A unifying phase diagram with correlation-driven superconductor-to-insulator transition for the 122∗ series of iron-chalcogenides

X. H. Niu,1, 2 S. D. Chen,1 J. Jiang,1, 2 Z. R. Ye,1, 2 T. L. Yu,1, 2 D. F. Xu,1, 2 M. Xu,1, 2 Y. Feng,1, 2 Y. J. Yan,1, 2 B. P. Xie,1, 2 J. Zhao,1, 2 D. C. Gu,3 L. L. Sun,3, 4 Qianhui Mao,5 Hangdong Wang,5 Minghu Fang,5, 2 C. J. Zhang,6, 2 J. P. Hu,3, 4 Z. Sun,7, 2, ∗ and D. L. Feng1, 2, † 1State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China 2Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, People’s Republic of China 3Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 4Collaborative Innovation Center of Quantum Matter, Beijing 100190, People’s Republic of China 5Department of Physics, Zhejiang University, Hangzhou, 310027, People’s Republic of China 6High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei 230026, People’s Republic of China 7National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China The 122∗ series of iron-chalcogenide superconductors, for example KxFe2−ySe2, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the 122∗ series of iron-chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of 122∗ iron-chalcogenides, including the isovalently ‘doped’ KxFe2−ySe2−zSz, RbxFe2−ySe2−zTez and (Tl,K)xFe2−ySe2−zSz. We found that the bandwidths of the low energy Fe 3d bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known 122∗-series iron chalcogenides can be understood via one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.