Cation exchange: synthesis and epitaxy of Hg-based superconducting thin films via atomic perturbations

A novel cation-exchange process has been developed for synthesis of bulks and epitaxy of thin films of highly volatile Hg-based high-temperature superconductors (Hg-HTS's). Unlike in a conventional thermal reaction process, precursor matrices of Tl-based high-temperature superconductors were employed in the cation exchange process and annealed in Hg vapors, in which Tl cations were perturbed thermally and replaced subsequently by the Hg cations. A large processing window has been observed suitable for such a atomic perturbation process. This allows Hg-HTS's to inherit high-quality crystalline structures, surface and film/substrate interface morphologies from their TI-HTS's precursor matrices, eliminating difficulties in synthesis and epitaxy of Hg-HTS's associated with the high volatility of Hg-related compounds. Nearly pure phase HgBa2CaCu2O6+delta and HgBa2Ca2Cu3O8+delta bulks and epitaxial thin films were obtained reproducibly with exciting physical properties including high T-c's, high J(c)'s, and low microwave surface resistance. This cation-exchange technique may also provide a general scheme for synthesis of other volatile compounds with pre-designed structure and composition.