High quality YBa2Cu3Ox ultra-thin films and Y/Pr/Y multilayers made by a modified RF-magnetron sputtering technique

High quality ĉ-axis oriented thin and ultra-thin films have been grown in situ on (100) surfaces of ZrO2, SrTiO3 and MgO. Sharp transitions were observed with Tc,zero of 87–91 K for films thicker than 70 Å. On atomically polished MgO substrates films as thin as 15 Å revealed a full transition to superconductivity above 45.5 K. The critical current density at 77 K was found to be strongly dependent on film thickness. A maximum value was found for a 100 Å film with 8 × 106A/cm2 at 77 K. Also, YBCO/PBCO/YBCO multilayer thin films have been fabricated in situ by the same technique. The epitaxy is maintained throughout the whole multilayer system. The superconducting properties of YBa2Cu3Ox layers do not change compared to single layers. Interdiffusion and possible chemical reaction close to the interfaces can be neglected.\ud \u

Properties of lift-off structured high Tc microbridges

Microbridges and DC SQUIDs (superconducting quantum interference devices) were fabricated, using a lift-off technique, from RF sputtered YBaCuO films on MgO single-crystal substrates. Microwave measurements at 9 GHz on microbridges and the magnetic-field dependence of their critical current reveal wide bridge behaviour up to temperatures near the maximum operating temperature of the bridge. Mostly, a linear dependence of the critical current on the temperature is found, which is connected with high intrinsic 1/f noise if the bridge is constant-current-biased slightly above the critical current. In some bridges and DC SQUIDs, regimes with a temperature dependence proportional to (1-T/Tc)1.5 are found. In this case the 1/f noise level is much smaller and SQUID modulation can be followed to about 65

Fabrication of 45 degrees template grain boundary junctions using a CaO lift-off technique

45 degrees grain boundary junctions have been made using (100) MgO substrates, a CeO2 template layer and an YBa2Cu3O7 top layer. To minimize the damage to the MgO surface, which will occur if the CeO2 is structured using ion milling, the CeO2 layer has been structured using the CaO lift-off technique. Electrical measurements of these junctions as a function of temperature, microwave irradiation and magnetic field will be discussed in this paper

On the suppression of the sidelobes of the supercurrent in small Josephson tunnel junctions

The critical currents of Nb/Al, Al-oxide, Al/Nb tunnel junctions of various shapes have been measured as a function of the applied magnetic field. For the square junction and for some special shapes like the diamond, ¿1 + cosine¿ and quartic junctions the Ic(B) pattern falls off theoretically as 1/Bn, with n respectively equal to 1, 2, 3 and 4. In general the measurements are in good agreement with the theoretical predictions. For the "1 + cosine" and quartic shapes we found a sidelobe suppression that is even larger than that obtained in theory. For the quartic junction the first sidelobe is only 0.3% of the zero-field current. An Ic(B) modulation with a small, only slowly decreasing amplitude is observed for the diamond, "1 + cosine", and quartic junctions, that can be explained by rounding of the sharp edges of the junction shapes, due to the fabrication process

Monolithic flux transformer-coupled high-Tc dc SQUID magnetometers

YBa/sub 2/Cu/sub 3/O/sub 7-x/ based monolithic flux transformer-coupled high-T/sub c/ DC SQUID magnetometers operating up to 73 K have been realized. The devices are characterized by high values of the modulation voltage, up to 32 /spl mu/V at 40 K. A minimal white noise level of 0.10 pT//spl radic/Hz was obtained above 200 Hz, and 0.64 pT//spl radic/Hz at 1 Hz and 55 K. The temperature dependence of the modulation voltage, the effective sensing area and the field sensitivity are discussed. Model-calculations have been performed to investigate high frequency resonances in the washer-input coil structure. Methods for damping are considered

ARXPS-studies ofcˆ-axis textured YBa2Cu3Ox-films

YBa2Cu3Ox sputter deposited cold on MgO grows in O2 annealing epitaxially to a transparent, superconducting film with Tc 80K. The unscraped surfaces of these films are smooth showing XPS lines changing with photoelectron take-off angle. This enhanced data base allows to separate the different chemical compounds (hydroxide, peroxide, carbonate, carboxyle, cuprate, graphite ...) and to obtain their spatial distribution. This yields the compounds, their amount and distribution making up the cinder growing with O2-anneal at internal and external surfaces. The cinder stoichiometry gives insights in the chemistry going on in O2 annealing. Below the cinder the signature ofcˆ-axis oriented YBa2Cu3Ox is identified, showing that a Ba-oxide layer forms the stable surface. This coats insulating CuO2 and Y-oxide layers yielding so an intrinsic dead layer

In-situ growth studies of sputtered ybco thin films by spectroscopic ellipsometry

Using spectroscopic ellipsometry we studied in-situ the growth of off-axis sputtered YBa2Cu3O6+x thin films on (001) SrTiO3 as a function of the deposition parameters. Especially in the very first growth stage (<5 nm) we observed that the optical properties of the grown layer differs from the "bulk" optical properties of YBCO and strongly depends on, both, the deposition temperature and the oxygen partial pressure. Both properties are well established as influencing the superconducting properties of thin YBCO films. YBCO thin film growth under optimal deposition conditions (Tcnot, vert, similar90 K; jc>106A cm¿2 @ 77 K) is smooth and homogeneous, except for the first unit cell layer (initial stage regime). The smoothness of the response is indicative for a step-mode like growth mechanism. In contrast, the initial stage regime is governed by a 2D nucleation mechanism. This behaviour changes when the deposition temperature is lowered. Due to increased disorder, the initial stage regime is extended to larger thicknesses and a true 2D growth mode is no longer apparent. Similar behaviour is observed with increasing oxygen partial pressure, where the optical response is shifted from a step-flow mode like mechanism to an island-growth mode

Smart SQUIDs based on Relaxation Oscillation SQUIDs

Smart SQUIDs based on double Relaxation Oscillation SQUIDs (DROS) and a superconducting up-down counter have been developed. DROS and counter form a flux locked loop on one single chip. The DROS output consists of a series of pulses that controls the two up and down write gates of the counter. The pulsed output structure of the DROS constitutes the internal clock for this single-chip device. Several prototypes were built with a clock frequency of 100 MHz, a linear operation flux range of about 2.5 ¿0, and a white noise level of 6.5 ¿¿0/¿Hz. The smart SQUID is in principle a promising device for application in multichannel SQUID system