High-Tc Superconductivity of La-Ba(Sr)-Cu Oxides. IV –Critical Magnetic Fields

Magnetic field dependence of the resistivity was measured up to 33T for high T c superconductors (La 1- x A x ) 2 CuO 4 with A=Ba and Sr. Although the transition from the superconducting to normal state was broad, the upper critical field defined as the field at 50% of the normal resistance was estimated to be larger than 50 T at liquid helium temperature. The critical field probably exceeds 10 T at liquid hydrogen temperature and also at liquid neon temperature for (La 1- x Sr x ) 2 CuO 4 .