• Neeraj Khare

      Articles written in Pramana – Journal of Physics

    • Observation of rf SQUID characteristics in YBCO bulk at 77 K

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

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      RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowIc are found to show this characteristic whereas dense samples with higherIc do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.

    • Observation of rf SQUID characteristics in YBCO bulk at 77 K

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

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    • Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

      S K Arora A K Gupta Neeraj Khare Ramesh Chandra V S Tomar

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      A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10−4 Φ0/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.

    • Harmonic generation studies in laser ablated YBCO thin film grown on 〈100〉 MgO

      Neeraj Khare J R Buckley R M Bowman G B Donaldson C M Pegrum

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      The generation of harmonics in a laser ablated YBCO film deposited on a 〈100〉 MgO substrate is reported. Higher odd harmonics appeared when the film was subjected to an ac field. The presence of a dc field induces only the second harmonic with a small value of slope ofV2Hdc curve (δV2/δHdc) compared to bulk YBCO. The variation of the amplitude of third harmonic (V3) withHac and temperature was studied. These results are explained in terms of a critical state model. The observation of only a small amplitude of second harmonic (V2) with a smallδV2/δHdc is explained in terms of a special kind of clean grain boundary present in YBCO laser ablated films on 〈100〉 MgO.

    • Harmonic generation in superconducting Bi(Pb)SrCaCuO thick films

      S K Shrivastava Neeraj Khare A K Gupta

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      We present a detailed study of harmonic generation in Bi(Pb)SrCaCuO superconducting thick film prepared by screen printing technique. A comparative study of amplitude of harmonics of two films of different Jc have been carried out. The variation of amplitudes of the harmonics are studied as a function of magnitude of ac and dc field. The temperature dependence of amplitude of third harmonic (V3) is studied with increasing amplitude as well as frequency of ac field. These results are analyzed in the frame work of critical state model. V3-T curve also indicates the presence of two phases in Bi(Pb)SrCaCuO film.

    • Study of magnetoresistance and conductance of bicrystal grain boundary in La0.67Ba0.33MnO3 thin film

      Neeraj Khare AK Gupta UP Moharil AK Raychaudhuri SP Pai R Pinto

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      La0.67Ba0.33MnO3 (LBMO) thin film is deposited on a 36.7°C SrTiO3 bicrystal substrate using laser ablation technique. A microbridge is created across bicrystal grain boundary and its characteristics are compared with a microbridge on the LBMO film having no grain boundary. Presence of grain boundary exhibits substantial magnetoresistance ratio (MRR) in the low field and low temperature region. Bicrystal grain boundary contribution in MRR disappears at temperature T>175 K. At low temperature, I-V characteristic of the microbridge across bicrystal grain boundary is nonlinear. Analysis of temperature dependence of dynamic conductance-voltage characteristics of the bicrystal grain boundary indicates that at low temperatures (T<175 K) carrier transport across the grain boundary in LBMO film is dominated by inelastic tunneling via pairs of manganese atoms and tunneling through disordered oxides. At higher temperatures (T>175 K), magnetic scattering process is dominating. Decrease of bicrystal grain boundary contribution in magnetoresistance with the increase in temperature is due to enhanced spin-flip scattering process.

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