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Notes: Josephson junctions and dc superconducting quantum interference devices (SQUIDs) have been fabricated in ex situ epitaxial Tl2Ba2CaCu2O8 films on bicrystal LaAlO3 substrates with symmetric 32° [001] tilt grain boundaries. The critical temperature Tc, of the junctions was in the range 105–107 K and the critical current densities at 77 K varied between 3×102 and 3×104 A/cm2, two or three orders of magnitude less than those of the film. The I–V curves are described by a resistively shunted junction model. Close to Tc, the temperature dependence of the critical current was described by (1−T/Tc)2. The flux noise spectra SΦ(f) of dc SQUIDs were measured in the locked-loop regime with constant current bias at temperatures up to 94 K. The white noise level was 50μΦ0/(square root of)Hz at 77 K. The crossover frequency to 1/f noise was low, about 5 Hz, and the flux noise level at 1 Hz was 440μΦ0/(square root of)Hz. © 1996 American Institute of Physics.

Notes: Sharp and straight step edges on (001) LaAlO3 (LAO) substrates were ion milled by using an electron beam defined amorphous carbon thin film mask. YBa2Cu3O7 (YBCO) thin films patterned to narrow strips across the step edges gave high quality Josephson junctions. Their current–voltage (I–V) curves could be well described by the resistively shunted junction model with or without excess current. By varying the YBCO film thickness over a fixed step height, the critical current density (jc) of the junction could be changed by several orders of magnitude. For junctions with high jc, typical IcRn (product of critical current and normal resistance) values of around 100 μV at 77 K and more than 1 mV at 4.2 K were obtained. Some excess current was observed. For junctions with low jc, the dependence of Ic on an applied magnetic field was strong even at low temperatures. The Ic showed a main peak in the center and well-defined periods as a function of applied magnetic field. The minimum Ic value suppressed by the magnetic field was about 20% of its maximum value at 4.2 K. Junctions with low jc usually showed hysteretic I–V curves at low temperatures. The McCumber constant βc fell in the range of 0.8–2. Fiske and flux-flow resonances were observed for some junctions. The shunting capacitances of the junctions were estimated from the McCumber constant βc, Fiske resonances, and flux-flow resonances. A shunting capacitance value per unit area of 12–35 fF/μm2 was obtained. High resolution cross-sectional transmission electron microscopy was used to study YBCO films grown across straight and wavy step edges. Two 90° tilt boundaries were formed at the edge of a step. The top and bottom YBCO films had their c axis oriented normal to the (001) plane of LAO. In the edge region, the c axis of the YBCO film was tilted by 90°, the a axis was normal to the (001) plane of LAO, and the b axis was lying along the step edge. For wavy step edges, second phase particles usually appeared in the YBCO film along the step edge region. Defects were found on the surface of the LAO substrate along the wavy step edge region. These defects might act as nucleation centers for the second phase particles in the YBCO film. © 1996 American Institute of Physics.

Notes: Epitaxial trilayer heterostructures of YBa2Cu3O7−δ/BaxSr1−xTiO3/YBa2Cu3O7−δ were grown on silicon-on-sapphire buffered by a double layer of CeO2/Y–ZrO2. Such structures may be considered for tunable microwave filters. The top and bottom YBa2Cu3O7−δ films were well c-axis oriented, free from microcracks and had superconducting transitions Tc's in the range 86–90 K. A thin antidiffusion layer of SrTiO3 (d≈70 A(ring)) between YBa2Cu3O7−δ and BaxSr1−xTiO3 (x=0.25–0.9) promoted better crystallinity and higher Tc of the top superconducting film. An Ag/BaxSr1−xTiO3/YBa2Cu3O7−δ capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan δ of the BaxSr1−xTiO3 layer. Maximum values of the permittivity of the BaxSr1−xTiO3 layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1−xTiO3 (x=0.25–0.75) layers depended strongly (≈20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan δ was much higher in films than in bulk crystals. © 1995 American Institute of Physics.

Notes: The combination of a low loss, high-Tc superconductor and a polarizable dielectric looks interesting from the point of tunable, high Q filters for cellular communication. Epitaxial heterostructures of YBa2Cu3O7−∂/SrTiO3/YBa2Cu3O7−∂/CeO2 and SrTiO3/YBa2Cu3O7−∂/CeO2 were grown on sapphire substrates in this study. Superconducting transition temperatures of 88–90 K and a critical current density of about 2×106 A/cm2 at 77 K were determined for YBa2Cu3O7−∂ films on CeO2 buffered sapphire. An effective permittivity of 340 at 300 K was measured capacitively for the SrTiO3 layer between YBa2Cu3O7−∂ electrodes; it increased three times at 50 K. The dielectric constant of SrTiO3 was observed to decrease a factor of 2 as a dc bias voltage of ±2.5 V was applied between the electrodes. Much smaller nonlinearities and temperature dependences of the dielectric constant were noted if the top YBa2Cu3O7−∂ electrode was replaced by an Ag one. © 1997 American Institute of Physics.

Notes: High-current-density Josephson junctions have been produced in high-temperature superconducting YBa2Cu3O7−δ films deposited on silicon on sapphire and using biepitaxial grain boundaries. The technique is estimated to be useful in integrating superconducting and semiconducting components. A multilayer system of epitaxially grown films was used to form the junctions. A double buffer layer CeO2/ZrO2(9.5% Y2O3) prevented interactions between YBa2Cu3O7−δ and Si during the high-temperature deposition and promoted formation microcrack-free films with a critical current density of 2×106 A/cm2 at 77 K. A MgO seed layer, with (001)MgO(parallel)(001)CeO2 orientation, was used to induce a 45° crystallographic grain boundary in YBa2Cu3O7−δ at its edge. An additional epitaxial buffer double layer of YBa2Cu3O7−δ and SrTiO3 on top of the seed layer promoted the formation of a grain boundary of better crystallinity and stoichiometry. It improved the critical current of the junction about tenfold and resulted in characteristic IcRn products of 150 μV at 77 K in microbridges crossing the grain boundary. Microwave-induced steps were detected at 77 K up to voltages corresponding to the characteristic IcRn value. Peak-to-peak responses to superconducting quantum interference devices reached values of 7 μV at 77 K. © 1995 American Institute of Physics.

Notes: An extensive investigation of low frequency noise in single electron transistors as a function of gain is presented. Comparing the output noise with gain for a large number of bias points, it is found that the noise is dominated by external charge noise. For low gains we find an additional noise contribution which is compared to a model including resistance fluctuations. We conclude that this excess noise is not primarily due to resistance fluctuations. For one sample, we find a low minimum charge noise of qn(approximate)2×10−5 e/Hz at a frequency of 4.4 kHz. © 1999 American Institute of Physics.

Notes: Combinations of high TC superconducting and ferroelectric films may give rise to tunable, high-Q microwave components. c-axis oriented YBa2Cu3O7−δ films were grown by laser ablation on (001) and vicinically cut KTaO3 substrates and studied by x-ray diffraction and electron microscopy. Competitive superconducting properties were registered. YBa2Cu3O7−δ/KTaO3/YBa2Cu3O7−δ trilayers were deposited on silicon-on-sapphire buffered by CeO2/Y-ZrO2. The dielectric permittivity of KTO3 at 15–100 K decreased considerably when the layer was polarized by a dc voltage. A loss factor tan δ=0.007 was measured at 100 kHz and T=50–100 K. © 1995 American Institute of Physics.

Notes: Experiments on a model of rapid single flux quantum (RSFQ) flip-flop cell, based on high-Tc (HTS) Josephson junctions show that it can operate as a voltage divider at frequency up to 400 GHz. The junctions were formed in YBaCuO film, deposited on novel Y–ZrO2 bicrystals with two asymmetric 32° grain boundaries, about 10 μm apart, and allow a new design of RSFQ logic based on a single HTS layer. Small inductances ((approximately-equal-to)10 pH) were made as narrow, submicron size slits. The junction widths were between 4 and 10 μm and for ten junctions located close to the tested circuits, the linear critical current densities at T=4.4 K were 10.7 μA/μm±50% for one grain boundary and 8.3 μA/μm±50% for the other one. IcRn was about 1 mV±50%. A current density of half the expected value meant that the test circuit did not act as an ideal flip–flop down to the lowest frequency. As a voltage divider it gave a half value division up to 0.82 mV at T=4.4 K and to 0.4 mV at 30 K. © 1995 American Institute of Physics.

Notes: We present a method to study separately the electrical transport properties of the grain boundaries (GBs) formed at the top and at the bottom edges of YBa2Cu3O7−δ(YBCO) step-edge Josephson junctions. The step-edge junctions were fabricated on (100) LaAlO3 steps using tilted Ar ion milling to define the electrodes and the microbridges. Due to the shadowing effect of the step, a continuous YBCO stripe remains along and at the bottom of the step on both sides of a microbridge. We found that the top GB is responsible for the weak link behavior of our step-edge junctions. The transport properties were correlated with the different microstructural properties of the two GBs formed at the edges of the step. © 1998 American Institute of Physics.

Notes: The microstructure of YBa2Cu3O7−δ(YBCO) grain boundaries obtained at step edges on (001) LaAlO3 substrates have been studied by scanning and transmission electron microscopy. A typical morphology of the YBCO grain boundaries is distinguished by two parts where the parts are expected to have different junction properties. One part of the boundary is caused by the difference in nucleation probability between the (001) surface and the inclined step facet. The second morphology is developing as a result of minimization of the YBCO surface energy. © 1997 American Institute of Physics.