How to optimize high-Tc superconductive cuprates

Different ways to optimizeT_c's and the magnetic properties of high-T_c superconductive cuprates are described and discussed. Oxygen intercalation-desintercalation phenomena and cationic substitutions (M³⁺ for M²⁺ or vice versa) lead to a variation of the hole carrier density and can have a drastic influence onT_c's, as shown for bismuth, thallium, and lead-based cuprates, as well as for La₂CuO₄ and 123-type superconductors. In the former, the role of hole reservoirs for the rock salt type layers is outlined. The modification of the hybridization of orbitals can also explain the variations ofT_c's observed in some systems like Y_1–xCa_xBa₂Cu_3–x:Fe_xO₇ in which the oxidation state of copper remains constant. A critical current density enhancement can be realized by irradiating high-T_c materials by high-energy heavy ions because of the peculiar columnar structure of the heavy ion-induced tracks. The effects of such columnar defects, 70 Å in diameter, on flux pinning, magnetic relaxation, and location of the irreversibility line of bismuth-based 2212 crystals irradiated by 6-GeV Pb ions are reported. We observe a strong shift of the irreversibility line toward high fields and temperatures, indicating that pinning effects must be taken into account in the vortex lattice motion. Such induced changes are accompanied by a strong enhancement of the critical current density and a significant enlargement of the irreversibility region in theH,T plane.