Superconducting Hg-Based Single Crystals

Research project P 14222 Superconducting Hg-Based Single Crystals Harald W. WEBER 06.03.2000 The last substantial increase of the "highest known" transition temperature into the superconducting state occurred in 1993, when the homologous series of Hg-based cuprates was discoverd. An enormous amount of scientific papers on the subject has become available in the mean time, but many fundamental issues remain ambiguous because of the unexpected difficulties with the fabrication of these substances in the form of single crystals. The present proposal addresses fundamental aspects of the mixed state properties in this class of materials and is aimed at a clarification of many of the open issues through a concerted effort of four research laboratories over the next three years. The results will not only contribute towards a better understanding of the physics in Hg-based superconductors and in layered high temperature superconducting compounds in general, but also lead to a better assessment of the technical potential of this class of materials. The approach is based on the successful synthesis of Hg-based single crystals with well defined properties and property variations, such as the doping level, certain substitutions and the oxygen content, and the subsequent assessment of the resulting effects on the anistropy and on other basic mixed state properties. Various series of these single crystals will be subjected to well defined radiation and annealing treatments, in order to probe (mostly by angular dependent experimental techniques) the influence of the mean free path and / or of various deliberately introduced structural defects on the "isotropy of the critical fields, the characteristic lengths, the mass anisotropy and the critical current densities. The inclusion of such hysteretic (non-equilibrium) effects into this kind of fundamental chafacterisation programme is based on the feasibility of deriving information on the inter-layer coupling properties from the flux pinning behaviour of the superconductor. In summary, we believe that a systematic` study of Hg-based single crystals (with a broad range of initial physical properties and with well defined subsequent structural modifications) by angular dependent methods will considerably advance our current knowledge of the mixed state in high temperature superconductors. re superconductors.

Superconductivity research has always strived to discover new materials with higher "transition temperatures" into the superconducting state ever since the first discovery of loss-free current transport in a certain metal below 4.2 K in 1911. Progress was slow, but steady, and came to a temporary halt in 1972, when the metallic compound Nb 3 Ge was found to superconduct below 23 K. After the "gold rush" period of the late 80`s, when ceramic superconductors were discovered and the highest known transition temperatures rocketed up to over 130 K within a very short period of time, the emphasis of research moved more towards the issue of making them suitable for practical applications, to form these brittle materials into wires and tapes, and to find methods for engineering their crystalline structure in such a way that loss-free current transport would be feasible at "high" operating temperatures (77 K) and in the presence of strong magnetic fields, all extremely difficult tasks that are still being pursued at present. In view of these developments, the discovery in 2001 of a most simple metallic compound, MgB2 , that becomes superconducting at the relatively high temperature of 40 K, was not only completely unexpected, but also spurred worldwide activities to develop this material into a candidate for practical applications within a very short period of time. We immediately decided to participate in this exciting development in the framework of the current project. First experiments on bulk polycrystalline materials and wires demonstrated a fundamental path to applications by showing that the transition fields into the normal conducting state could be substantially moved up by reducing the mean free path of the charge carriers, much in the same way as in classical metallic superconductors, and that the current was merely "percolating" between neighbouring grains and limited by the intrinsic anisotropy of the superconductive properties with respect to the magnetic field direction. Only shortly later, excellent single crystals became available, which were used to fundamentally characterize the material in terms of its inherent solid state properties and to confirm the basic ideas put forward initially to explain the results on the polycrystals. Detailed experiments made to modify their crystalline microstructure, led to spectacular enhancements of the transition fields into the normal conducting state and to fundamental insights into the nature of the loss-free transport capability of this material. Similar experiments on single crystals of the Hg-based high temperature superconductors (the initial research plan of this project) revealed a number of stunning similarities to the results obtained on MgB2 , particularly with respect to anisotropy and current transport.