• A K Gupta

      Articles written in Bulletin of Materials Science

    • Thin films of Y-Ba-Cu-O by rf sputtering and study of Josephson effect

      Mukesh Kumar N D Kataria V S Tomar V N Ojha N Khare S U M Rao G S N Reddy A K Gupta

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      Thin films of Y-Ba-Cu-O material have been prepared by rf diode sputtering using a single oxide target on strontium titanate substrates kept at an ambient temperature. It was found difficult to attain stoichiometry of the films identical to that of the target due to resputtering of the films. The influence of sputtering parameters such as target-substrate distance, rf power, gas pressure and substrate temperature in attaining a particular stoichiometry of the film has been studied. The I–V characteristics of the bridges show Shapiro steps when the bridges are irradiated with microwave radiations. The thin film bridges have the dimensions larger than coherence length; thus all the Josephson effect features are understood to arise due to intergranular junctions.

    • Growth of highTc Bi-Sr-Ca-Cu-O thick films

      Sangeeta Chaudhry Neeraj Khare A K Gupta K C Nagpal V N Ojha G S N Reddy V S Tomar

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      Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyTc (R=0) increased from 77 to 103 K as the annealing duration was increased after whichTc decreased. Kinetics of the growth of highTc phase is discussed in the light of our results.

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

      D K Walia A K Gupta G S N Reddy V N Ojha V S Tomar Neeraj Khare N D Kataria K C Nagpal

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      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.

    • Josephson effects and dc SQUID behaviour in break junction of Y-Ba-Cu-O at 77 K

      S K Arora A K Gupta Neeraj Khare N D Kataria G S N Reddy V N Ojha V S Tomar

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      Break junctions of Y-Ba-Cu-O superconductor were realized by creating fresh crack in the bar shape bulk samples mounted in a specially designed probe kept in the liquid nitrogen bath. While the bulk sample was kept static, a sharp tip of a shaft was moved downward from the outside of the dewar in order to break the sample. It was possible to change the critical current of the break junctions by adjusting pressure on the tip of the shaft. The effects of magnetic field and microwave power on the I-V characteristic of the junction were studied. The junction was found to show periodic V-B behaviour and the microwave-induced Shapiro steps were clearly observed at 77 K. The flux noise spectrum of the junction was also studied.

    • Performance of a 2-hole Y-Ba-Cu-O rf SQUID

      S K Arora A K Gupta Neeraj Khare V S Tomar V N Ojha

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      A two-hole rf SQUID has been fabricated out of bulk YBCO by drilling two holes and careful erosion of the wall between the holes. Commercial SQUID electronics is inductively coupled to the SQUID through a copper coil glued in one of the holes. Periodic oscillations in V-B characteristic of the SQUID are observed at 77 K. The spectral density of the flux noise in the white noise region is 5·5 × 10−4Φ0 / √Hz. The flux noise is frequency-dependent below 200 Hz.

    • Study of galvanomagnetic effects in Bi-Sr-Ca-Cu-O films

      Ramesh Chandra Neeraj Khare A K Gupta D K Walia Sangeeta Chaudhry V N Ojha V S Tomar

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      Galvanomagnetic effect has been studied at 77 K on Bi-Sr-Ca-Cu-O films as a function of d.c. bias current. These films were prepared by spray pyrolysis and screen-printing techniques, withTc (R=0) 80 K and 100 K respectively. Magnetic field dependence ofIc of sprayed-film showed very small hysteresis whereas screen-printed film showed greater hysteresis. Application of small magnetic field on these films destroyed the zero-resistance state and a finite resistance was developed. The slope of the resistance vs magnetic field curveΔR/ΔH depended on the current flowing through the sample. To increaseΔR/ΔH, meander-shape pattern was prepared on the sprayed film. In generalΔR/ΔH increased by a factor of 103 after sample patterning. The possibility of using these films as a sensitive magnetic field sensor is discussed.

    • 1/f Noise properties of swift heavy ion irradiated epitaxial thin films of YBCO

      S K Arora Ravi Kumar D Kanjilal G K Mehta S Khatua R Pinto Vijay Kumar A K Gupta

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      Effect of 250 MeV107Ag ion irradiation induced columnar defects on the noise properties of the YBCO superconductor in the normal and superconducting state have been investigated. Magnitude of the spectral density of the noise is found to scale inversely with the frequency and exhibit a quadratic dependence on the bias current confirming that the noise arises due to the resistance fluctuations. The magnitude ofSv has been found to decrease with decrease in temperature and shows a noise peak in the transition region. The noise performance of these materials in the vicinity of the superconducting transition as well as in the normal state is found to improve by an order of magnitude after irradiation with 250 MeV107Ag ions. The decrease in the magnitude of 1/f noise peak is due the irradiation induced enhanced flux pinning of the material which suppresses the flux motion induced noise in the vicinity ofTc.

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