Superconducting Upper Critical Field Hc2 of High-Tc Y-Ba-Cu-O Compound System

Superconducting transition in the magnetic field up to 9 T and upper critical field H c2 near T c were investigated for several samples which have the nominal composition Y 1- y Ba y CuO x with (a) y =0.60 or (b) y =0.65 and x undetermined. (a)- and (b)-samples show zero-resistance around 88 K and multi-phase and almost-single-phase behaviors, respectively, in the magnetic field, being consistent with a recently proposed model of YBa 2 Cu 3 O x for a 90 K-range phase. In a (b)-sample, a very large value of (-d H c2 /d T ) of 3.0∼3.5 T/K or 4.0∼4.5 T/K was observed, depending on if H c2 was taken at the 50% of the normal state resistance R n or the 90%. Based on a theory assuming no paramagnetic limiting, this means H c2 (0) of 200 or 250 T, the highest one among the reported values.