N D Kataria

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.

Laterally resolved microwave surface-resistance measurement of high-T/sub c/ superconductor samples by cavity substitution technique

Microwave surface impedance measurement of a high-temperature superconductor (HTS) is a sensitive probe to test its quality, particularly if a microwave device is to be fabricated. Most microwave characterization employs resonance techniques in which the components of the surface impedance are extracted from the measured Q value and the shift in the resonance frequency. In this paper, we present a modification of the widely used complete end-plate substitution technique to measure the surface resistance of samples having dimensions smaller than the dimension of the end plate at 20 GHz, as well as to facilitate the laterally resolved surface resistance measurement of large-area HTS samples. From the knowledge of the electromagnetic-field configuration in a TE/sub 011/-mode cylindrical cavity, the loss contribution from the HTS sample is analyzed theoretically and measured experimentally in the temperature range of 20-100 K. The design of the cavity is discussed to optimize the sensitivity of the measurement by the placement of the sample and to maximize the difference in the measured Q value.

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.

Josephson voltage standard at National Physical Laboratory, New Delhi

A C Josephson effect is now used by several countries as the reference standard for the unit of d.c. voltage. This paper describes the work done at the National Physical Laboratory (NPL), New Delhi in the realization of the unit of volt based on the a.c Josephson effect. A voltage standard at 1 mV level using a Nb-Nb point contact junction has been established and the as-maintained volt based on a bank of standard cells has been intercompared against it using a 1:1000 voltage divider. The experimental set-up used in this comparison and the results of recent measurements are described. The overall uncertainty in assigning the value of emf to a standard cell is about 1 ppm. The as-maintained volt has been found to agree with the Josephson voltage within overall uncertainty.

Study of Josephson effects in microbridges of Bi-Sr-Ca-Cu-O films made by spray pyrolysis

Superconducting Bi-Sr-Ca-Cu-O (2212) films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. TheR-T curves of the films show metallic behaviour above the superconducting transition temperature.Tc (R=0) is observed between 80 and 85 K. Annealing temperature has a profound effect onTc (R=0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films withc-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50 µm × 50 µm have been patterned photolithographically followed with chemical etching. The 1 V characteristics of the microbridges show Josephson effects due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.

GROWTH OF ORIENTED SUPERCONDUCTING FILMS OF Bi–Sr–Ca–Cu–O BY SPRAY PYROLYSIS TECHNIQUE

Highly oriented superconducting thick films of Bi2Sr2CaCu2O8+x have been prepared by low cost spray pyrolysis technique. Aqueous solution containing nitrates of Bi, Sr, Ca, and Cu in the atomic ratio of 2:2:1:2 is sprayed on MgO (100) substrates which are kept at elevated temperature. After annealing in air near the melting temperature, highly oriented films with c-axis perpendicular to the substrate are obtained. The films show a sharp superconducting transition with zero resistance at 85 K. The transport critical current density of the film at 77 K and zero field is of the order of 4000 A/cm2.