A. Di Chiara

YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain boundary Josephson junctions with a MgO seed layer

A YBa/sub 2/Cu/sub 3/O/sub 7-x/ artificial grain boundary Josephson junction mas fabricated on to a [110] SrTiO/sub 3/ substrate employing a MgO seed layer. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grows mainly oriented (103) on SrTiO/sub 3/ and [001] on the seed layer. An artificial 45/spl deg/ a or b axis tilt or twist boundary can occur according to the orientation of the MgO edge with respect to the [001] in plane direction of the substrate. Josephson junctions of good quality and excellent reproducibility were obtained in microbridges patterned across the grain boundary. Typical features of the phenomenology of high critical temperature superconductor (HTCS) Josephson junction were observed. The maximum value of the I/sub C/R/sub N/ product is of the order of 2-3 millivolts at T=4.2 K and of the order of 50 /spl mu/V at T=77 K respectively. The modulation of the critical current with the external applied magnetic field confirms the Josephson nature of the current in the weak link.

Phase competition between Y2BaCuO5 and Y2O3 precipitates in Y-rich YBCO thin films

Abstract Segregation and competition of Y-based impurities in Y-rich biepitaxial YBCO samples is investigated. The analysis of distribution and microstructure of the precipitates is performed by high resolution electron microscopy (HREM). Y 2 O 3 precipitates are found in (103) YBCO films grown on (110) SrTiO 3 (STO) substrates. On the same biepitaxial samples, Y 2 BaCuO 5 (Y211) precipitates are found in (001) YBCO films deposited on an intermediate (110) MgO seed layer. Agglomeration of Y211 precipitates is observed near the edge in the MgO seed layer. The experimental data are discussed in the context of phase competition, nucleation, epitaxy and surface migration in the Y–Ba–Cu–O system. It is deduced that the change of supersaturation and surface energy, also governed by the favorable epitaxy of the secondary phase with the substrate or the YBCO matrix, plays a major role in determining the segregation of either Y211 or Y 2 O 3 precipitates.

Electron beam irradiation of Y1Ba2Cu3O7−x grain boundary Josephson junctions

The properties of the Y1Ba2Cu3O7−x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O7−x thin film by cylindrical magnetron sputtering technique on the (110) SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions IC and increased the normal state resistance times area to values of the order of 1(μ Ω cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions.

Structure and morphology of MgO/YBCO bilayers for biepitaxial junctions

Abstract (110) MgO thin films have been deposited by RF sputtering on (110) SrTiO 3 and used as buffer layers for YBCO deposition. The MgO films show high morphological quality, as confirmed by X-ray specular reflectivity, and narrow (≈ 1°) X-ray diffraction peaks in the rocking curves measurements. These results are discussed in the framework of an ionic oxide growth model. XRD analyses performed on the bilayer YBCO/MgO (110) confirm the epitaxial growth of the films, with (001) YBCO//(110) MgO. XRD, AFM, SEM measurements are compared with data relative to bilayers deposited on (100) SrTiO 3 .

Electron beam irradiation of Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}x} grain boundary Josephson junctions

The properties of the Y1Ba2Cu3O7−x biepitaxial Josephson junctions were reproducibly modified by a focused electron beam irradiation of the interface region. The junctions were fabricated by depositing Y1Ba2Cu3O7−x thin film by cylindrical magnetron sputtering technique on the (110) SrTiO3 substrate, partially covered by a pregrown MgO seed layer. The junction parameters can be adjusted controllably by applying an appropriate dose. Electron irradiation decreased the critical current of the junctions IC and increased the normal state resistance times area to values of the order of 1(μ Ω cm2). Some other effects, such as the disappearance of the excess current, were also observed. The original properties of the junctions could be partly restored by isothermal annealing. We also speculate that some aspects of the nature of the grain boundary barriers can be better understood from the study of the properties of irradiated junctions.

A new type of biepitaxialc-axis tilted YBCO Josephson junction

A novel fabrication procedure of high critical temperature superconductor Josephson junctions (HTCSJJ) has been developed by a 90° rotation of YBa2Cu3O7−xc-axis around an in-plane direction, on the basis of concepts of the biepitaxial technique. YBa2Cu3O7−x grows oriented along (001) direction on a MgO seed layer deposited on a (110) SrTiO3 substrate and along (110) direction on the bare substrate. Josephson junctions of good quality were obtained exhibiting RSJ behavior and features characteristic of HTCSJJ phenomenology. Even if not uniform, in some samples, the nature of the critical currentIc is completely Josephson, as resulting from theIc dependence on the applied magnetic field. The maximum measuredIcRN value atT=4.2K is 2mV.

Fabrication of YBCO step-edge Josephson junctions by inverted cylindrical magnetron sputtering technique

The Inverted Cylindrical Magnetron Sputtering (ICMS) technique been used to fabricate YBCO step-edge dc SQUIDs. Steps are obtained by standard ion milling procedure on LaAlO/sub 3/ (100) substrates using Nb-masks patterned by reactive ion etching. Thin films are then deposited under different conditions (both step and deposition angles) and patterned by standard lithography and wet chemical etching. An analysis of junction properties in terms of step height and film thickness has been carried out. Measurements on current vs magnetic field and SQUID voltage response measurements have been performed. Their temperature dependences have been also considered. Operating temperature as high as 77 K has been achieved. At 4.2 K the SQUIDs show a maximum voltage to flux transfer function (/spl part/V//spl part//spl phi/)/sub max/=870 /spl mu/V//spl Phi//sub 0/ and a good periodicity of the V-/spl phi/ modulation up to 20 /spl Phi//sub 0/ without any sign of hysteresis.<<ETX>>

A qualitative explanation of tunneling characteristics of high-Tc junctions in terms of the McMillan proximity model

In this paper the influence of proximity effects on the transport properties of high-Tc superconductors is considered. On the basis of the experimental results on tunneling spectroscopy reported in the current literature, we discuss some of the main features ofdI/dV vs.V characteristics. More particularly, attention is focused on the depression of gap structure, the presence of conductance peaks due to the counterelectrode gap, and “zero-bias anomalies.” A possible explanation of those features in terms of the McMillan proximity model is given along with some examples of application of the model to experimental data.

A study on the stability of superconducting YBa2Cu3O7−x phase

The thermal stability of the superconducting phase of nominal composition YBa2Cu3O7−x-sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10−4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO2 (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.

Mo-Re superconducting thin films by single target magnetron sputtering

The authors prepared Mo/sub 60/Re/sub 40/ thin films by single target magnetron sputtering. All the films analyzed exhibited a sharp superconductive transition and values of the critical temperature T/sub c/ up to 13 K were achieved. Structural analyses (EDS and X-rays) were performed on the samples. A15 phase was obtained even at low deposition temperatures. From the measured low temperature resistivity O/sub c/ and critical temperature T/sub c/ an estimate of the theoretical BCS surface resistance is inferred and compared with that of niobium. >