Uri Dai

Preparation and patterning of YBaCuO thin films obtained by sequential deposition of CuOx/Y2O3/BaF2

Superconducting thin films of YBaCuO were prepared by stacking layers of CuOx , Y2O3, and BaF2 onto substrates held at room temperature. The CuOx layer was obtained by oxidizing in air a previously deposited metallic Cu film. After deposition of the subsequent layers, the films were annealed at 880 °C in flowing oxygen. Samples 1 μm thick on sapphire showed an onset at Tonsetc =85 K and zero resistance at T0c =70 K; on yttria‐stabilized zirconia (YSZ), Tonsetc =90 K and T0c =80 K. Patterned films are easily obtained by applying conventional photolithographic methods to the Cu film.

Strong magnetic field dependence of the critical currents correlated to the microstructure of YBaCuO ceramics

Critical currents of YBaCuO ceramics were measured at 77 K by transport measurements in an applied magnetic field H. A hysteretic behavior between 10 and 10 000 G was observed. For decreasing fields, the critical current Jc obeys a power law H−n with n ranging between 0.6 and 2.8. The exponent n is correlated with the microstructure and the average grain size of the ceramic. This power law behavior is discussed in terms of modified flux creep and flux flow regimes related to the microstructure.

Critical current of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ in strong applied fields

Critical currents of the new high‐temperature superconductor Y1Ba2Cu3O7 have been measured in applied fields of up to 7 T and for temperatures down to 70 K. We find that the critical current is drastically reduced by the application of magnetic fields much smaller than the upper critical field of the samples, Hc2. This anomalous behavior might be due to very weak pinning, or to a very strong anisotropy of Hc2. Hc2 is found to follow a linear temperature dependence that however extrapolates to a critical temperature higher than that measured directly. This might result from the existence of a percolative structure, or from the presence of a small volume fraction of high critical temperature, high critical field regions.

Critical current of Y1Ba2Cu3O7 in strong applied fields

Critical currents of the new high‐temperature superconductor Y1Ba2Cu3O7 have been measured in applied fields of up to 7 T and for temperatures down to 70 K. We find that the critical current is drastically reduced by the application of magnetic fields much smaller than the upper critical field of the samples, Hc2. This anomalous behavior might be due to very weak pinning, or to a very strong anisotropy of Hc2. Hc2 is found to follow a linear temperature dependence that however extrapolates to a critical temperature higher than that measured directly. This might result from the existence of a percolative structure, or from the presence of a small volume fraction of high critical temperature, high critical field regions.

Cathodoluminescence study of thin films of high Tc superconductors

Cathodoluminescence (CL) of thin films of high Tc superconductors was studied in the scanning electron microscope. The depth and the lateral locations of the different phases can be revealed. In thin films, unlike the bulk superconductors, the CL information can be obtained either from the film itself or the substrate by varying the primary beam energy. At high beam energy, substrate defects and slight thickness variations of a single high Tc phase are observed. The resolution of the CL measurements improves at low temperatures.

Flux‐creep‐limited critical currents in YBa2Cu3O7−δ ceramics

Critical currents of YBa2Cu3O7−δ bulk‐sintered samples were measured at and above 77 K in the presence of magnetic fields. In moderate magnetic fields a H−n dependence with n<1 was observed. Transport measurements give n≂0.5, indicating that critical currents are limited by flux creep rather than by field quenching of Josephson junctions.

Long coherence lengths in c-axis oriented Y0.6Pr0.4Ba2Cu3O7 thin films determined by the resistive transitions under applied magnetic fields

Abstract We report the measurement and analysis of the resistive transitions under applied magnetic fields for c axis oriented thin films of Y 0.6 Pr 0.4 Ba 2 Cu3O7 (PYBCO). We estimate the in-plane and out-of-plane coherence lengths to be much larger YBCO. Our data could not be fitted in a satisfactory way to the ‘giant flux-creep’ model. We also discuss the change in the superconducting properties of this system as compared to YBCO in terms of the double pair-breaking theory (both for magnetic impurities and magnetic field) and find it to be insufficient to explain the observed data. We discuss the large coherence lengths ξab ξc in PYBCO in terms of the van-Hove scenario for HTC.

Contacts of very low resistivity to YBaCuO thin films

The authors report a method for the preparation of contacts of very low surface resistivity (contact resistance multiplied by contact area) to polycrystalline YBaCuO thin films. The method provides contacts with resistivities smaller than 3*10-7 Omega cm2, which are obtained by the application of silver paint dots onto the film surface prior to annealing. Only one annealing procedure is required for the preparation of samples and contacts. Above the critical current both the four-terminal and two-terminal resistances obeyed Ohms law for several current decades.

Electrochemical room temperature reduction and reoxydation of thin films and pellets of YBa2Cu3O7−x

Abstract YBa 2 Cu 3 O 7 (1237) was reduced and reoxydated quantitatively, in a controlled manner, at room temperature (RT), by way of extraction or insertion of oxygen, using an electrochemical set-up. RT reduction and reoxygenation of thin films, which led to homogeneous products, as judged by X ray diffraction. Since reduction leads to loss of superconductivity and re-oxidation restores it, this method allows room temperature control over superconducting properties of (parts of) films. This permits patterning of (1237) thin films. SNS-like structures were already prepared by this method. The method affords control not only over n(E F ) via [oxygen] bulk , but also over the quality of the intergranular contact, possibly via control of [oxygen] in the outermost layers of grains (in films and pellets).

Hall voltage measurements using ac lock‐in detection

The measurement of the Hall constant in insulating highly resistive films is a difficult and challenging task. Owing to the small currents that may be passed through the insulating films, the Hall voltages are often very small in magnitude. A simple electronic circuit using commercial lock‐in amplifiers is described which allows measuring these minute Hall voltages. Other practical measuring suggestions are also presented.