High field properties of superconducting Y_xBa_1-xCu-Oxides

The transition temperature, T_cthe upper critical field, H_c2(T), and the critical current density, J_c(H), at 4.2 K have been studied in high-T_c Y_xBa_1-xCu-Oxide system. Two maxima in T_c around x=0.33 and 0.4 have been observed. The slope dH_c2^middT near T_c for the sample x=0.4 is -1.5 T/K, resulting in H_c2 at 0 K higher than 85 T by an extrapolation according to the WHH theory. Although the J_c value of ∼3.5 A/cm² at 15 T is very low, it decreases only 10% even at 27 T, showing high potential for practical use at high fields when J_c is raised by the introduction of strong pinning centers in this material.