Over 100 previously unknown superconducting materials ranging from new iron-based superconductors (IBSCs or pnictide semiconductors) to titanium- and cobalt-based ones are listed in a review article by Hideo Hosono and colleagues, published in the June issue of Science and Technology of Advanced Materials (DOI: 10.1088/1468-6996/16/3/033503). The article summarizes the results of a four-year long research project funded as part of FIRST (Funding Program for World-Leading Innovative R&D on Science and Technology), a Japanese government-funded program. Six Japanese research groups collaborated under the leadership of Hosono from the Tokyo Institute of Technology to pursue multiple goals in the field of superconducting materials, the fabrication of prototype devices based on superconducting wires and tapes, and exploration of new functionality of relevant materials. The article encompasses detailed discussions on crystal structure investigations. The electronic structure and phase diagrams are shown for many of the materials discovered from 2010 to 2014 as part of the FIRST project.
Electronic phase diagram of SmFeAsO1–x H x superimposed over that of SmFeAsO1–x F x . Reprinted with permission from Phys. Rev. B 84, 024521. © 2011 American Physical Society.
Among other goals, the researchers had proposed to discover a new iron-based superconductor with a critical temperature (T c) >77 K. The researchers did not meet this particular self-set goal, but one research highlight of the study is the observation that hydride ions (H–) can be used as an electron dopant in place of fluoride (F–) to induce superconductivity in IBSC materials. This approach yields, for example, SmFeAsO1–x H x with a critical temperature T c of 56 K, the superconducting material with the highest T c reported in the review article (see Figure).
IBSCs exhibit a layered crystal structure. Taking this into consideration, a significant portion of the review article describes the syntheses and properties of various superconducting intercalation compounds, not limited to IBSCs. The section is followed by studies on changes in critical current density based on microstructural grain variations in tape connectors, with the article culminating in describing devices such as the first bipolar oxide thin-film transistors that can be applied to complementary metal–oxide–semiconductor circuits and a new catalyst based on an electride superconductor (heavily electron-doped 12CaO·7Al2O3) for ammonia synthesis at ambient pressure.
It is noteworthy that the researchers not only report the new superconductor materials they found, but they also list ∼700 materials they either unsuccessfully attempted to synthesize or that did not exhibit the desired superconductivity. Presentation of tables of null results like these are a return to the early days of intermetallic superconductivity, when it was common practice to present summaries of measurements on failed superconductors as well as successful ones. “These tables are very useful,” says Paul C. Canfield, Distinguished Professor of Physics at Iowa State University, a Senior Physicist at Ames Laboratory, and a long-time researcher in the field of superconductivity, who was not involved in the study. “They help researchers focus their searches for new materials. Across the globe there are efforts to find new superconducting compounds, but the scope of these efforts is miniscule compared to the number of known, as well as unknown, compounds. We are all exploring a wilderness of phase space, and any signs left on the trail by earlier researchers are valuable.”
