About 100 years ago, a Dutch physicist cooled mercury close to absolute zero, the lowest temperature theoretically achievable. Serendipitously, he discovered that its resistance had disappeared, demonstrating superconductivity for the first time. Since then, numerous superconducting materials, both metallic and oxide, have been discovered and studied, yet the mysteries of superconductivity abound. The newest kids on the block are the iron-based superconductors that open up exciting physics and other possible applications without supercooling. To date, such materials have been studied in bulk and polycrystalline form because of difficulties in synthesising epitaxial thin films. Partners with recent success in epitaxial growth of iron-based superconducting films joined forces on the EU-funded project IRON-SEA (Establishing the basic science and technology for iron-based superconducting electronics applications). Efforts were made to establish the scientific foundations for future exploitation in novel devices and applications. For sophisticated devices, high-quality epitaxial thin films are necessary. Carefully layered superconducting materials are increasingly important in superconducting quantum interference devices (SQUIDs) that could be used for quantum computing. Scientists worked on optimising and establishing the growth conditions of all studied iron-based superconducting thin films by using pulsed laser deposition and molecular beam epitaxy. Another important element for developing novel devices is sound technology for high-resolution patterning. Scientists successfully developed a photolithography method for precise patterning of thin films of iron-based superconducting materials. After successfully developing superconducting tunnel junctions (STJs), scientists found that titanium oxides and aluminium oxides were the most suitable materials as insulating barriers to junctions for device fabrication. A great variety of STJs were optimised to enable hybrid phase-sensitive devices in the near future. The superconducting energy gap and its symmetry in iron-based superconductors are different from that found in conventional or cuprate superconductors. Conventional superconductors require a symmetric gap. Through different spectroscopy techniques, the team performed significant research to understand the pairing mechanism between the energy gaps and symmetry to disclose the physics of superconductivity. Except for SQUIDs, project findings open the way for developing nanowire detectors and magnetic STJ memories for rapid single-flux quantum logic circuits based on iron-based superconductors.
Quantum devices, iron-based superconductors, superconductors, superconducting quantum interference devices