High- T c Superconductor Ba 0.5 Y 0.5 CuO x —Upper Critical Field and Phases

The critical field (Hc2) has been studied for the compound Ba0.5Y0.5CuOx. The behaviour of the lower temperature side Tcl of the resistive superconducting transition region is found to depend on the heat treatment given to the sample. The dependence is understood in terms of the coexisting phases i.e. tetragonal and orthorhombic, formed in the sample. The Hc2(0) for the orthorhombic phase is estimated to be as high as 197 T for the Ba0.5Y0.5CuOx compound.