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Heavily Hydride-ion-doped 1111-type Iron-based Superconductors: Synthesis, Physical Properties and Electronic Structure

Iron-based superconductors have grown to be a new continent of high Tc superconductors comparable to cuprates. The optimal critical temperature (Tc) of 56 K in electron-doped 1111-type iron oxypnictides attracts considerable attention of physicists and chemists. Carrier doping is not only essential to induce superconductivity but also is a critical parameter that governs the electronic, magnetic, and crystallographic properties of ground states in high-Tc superconductors. Hydride ion (H-) which is an anionic state of hydrogen acts as an efficient electron donor in the 1111-type, leading to several important discoveries such as two-dome structure of superconducting phase and bipartite parent phase. This article summarizes the synthesis, physical properties, and electronic structure of the H- bearing 1111-type iron-based superconductors along with the relevant phenomena of other superconductors. We show several common characteristics of iron-based superconductors with high-Tc over 50 K and suggest a way to achieve higher Tc.