We have g-own superconducting thin films of Ba, _ ,K,Bi03 by in situ pulsed laser deposition from a stoichiometric (x = 0.4) target. The best films exhibit an onset transition temperature of 28 K and have zero resistance as high as 26 K. Films are single phase and highly oriented in the (100) or (110) direction on MgO, SrTiO,, LaA103, and Al,O, substrates. We have observed high-quality normal-insulator-superconductor and superconductor-insulator-superconductor quasiparticle tunneling characteristics with the films.

The complex transmission coefficient for millimeter and submillimeter waves incident on a Ba0.6K0.4BiO3 thin film (82 nm) has been measured over a frequency range of 200-1200 GHz at temperatures above and below T-c using coherent time-domain spectroscopy. We observe a dramatic change in both the magnitude and phase of the terahertz transmission in the superconducting state caused by a rapid carrier condensation. Both the real (sigma(1)) and imaginary (sigma(2)) parts of the complex conductivity are determined directly from the amplitude and phase of the transmitted electric field without the need for a Kramers-Kronig analysis. By fitting at in the framework of BCS theory, a superconducting gap 2 Delta(0)=6.9 meV=3.8k(B)T(c) is obtained. Below T-c, the sigma(1) is rapidly enhanced for omega/2 pi<500 GHz, which is attributed to the BCS coherence effects. However, the conductivity exhibits monotonic temperature dependence and no clear sigma(1)(T) peak is observed throughout the frequency range measured. The high-frequency penetration depth (similar to 600 nm) is also extracted and discussed. Our results are consistent with a picture of BCS moderate coupling superconductivity in an intermediate to dirty limit.

This work directly compares coplanar superconducting transmission lines and single-pole resonators patterned from YBCO to aluminum structures for use in GaAs/YBCO hybrid circuitry. A cryogenic on-wafer station was used to make s-parameter measurements of passive coplanar circuits as well as to characterize the performance of GaAs MESFETs at 80 K. Comparisons were made between measured data and theoretical results for passive YBCO and aluminum structures. The YBCO film was also measured using a parallel plate technique to determine microwave surface resistance to establish a correlation between patterned film and thin film microwave properties. Small-signal models were constructed to accurately predict the operation of 0.25 micrometers gate length GaAs MESFETs at 80 K under a variety of bias conditions. The cutoff frequency and maximum frequency of operation of the GaAs MESFETs increased by 29% and 13% respectively under a drain-source voltage of 2.0 V (Id equals 100% Idss) as the temperature was lowered from 300 K to 80 K.

The microstructure of Ba1−xKxBiO3 (BKBO) thin films from three different sources has been extensively compared by transmission electron microscopy studies. The three films were prepared independently in three different laboratories on three different substrates of (100) orientation and displayed excellent superconducting properties. The observed microstructure is remarkably similar in the three films. They are epitaxial with (100) orientation through all their extension and no cracks have been observed. Their defect density is similar and the resulting extension of defect-free regions is of the order of 50–80 nm.

The effects of magnetic alignment, heat treatment, and substrate interactions on the microstructural development and properties of YBa2Cu3O7−x (Y123) thick films were studied. Aligned films were formed by vacuum filtrating a particulate suspension in a 7 T applied field. These films and nonaligned control films were fired on either platinum (Pt) foil or magnesium oxide (MgO) substrates to various maximum temperatures between 930 and 1040 °C. Optical microscopy revealed large differences in microstructural development between the various films. Aligned Y123 films fired on Pt foil exhibited the best microstructural properties. Via plasma emission spectroscopy and secondary ion mass spectroscopy, approximately 0.1 wt % Pt was found distributed throughout the films fired on platinum, while negligible amounts of Mg were detected in the films fired on MgO substrates. Differential thermal analysis revealed that, in the presence of Pt, the peritectic temperature (1030 °C for pure Y123 in O2) is reduced 70 °C, thereby opening a substantial thermal processing window for partial melt assisted growth of textured Y123. SQUID measurements of magnetic hysteresis and Tc provided quantitative evidence that, relative to the films fired on MgO, those fired on Pt exhibited enhanced texture development [ΔM(Happ∥c axis)/ΔM(Happ⊥c axis)=2.6 at 5 K, 1 T] and properties (Bean model Jc,m=5×104 A/cm2 at 5 K, 4 T) without degradation of the Tc characteristics.

Grain-aligned YBa2Cu3O7−x films were fabricated by vacuum filtration in an applied magnetic field (7 T). Platinum (Pt) was shown to lower the peritectic temperature of YBa2Cu3O7−x by almost 75 °C, leading to dramatic microstructural differences between films densified on MgO (single crystal) substrates and those densified on Pt foil to a maximum temperature of 1020 °C in oxygen. Superconducting quantum-interference device hysteresis loops measured at 0–5.5 T at 5 K showed that films fired on Pt had ΔM (Happ∥c axis) values 15–60 times larger than those fired on MgO.

ABSTRACT We investigate the superfluid and quasiparticle response for high temperature superconductor films of YBa2Cu3O7 and Ba06K04BiO3 under optical radiation. This is done through direct measurement of the complex conductivity of the illuminated superconductors using a coherent terahertz- bandwidth spectroscopy technique. The nature of the detected signals is discussed in the context of bolometric and nonbolometric response.1, INTRODUCTION Since the discovery of high-Ta superconductors, the study of the photoresponse of superconducting thin films has been of considerable interest. The possibility that a nonbolometricresponse may be observed, as well as an interest in identifying the physical origin of the opticalresponse and potential fast, broadband optical detectors16, have driven the study of the interaction ofoptical radiation with thin superconducting films. The scheme most commonly used to investigate thesephenomena is the generation and measurement of transient voltage pulses in a superconducting bridgeby pulse laser radiation. However, the interpretation of these measurements is not straightforward andmuch controversy exits about the nature of the response mechanism. While some reports attribute fastsignals to a rapid bolometric response4, most other researchers have attributed short duration response

Atomic resolution images of Ba0.6K0.4BiO3 have been obtained at electron-dose levels of 102 e-/Å2 using a slow-scan CCD camera on a high-resolution electron microscope; at this dose level electron-beam damage can be avoided. The superconducting material is a clean perovskite without evidence for any distortions such as charge-density waves. At higher dose levels modulations appear which, from measurements of the desorbing species under low-energy electron radiation, are due to ionization damage and loss of oxygen.

We have used a bolometric technique to obtain accurate low temperature loss data for epitaxial thin films of Ba[sub 0.6]K[sub 0.4]BiO[sub 3] from 30 to 700 cm[sup [minus]1]. These films were grown on MgO and SrTiO[sub 3] substrates by MBE, off-axis sputtering and laser deposition techniques. All films show a strong absorption onset near the BCS tunneling gap of 3.5k[sub B]T[sub c]. We have analyzed these data using a Kramers-Kronig transformation and have corrected for finite film thickness effects. Results indicate that the absorption onset is consistent with a superconducting energy gap. Comparison is made with predictions based on strong coupling Eliashberg theory using [alpha][sup 2]F([omega]) spectra obtained from the literature. While we are able to fit the overall measured absorptivity, we are unable to fit the structure observed in our data.

The complex conductivity of a Ba0.6K0.4BiO3 thin- film superconductor has been measured using terahertz- radiation spectroscopy. A BCS-like energy gap and a peak in the real part of the conductivity have been directly obtained without the need for Kramers-Kronig analysis.

Microwave structures utilizing superconducting thin films of Ba/sub 0.6/K/sub 0.4/BiO/sub 3/ (BKBO) have been fabricated and tested. A high-quality BKBO thin film (T/sub c/>28 K, J/sub c/>1 MA/cm/sup 2/) grown by pulsed laser deposition on a