Upper critical fields of the 40 K- and 90 K- superconductors, Sr-La-Cu-O and Ba-Y-Cu-O

Superconducting transitions and the upper critical fields, H c2 of 40 K- and 90 K-superconductors, Sr x La 2- x CuO 4- y ( x =0.6, y ∼1) and Ba x Y 1- x CuO 3- y ( x =0.6) in ceramic forms, were studied by resistivity measurement. The onset of superconductivity of the former (latter) was 43 K(93 K), and the midpoint was 35 K(89 K). The slopes, -d H c2 /d T were 1.48 T/K at the onst, and 0.92 T/K(1.79 T/K) at the midpoints. The Ginzburg-Landau coherence length, ζ GL , at 0 K is estimted to be 23 A (onset points) and 32 A (midpoints) for the former, and 14 A (midpoints) for the latter. The value of 14 A is close to lattice constant, 12 A. H c2 ( T =0 K) is discussed in relation to the small value of ζ GL . Two-step transition was observed.