Abstract

Fe-chalcogenide superconductors are the family of materials that contain iron and chalcogens. Since 2008, interest in this family of materials has grown dramatically due to the high transition temperature T _c, which rivals those of some cuprate superconductors. The first Fe chalcogenide discovered, FeSe, has tetragonal P4/nmm symmetry at room temperature and is composed of a stack of edge-sharing FeSe4-tetrahedra layer by layer, which makes it the simplest crystal structure of all the Fe-based compounds (Hsu et al., 2008). Despite this simplicity, T _c of FeSe increases dramatically from 8.5 K to 30–40 K when the system undergoes either external pressure (Mizuguchi et al., 2008) or internal pressure via chemical substitutions (Yeh et al., 2008) and intercalations (Guo et al., 2010). The materials discovered could be categorized into four major groups as tabulated in Table D5.1, and their crystal structures are shown in Figure D5.1.