Effects of silver doping in the high-T c superconductor system Y-Ba-Cu-O

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Abstract
Changes in various superconducting properties as a function of silver substitution in the bulk granular high‐Tc system Y‐Ba‐Cu‐O have been investigated. In the low doping limit, Ag impurities behave as an excellent oxygen stabilizer and the critical current density is enhanced by an order of magnitude while the Tc remains practically unchanged. When the Ag content is increased to above 20%, the Tc starts to drop and the effects of Ag clusters as well as impurity phases become more important. However, the compound is still a superconductor with a Tc around 20 K by full substitution of Ag for Cu. The problems concerning the location of Ag atoms and formation of Ag clusters are examined with measurements of x‐ray absorption fine structure using synchrotron radiation. Our results indicate that at least some Ag atoms occupy the Cu(1) and Cu(2) sites in the material. Magnetic field dependence shows that the critical current density of a bulk sintered high‐Tc superconductor below a characteristic value H’c is mainly controlled by intergranular weak‐link coupling. A shift of Hc with the addition of Ag helps explain the variation of critical current density in this compound system.