Browsing by Author "Kishan, H."

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    Phase formation and superconductivity of fe-tube encapsulated and vacuum-annealed MgB2
    (World Scienti fic Publishing Company, 2006) Singh, K. P.; Awana, V. P. S.; Shahabuddin, M. D.; Husain, M.; Saxena, R. B.; Nigam, Rashmi; Ansari, M. A.; Gupta, Anurag; Narayan, Himanshu; Halder, S. K.; Kishan, H.
    We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum-annealed (10􀀀5 Torr) and LN2 (liquid nitrogen) quenched MgB2 compound. These are single-phase compounds crystallizing in the hexagonal struc- ture (space group P6=mmm) at room temperature. Our XRD results indicated that for phase-pure MgB2, the TH for 10􀀀5 Torr annealed and LN2-quenched samples is 750 C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10􀀀5 Torr and TH of 750 C is found for the hold time (thold) of 2.30 hours. With varying thold from 1{4 hours at xed TH (750 C) and vacuum (10􀀀5 Torr), the c-lattice parameter decreases rst and later increases with thold (hours) before a near saturation, while the a-lattice parameter rst increases and later decreases beyond a thold of 2.30 hours. The c=a ratio versus thold plot showed an inverted bell-shaped curve, touching the lowest value of 1.141, which is the reported value for perfect stoichiometry of MgB2. The optimized stoichi- metric MgB2 compound exhibited superconductivity at 39.2 K with a transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum- annealed MgB2 compound are optimized and are compared with widely-reported Ta tube encapsulated samples.
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    Swift heavy ion irradiation of MgB2 thin films: a comparison between gold and silver ion irradiations
    (2007) Narayan, Himanshu; Agrawal, H. M.; Kushwaha, R. P. S.; Kishan, H.; Bhatt, Ravindra K.
    The effect of 200 MeV Au ion irradiation on the temperature and field dependence of the critical current density, Jc, of MgB2 thin films on sapphire substrates is reported. The results have been presented in comparison with those obtained after 200 MeV Ag ion irradiation of a similar film. After irradiation, the critical temperature Tc decreased for all samples except for the one with a higher dose of gold ions, where it increased marginally. This observation was also confirmed from measurements of magnetization as a function of temperature with a constant applied field of 10−2 T. The critical current density, Jc, was estimated from the widths of magnetization loops using Bean’s critical state model. It has been found that Jc increases after irradiation, the enhancement being more pronounced for the Au ion irradiation. The higher enhancement of Jc by Au ion irradiation may be attributed to higher flux-pinning efficiency of the irradiated samples. It has been concluded that, although the flux-line shear (FLS) model satisfactorily explains the silver ion irradiation induced enhancement of Jc, some other mechanisms also seem to play a role in the Jc enhancement by gold ion irradiation.