D. Abukay

Effect of YBa2Cu3O7-x thin films on the textures and orientational properties of liquid crystals

Abstract.In the present work, the effect of thin films of YBa2Cu3O7-x complex compound on the mesomorphic and orientational properties of thermotropic nematic liquid crystals has been studied. Homogeneous, stable and reproducible homeotropic and tilted oriented textures of nematic liquid crystals were obtained. The effect of YBa2Cu3O7-x thin films on the morphologic, orientational and optical properties of thermotropic nematic liquid crystals are discussed.

Investigation of the effect of thermal cycling on the device performance of YBa2Cu3O7−δ DC-SQUIDs

We investigated the effect of thermal cycling on the operational performance of YBa2Cu3O7?? (YBCO) direct current superconducting quantum interference devices (DC-SQUIDs) fabricated onto 24??SrTiO3 (STO) bicrystal substrates. The devices under investigation consist of directly coupled DC-SQUID magnetometer configurations. Thin films having 200?nm thicknesses were deposited by dc-magnetron sputtering and device patterns were made by a standard lithography process and chemical etching. The SQUIDs having 4??m-wide grain boundary Josephson junctions (GBJJs) were characterized by means of critical currents, peak-to-peak output voltages and noise levels, depending on the thermal cycles. In order to achieve a protective layer for the junctions against the undesired effects of thermal cycles and ambient atmosphere during the room temperature storage, the devices were coated with a 400?nm thick YBCO layer at room temperature. Since the second layer of amorphous YBCO is completely electrically insulating, it does not affect the operation of the junctions and pick-up coils of magnetometers. This two-layered configuration ensures the protection of the junctions from ambient atmosphere as well as from the effect of water molecules interacting with the film structure during each thermal cycle.

Flux propagation in tubular high-Tc ceramics

Abstract We present an experimental and theoretical study of the dynamics of low-field penetration in tubular YBaCuO ceramics when an axial magnetic field is suddenly applied. The main characteristics times of the phenomenon are calculated assuming a flux-flow process through the Josephson medium. The theoretical predictions are in good agreement with the experimental results at 77K.

Thin film growth of high-Tc YBa2Cu3O7-δ phase by an oxygen refilling process

Abstract The high-T c . YBa 2 Cu 3 O 7-δ phase in thin films has been grown by an oxygen refilling process and low-temperature annealing. Such samples on an MgO substrate showed a small electrical resistivity with exactly linear temperature-dependence below room temperature and sharp transition with high T c .

The formation of high Tc YBa2Cu3O7 − δ by an oxygen refilling process on thin films made by r.f. magnetron sputtering

Abstract Superconducting films of YBa2Cu3O7 − δ were grown on MgO and sapphire substrates by single-target r.f. magnetron sputtering in an argon atmosphere. A special post-annealing process was used which firstly increases the oxygen deficiency in the film layer and then refills the oxygen vacancies in a controlled manner to give a narrow and high superconducting transition temperature Tc. The onset point was 90 K for the films on MgO substrates and zero resistance was obtained at 83 K. For the films on sapphire an onset temperature of 80 K was found.

The Effects of High Reaction Temperatures on the Tc of Multifilamentary Nb-Al Wires

Powder metallurgy processed multifilamentary Nb-8 wt.% Al wires were heat-treated at temperatures >2000°C for very short times. Values of Tc ranging from 12 to 20 K have been resistively measured from these wires after annealing them at 700 to 750°C for 48 to 185 h. The Bc2(T) values of the same samples were also measured, and the values of their magnetic slope (-dBc2/dT) at Tc varied in the range 2.38 to 2.58 T/K.

Dependence of Josephson Junction Critical Current on The Deposition Rate of YBa2Cu3O7−δ Thin Films

We have reported the effect of YBa2Cu3O7−δ (YBCO) thin film deposition rate on the 24 and 30 degree STO bicrystal Josephson junctions critical currents by fabricating series of junctions with different deposition rates. Dependence of YBCO thin film structures on the deposition rate was investigated. We have observed that the critical currents of junctions are strongly affected by the thin film deposition rate.

Selection of the Best Proper DC-SQUIDs in a Multi-SQUID Configuration

We have carried out experimental investigation of multi-DC-SQUID magnetometer configuration fabricated on thin films onto 24 degree bicrystal substrates by directly coupling the pick-up loop to DC-SQUIDs. The layout of the magnetometer pick-up loop was chosen as a square washer configuration by maximizing loop effective area and minimizing loop inductance. We have used Dc-Magnetron Sputtering technique for deposition of the films and chemical etching process for patterning the Josephson junctions having 4 widths. The use of multi-SQUID configuration is related to the selection of the best proper junctions for SQUID to improve the chip sensitivity with selectivity option of choosing the squid junctions rather than multichannel operation. Selection of the best junctions compared to each other depending on the junction critical currents and noise levels caused by the fabrication process and placements of the junctions on the grain boundary enable having an increased output signal of the DC-SQUID.

Developing a trilayer processing technique for superconducting Y Ba2Cu3O7−δ thin films by using Ge ion implantation

For making trilayer superconducting devices based on Y Ba2Cu3O7?? (YBCO) thin film processing, we developed a new technique by employing Ge ion implantation. A YBCO thin film of 150?nm thickness having high c-axis orientation and a transition temperature, Tc, of 90?K was implanted with 80?keV, 1 ? 1016?Ge?ions?cm?2 at room temperature. By the result of TRIM calculation, Ge ions were found to penetrate into the YBCO thin film approximately 60 nm below the surface of the film, thus leaving the lower part of the film as a superconductor. Upon implantation with Ge ions, the implanted upper part of the sample lost its electrical conductivity and diamagnetism while its original crystalline structure was preserved. The implanted ions we found did not alter the overall crystal structure of the YBCO thin film; this allowed us to grow an epitaxial superconducting upper layer of YBCO on top of the implanted area, leaving no need to use any buffer layer. The superconducting properties of the upper layer were similar to those of the pure YBCO base layer with an increased room temperature resistivity and a lowered Tc (88?K). This process provides an effective method for fabrication of a trilayer HTS device structure.

Preparation of High Tc Bi-System Film with Magnetron Sputtering and Physical Properties

High Tc Bi-system film were deposited on the (100) face of MgO single crystal with rf magnetron sputtering method using the single target Bi-(Pb)-Sr-Ca-Cu-O and then annealed. From the temperature dependence of resistivity and the electron microscope photograph, it was found that the critical temperature T c of Bi-system film depends on the annealing condition and the film thickness, and that the grain size grows with doping Pb into Bi-Sr-Ca-Cu-O. Further, from the currernt-voltage (I – V) characteristic of Bi-system films under microwave radiation and magnetic field and tunneling spectroscopy, the physical properties of these films were investigated. The first Shapiro step was observed in the V-dl/dV characteristic under the microwave radiation (50GHz). The one electron tunneling spectrum at 38K gave the superconducting gap 2△=60meV.