Magnetic Field Effects in the High-Tc Superconductor Y-Ba-Cu-O

Abstract
The single-phase compound YBa2Cu3O7 prepared in pure oxygen atmosphere exhibited an extremely high upper critical field H c2 (0 K) above 110 T (midpoint) as well as high T c above 92 K (end point). Magnetic field dependence of resistance R has been measured as a function of temperature T. Shoulders appeared in the R-T transition curves of (Y1-x Ba x )CuO3-y (x=0.6) prepared in air, suggesting the coexistence of two kind of phases, i.e., a high-H c2 phase and a low-H c2 phase. H c2(77 K) defined by a zero-resistance state was at most ∼0.05 T in the low-H c2 phase.