This special issue reports 18 papers on iron-based superconductors. Each of them is an invited paper presented at the International Conference on Novel Superconductors and Supermaterials (N2S) at Tokyo, Japan, for March 6–8, 2011, just a week before the giant earthquake and tsunami of March 11. This conference was very exciting and fruitful owing to excellent talks on many new materials and new insights into the pairing mechanism. We asked each invited speaker to contribute an article to this issue. Each paper has been reviewed by referees and a Guest Editor. It was our editorial policy to respect the authors’ view unless the description was obviously incorrect, misleading or unclear. We hope this issue would be helpful for readers to get acquainted with the current status of this rapidly advancing subject.
Iron had been believed to be the most harmful element for the emergence of superconductivity. However, the situation was totally changed since the discovery of an iron oxy-pnictide superconductor LaFeAsO1−xFx with Tc=26 K in early 2008. This discovery sparked intense research activity on superconductivity in this system worldwide. As a consequence, more than 4000 printed papers have been published to date along with comprehensive review articles listed at the end of this preface.
What is the impact of iron-based superconductors? There will be two answers, i.e.
The first is the breaking of a widely accepted belief of “iron is antagonistic against superconductivity”, which leads to the opening of a versatile material frontier for new superconductors. We have learned a fact through both extensive and intensive studies in the past 4 years that iron can be a good friend for high Tc superconductors under certain conditions. The second is finding a rich variety in material candidate and in pairing interaction. It has turned out that there are many material varieties in iron-based superconductors such as 5-types of parent materials, 1111, 122, 111, 11, and thick-blocking layer materials, and each type has rather different superconducting properties. Recently discovered alkali ion-intercalated FeSe compounds appear to contain a Mott-insulating compound with a high Neel temperature (∼500 °C). This finding is totally surprising, demonstrating that iron-based superconductors have a much larger spread than people felt at the initial stage. All the major actors are ready in this system associated with charge, spin, and orbitals. The research is now entering into a new phase in which materials are required to be tuned/designed so as to enable these actors to play their characteristic roles toward higher Tc, eventually, room-temperature superconductors. The next century of superconductors starts from 2012!
Editing this special issue started upon a keen request by Prof. A. Pinczuk, the Editor-in-Chief, but we learnt a lot though this work. We would like to express our sincere thanks to all the contributors of excellent papers for this special issue. Special thanks are due to Dr. Satoru Fujitsu of the Tokyo Institute of Technology for his effort to manage the manuscripts. The N2S conference was supported by the JSPS FIRST program and JST TRIP project.