V S Tomar

RF SQUID behaviour in YBCO thick films up to 85 K

RF SQUID behaviour has been observed in screen-printed thick film of YBCO up to 85 K. A hole shunted with a microbridge type of geometry has been made manually on the film. Phase reversal in V- Phi behaviour, which is a characteristic feature of the RF SQUID, is also observed at 77 K. The flux noise spectrum has been found to depend sensitively on the quality of the film. Films containing 123 phase as well as 211 phase show more 1/f noise than films containing only 123 phase.

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.

Performance of Bi-Sr-Ca-Cu-O thick film rf SQUIDs at liquid nitrogen temperatures

Periodic oscillations in the V‐Φ characteristics have been observed up to 97 K in radio frequency SQUIDs fabricated from screen printed Bi‐Sr‐Ca‐Cu‐O thick films. The SQUID has been operated successfully in the flux‐locked‐loop mode at 77 K. Stability of the SQUID output and its noise performance have been found to improve considerably when the SQUID is operated in the flux‐locked‐loop mode as compared to that in the open‐loop mode. The spectral noise density, in the white noise region, is 5×10−4 Φ0/√Hz at 77 K for the flux‐locked‐loop mode.

Behaviour of thick film BiSrCaCuO rf SQUID at 77 K

Abstract Rf SQUID effect has been observed in BiSrCaCuO thick film at 77 K. The films were prepared using screen printing technique on MgO (100) substrate with the starting composition of 1112. The superconducting film has Tc(R=0) ∼ 100 K. A hole shunted with a microbridge is carved manually for observing rf SQUID effect. Flux noise spectrum of the SQUID has been studied at 77 K. The flux noise in this device is relatively less than that observed in thick film YBaCuO rf SQUID.

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.