Ronald H. Ono

Characterization of a tunable thin film microwave YBa2Cu3O7−x/SrTiO3 coplanar capacitor

We have fabricated and characterized electrically tunable high temperature superconductor coplanar microstrip resonators incorporating tunable SrTiO3 ferroelectric thin films. The low frequency capacitance of the SrTiO3 capacitor is measured directly. High frequency capacitance and loss information are extracted from the observed resonances and compared with the low frequency data. Hysteresis loops display an onset of ferroelectricity at 160 K. The spontaneous charge and coercive voltage (at 10 kHz) as a function of temperature are extracted from these loops.

High‐TC superconductor‐normal metal‐superconductor Josephson microbridges with high‐resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor‐normal metal‐superconductor microbridges using a step edge to define the normal metal length. Critical current‐normal resistance products over 1 mV have been measured at low temperature in devices with high‐resistivity Ag‐Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.

Operation of a Y‐Ba‐Cu‐O rf SQUID at 81 K

An rf superconducting quantum interference device (SQUID) has been made from bulk Y‐Ba‐Cu‐O. The device displays quantum interference effects and operates with useful signal levels up to 81 K. The SQUID is formed from a ring of Y‐Ba‐Cu‐O which is broken in the cryogenic environment and then recontacted. Estimates of the SQUID noise performance are given.

Scanning tunneling microscopy of the surface morphology of YBa2Cu3Ox thin films between 300 and 76 K

Scanning tunneling microscopy (STM) images of YBa2Cu3Ox (YBCO) thin films show different growth mechanisms depending on the deposition method and substrate material. We present images of YBCO films sputter deposited onto MgO and SrTiO3, and laser ablated onto LaAlO3 showing screw dislocation and ledge growth mechanisms. At room temperature we observed an anomalous tunneling conductance near the edge of growth steps which causes a large apparent step‐edge height in the STM image. This effect decreases with decreasing temperature, so that the step height approaches the expected value for one unit cell of 1.2 nm at 76 K. This phenomenon reflects changes in either the surface tunneling barrier or tunneling density of states upon cooling.

A Josephson array voltage standard at 10 V

The technology of Josephson voltage standards has been extended to an array of 14 184 junctions which is capable of generating over 150 000 quantized voltage levels spanning the range from -12 to +12 V. This makes possible the direct calibration of 10-V Zener reference standards without the use of a voltage divider.

Geometry dependence of nonlinear effects in high temperature superconducting transmission lines at microwave frequencies

We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonline...

High Temperature Superconductor-Normal Metal-Superconductor Josephson Junctions with High Characteristic Voltages

We have fabricated step edge superconductor‐normal metal‐superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current‐normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor‐normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current‐voltage characteristics with a 760 GHz beam from a far infrared laser.

Construction and performance of a high-temperature-superconductor composite bolometer

A high‐Tc superconducting bolometer has been constructed using a YBa2Cu3Ox thin‐film meander line 20 μm wide and 76 000 μm long, deposited on a SrTiO3 substrate. Radiation is absorbed by a thin film of Bi with well‐characterized absorption properties deposited on a Si substrate in contact with the SrTiO3. At 1.8 Hz the measured bolometer response to a 500‐K blackbody is 5.2 V/W (820 V/W extrapolated to dc), and the NEP is 5.7×10−8 W/(Hz)1/2 . The impact of apparent nonohmic behavior at the transition is discussed, as are ways of reducing the observed 1/f noise. The response time is 32 s and is dominated by the heat capacity of the SrTiO3 substrate.

High‐Tc multilayer step‐edge Josephson junctions and SQUIDs

We have developed a reliable process to fabricate high‐quality YBa2Cu3O7−x (YBCO) superconductor‐normal metal‐superconductor (SNS) step‐edge junctions and SQUIDs over YBCO ground planes. These multilevel circuits employ thin films of SrTiO3 and NdGaO3 as the insulating layer between the active device and the ground plane and use Ag as the normal metal in the Josephson junction. The reproducibility and uniformity of the junctions are better than our single‐level devices grown directly on step edges cut into single‐crystal substrates. Here the critical current variation among junctions on a single wafer is less than a factor of 2. Junctions grown on thin‐film step edges of SrTiO3 have critical currents near 2 mA at 4 K, while those grown on NdGaO3 step edges have critical currents near 0.5 mA at 4 K.

Effect of thermal noise on Shapiro steps in high‐Tc Josephson weak links

The amplitudes of Shapiro steps are measured at temperatures up to 77 K for a superconductor‐normal metal‐superconductor Josephson weak link fabricated from YBa2Cu3O7−δ with a Ag‐Au alloy bridge. The step amplitudes are found to be in agreement with calculations based on the resistively shunted‐junction model when the Johnson noise of the junction resistance is included.