Superconducting properties of orthorhombic YBa/sub 2/(Cu/sub 1-x/Zn/sub x/)/sub 3/O/sub 7/ and tetragonal YBa/sub 2/(Cu/sub 1-x/Ga/sub x/)/sub 3/O/sub 7/

The authors used Zn and Ga, two unique dopants having definite valence states and filled 3d levels, to probe the YBa/sub 2/Cu/sub 3/O/sub 7/ superconductor. A small doping of Ga induces an orthorhombic to tetragonal structural transition, but the values of T/sub c/ are as high as 81 K in the tetragonal phase. YBa/sub 2/(Cu/sub 1-x/Zn/sub x/)/sub 3/O/sub 7/ retains the same orthorhombic structure as the parent compound, but with highly depressed T/sub c/. The oxygen vacancy order, or the linear chain structure, is found to be not essential for the high T/sub c/. Our data suggest that Ga and Zn preferentially substitute Cu(1) and Cu(2) sites, which leads to the conclusion that the most important feature of the high T/sub c/ oxide superconductors is the Cu-O/sub 2/ planes