 5.2 kA/cm/sup 2/. AC Josephson steps were observed with the application of 10-GHz radiation. The temperature dependence of J/sub c/ and the observation of the AC Josephson effect suggest that true supercurrents are present and that they do indeed represent the characteristics of the YBa/sub 2/Cu/sub 3/O/sub 7- delta //Au/Nb structure. The best results were obtained when the devices were annealed at approximately 450 degrees C in O/sub 2/ for 30 min after Au deposition.

Modeling of planar quasi-TEM superconducting transmission lines

Design oriented modeling of high-temperature superconducting thin-film microwave circuits is difficult when film thickness is on the order of the penetration depth of the fields. Involved formulas for loss, phase velocity and characteristic impedance can be derived from the Bardeen-Cooper Schrieffer (BCS) theory of superconductivity. The parameters required by these formulas do not correspond to readily measurable observables that depend on the manufacturing process of the superconductor. An application of the phenomenological loss equivalence method in modeling the microwave behavior of planar quasi-TEM superconducting transmission lines is presented. Measured and modeled S-parameters of a superconducting coplanar waveguide lowpass filter agree to within 0.3 dB in magnitude and 0.5 radians in phase. Extracted values for the penetration depth and the real part of the conductivity of the superconducting film are within 10% of the findings of other researchers. >

X-ray photoelectron spectroscopy characterization of a nonsuperconducting Y-Ba-Cu-O superconductor-normal-metal-superconductor barrier material

X‐ray photoelectron spectroscopy (XPS) is used to characterize a new nonsuperconducting Y–Ba–Cu–O barrier material which has successfully been used in superconductor–normal‐metal–superconductor (SNS) device structures, grown under conditions similar to those reported to yield a cubic phase (c‐YBCO). Comparison of the XPS spectra to those obtained from the orthorhombic (o‐YBCO) and tetragonal (t‐YBCO) phases shows that the barrier material is distinct from either of the previously known phases. The O 1s spectrum from o‐YBCO shows a main peak at 528.0 eV associated with Cu–O planes, a clearly defined shoulder at 527.1 eV associated with Cu–O chains, and a peak at 528.7 eV which variable angle measurements show is a surface peak, tentatively assigned to reconstruction of the surface‐terminated Cu–O planes. The O 1s spectra from t‐YBCO and the YBCO barrier material exhibit single peaks at 528.8 and 528.1 eV, respectively. The valence band of the barrier material is similar to that of o‐YBCO, but narrower on t...