Excess conductivity,magneto-conductivy and AC susceptibility studies in La-Ba-Ca-Cu-O superconducturs

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The discovery of high transition temperature (Tc) superconductivity in (La-Ba)-Cu-O system with Tc = 30 K in1986 by Bednorz and Muller triggered the development of several high Tc Cu- O based superconductors. Among them, Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems are widely studied in literature. Tl-Ba-Ca-Cu-O and Hg-Ba-Ca-Cu-O systems exhibit superconductivity with large transition temperatures such as 120 and 135 K respectively. However, these two series are toxic in nature. There are several reports on the enhancement of critical current density, Jc by novel preparation techniques including melt processing technique. The temperature variations of precise electrical resistivity and magneto-resistivity measured on most of the above high Tc superconductors have been studied in terms of thermodynamic fluctuations induced excess conductivity above Tc. As a result of thermodynamic fluctuations in the amplitude of order parameter, there is a finite probability of Cooper pair formation above Tc and this leads to excess conductivity. The temperature and magnetic field variations of ac susceptibility and magnetization of high Tc superconductors are widely analyzed in terms of several critical state models. According to critical state models, the penetrated super current flows with a density equal to critical current density. ...
Supervisor: S. Ravi