SUPERCONDUCTIVITY OF OVERDOPED YBA2CU3OX SINGLE-CRYSTALS NEAR X=7

Magnetization measurements on high-quality YBa2Cu3Ox single crystals a re analyzed according to the model of Hao and Clem. The resulting ther modynamic critical field and the jump in the specific heat at T-c, cal culated from it, increase monotonically as the oxygen content is incre ased, even up to x = 7.0. In contrast to this behavior, the supercondu cting transition temperature reaches a maximum of about 91.5 K at x = 6.94 and decreases again to 87.6 K for x = 7.0. The different trends o f these quantities are attributed to the appearance of superconductivi ty in the Cu-O chains near x = 7.0.