Evidence of high-Tc superconductivity in optimally doped tetragonal YBCO micro-crystals

In the YBa2Cu3O7-x (Y-123) system, the orthorhombic phase is superconducting due to the formation of ordered chains that act as dopants for the two-dimensional CuO2 planes. In the tetragonal ordering, these chains are absent thus inhibiting the charge transfer from the CuO2 planes and making this phase insulating. Within this context, we have investigated the effect of chemical cationic substitutions as Al, Sb, Te and Ca, of anionic doping (O2) and of X-ray nanobeam irradiation on both the electrical and the structural properties of HTSC microcrystals. These microcrystals, characterized by a high structural ordering, low defects concentration and excellent superconducting features, represent ideal candidates for the study and the design of THz devices based on intrinsic Josephson Junctions. In this work, we present some structural and superconducting modifications induced on Y-123 by chemical doping with both Ca, Al and Te. In particular, we propose a fully ordered tetragonal crystal structure, with space group P4/mmm, which is obtained from the orthorhombic ordering of YBa2Cu3O6.5 but with the chains running along the a axis as well as along the b axis. In this way, the Cu(1) atoms in the plane of the chains have twofold, square-planar fourfold, and sixfold coordinations, in contrast to the purely square-planar coordination in orthorhombic Y-123. This structure, previously invoked by Gupta et al., results from single crystal X-ray diffraction and turns out to be superconducting from R vs T characterization.