V N Ojha

Superconductivity in BiSrCaCuO films made by spray pyrolysis

Abstract Thin films of BiSrCaCuO compound are made by spray pyrolysis of aqueous solution of nitrates of the metallic constituents on oriented (1 0 0) MgO substrates kept at elevated temperatures. Annealed films are superconducting with T c (on set) near 91 K with zero resistivity around 83.5 K. The films are granular with Josephson weaklinks formed through normal grain boundaries. The sensitivity of critical current to microwave power and its nearly square variation with reduced temperatures reveal the presence of the proximity induced intergranular weak links.

BEHAVIOR OF DOUBLE JUNCTION SQUID OF Y-Ba-Cu-O BULK SUPERCONDUCTOR

A dc SQUID like geometry with a ring and a pair of constrictions is carved out of bulk Y-Ba-Cu-O superconductor. The I—V curves exhibit hysteresis arising due to trapping of the self fields. The V—Φ curves show multiple flux periodicities with unequal amplitudes. There is twenty fold rise in the amplitude of voltage modulation in cooling from 77 to 4.2 K. The complications of presence of series and parallel arrays of granular junctions are attributed respectively to large and small periodicities observed in V—Φ curves. The modulation structure gets more resolved at 4.2 K due to reduction in thermal noise effects.

Microwave irradiated effects in microbridge of r.f. sputtered YBaCuO thin film

Abstract YBaCuO thin films were fabricated by r.f. diode sputtering using single composite target. Bridge type Josephson junctions were patterned by conventional photolithographic technique using dilute phosphoric acid as an etchant. Microwave irradiation on the bridge induces Shapiro steps due to the a.c. Josephson effect.

GROWTH OF SUPERCONDUCTING THICK FILMS OF YBaCuO BY SPRAY PYROLYSIS TECHNIQUE AND STUDY OF QUANTUM INTERFERENCE EFFECTS

Superconducting thick films of YBaCuO are made by spray pyrolysis technique on single crystal SrTiO3 and MgO substrates. The thickness of the film is about 10 µm. The best film shows Tc (R = 0) of the order of 86 K. No marked degradation in superconducting transition temperature is observed when the films are thermally cycled between room temperature and 4.2 K inside a closed liquid helium container. Temperature dependence of the critical current of microbridges fabricated on this film indicates the presence of SNS type weak links. dc SQUID configuration has been made in this film by scribing two bridges in parallel. V - Φ curves show periodic behavior superposed with additional structures.

Low-resistance silver contact on Bi-Sr-Ca-Cu-O film

A simple method of making low-resistivity contacts ( approximately 10-7 Omega cm2) is reported for the first time on Bi-Sr-Ca-Cu-O films. The method basically involves the laying of silver-paste pads on the surface of as-deposited film, their annealing and the subsequent attachment of copper leads. The annealing step thus serves a dual purpose. It not only renders the film superconducting with requisite oxygenation but also helps in the preparation of low-resistivity contacts. The contacts on the film are quite rugged and could withstand several thermal cyclings between 4.2 and 300 K over a period of one month.

Effect of heat treatment on Ag/Y-Ba-Cu-O contact resistance

The contact resistance (Rc) of the metal/YBCO interface has been studied in pressed indium, painted colloidal silver and thermally-evaporated silver contact pads. Indium contacts always show the highest resistance amongst these three systems. In thermally-evaporated Ag contacts, post-deposition thermal treatments show favourable effects on the reduction ofRc. Heat treatment in oxygen atmosphere in two steps is essential to reduce theRc values. Significant improvement in obtaining low resistivity contacts has been attributed to the diffusion of silver atoms to grain boundaries at the surface of YBCO and to the enrichment of oxygen-deficient layer at the interface during thermal treatment.

Effective passivation of YBaCuO superconducting bridges by photoresist overlayers

Abstract Degradation of the superconducting properties of microbridges of YBaCuO films and bulk samples have been effectively checked by covering their surface with protective layers of photoresist. The overlayers are sufficiently resistant to condensation of moisture on exposure of liquid nitrogen-cooled samples to ambient conditions and to crack formations. The samples could withstand several thermal cycles between room temperature and liquid nitrogen temperature without any degradation of their superconducting properties.

Superconducting thick films of Bi2Sr2CaCu2O8+x with zero resistivity at 96 K

Thick screen-printed films of Bi2Sr2CaCu2O8+x on (100) MgO substrate have been prepared with zero resistivity at 96 K. The processing technique involves two-step heat treatment at 880 degrees C and 864 degrees C followed with a rapid quenching to ambient temperature. The X-ray diffraction studies reveal formation of single-phase oriented films of composition 2212. The enhancement of Tc has been attributed to the favourable oxygenation of the usual 85 K phase.

Laser patterning of microbridges and the DC squid structure on Bi-Sr-Ca-Cu-O film

Patterning of BiSrCaCuO film by laser ablation to make microbridges and DC SQUID structures is reported. No degradation in the superconducting transition curve of the film is observed after patterning. The transport critical current density Jc for the microbridge is 2.4*103 A cm-2 at 77 K.

Fabrication and characterization of high T/sub c/ superconducting magnetic shield

Superconducting magnetic shields of Y-Ba-Cu-O superconductor were fabricated in tubular form with one end closed. These shields were used successfully to reduce the flux noises of a two-hole RF SQUID of Y-Ba-Cu-O superconductor. Hysteretic critical current density and penetrated field amplitude under increasing and decreasing magnetic fields reveal the presence of grain boundary weak links. The flux trapping behavior can be explained by using a modified version of C.P. Beans self-field-limiting model (1962).