Structures of high-temperature cuprate superconductors

Most of the known cuprate superconductors are shown to belong structurally to a single family and are closely related to each other, The relationships between the structures of cuprate superconductors are reviewed with emphasis on their similarities, Individual details of the structures are ignored and only idealized structures are discussed. A formula is used to describe the basic structures of the cuprate superconductors. Crystal structure analysis leads to three significant insights into the nature of, and requirements for, superconductivity in these phases: (1) A nearly planar sheet of CuO2 supports the supercurrent, (2) As the distance between the n ACuO(2) blocks in a crystal structure increases, the critical current density decreases at high reduced temperatures due to the magnetic flux lines becoming unpinned, (3) The strain energy generated by the lattice mismatch between the CuO2 layers and the nonsuperconducting layers controls the oxidation state of the Cu ions, which needs to be about +2.2 (in p-type superconductors) to get Cooper pair condensation.