The precursor of the copper-oxygen compound high-temperature superconductor is generally considered to be an anti-ferromagnetic Mott insulator. Superconductivity is generated by doping the carrier and suppressing the transition of the insulator-superconductor caused by the antiferromagnetic state. Unlike copper-oxygen compound high-temperature superconductors, although the matrix of iron-based superconductors mostly has an anti-ferromagnetic structure, it exhibits a poorly conductive metal property. Therefore, there has been controversy as to whether the electronic correlation in iron-based superconductors and whether Mott insulator images are still applicable to iron-based superconductors. Institute of Physics, Chinese Academy of Sciences / National Laboratory of Condensed Matter Physics, Beijing National Laboratory for Superconductivity He Junfeng, Liu Xu, Zhao Lin, Liu Defa, He Shaolong, etc. of the Zhou Xingjiang Research Group, and Xue Qikun/Tsinghua University Institute of Physics Xu Wencun of the Xucun Research Group cooperated with Li Fangsen and others to find out the change of the carrier-concentration induced insulator-superconductor transition in a single-layer FeSe/SrTiO3 thin film for the first time to understand iron-based systems using angular-resolved photoelectron spectroscopy. The electronic correlation effect in superconductors and its role provide important information. It was found that depending on the carrier concentration, there may be two phases with different electronic structures in the single-layer FeSe/SrTiO3 thin film: N-phase and S-phase. Through the in-situ annealing of single-layer FeSe/SrTiO3 thin film under ultra-high vacuum, the S-phase carrier concentration can be controlled, and the electronic structure and superconductivity with the evolution of carrier concentration can be studied. First, when the carrier concentration is low, the S-state electronic state density has almost no spectral weight near the Fermi level. The spectral weight near the Fermi level gradually increases as the carrier concentration increases (Figure 1). Secondly, the measurement of the energy gap found that at the low carrier concentration (less than 0.089 electrons/Fe), there is a band gap opening on the Fermi surface of the S phase. The energy gap decreases as the carrier concentration increases, and decreases to zero when the carrier concentration reaches 0.089 electrons/Fe. As the carrier concentration further increases (greater than 0.089 electrons/Fe), a new energy gap opens up on the S-phase Fermi surface, and its size increases with increasing carrier concentration (Figure 2). Detailed temperature change measurements and other characteristics show that the energy gaps at low carrier concentrations correspond to the insulating energy gaps and the energy gaps at high carrier concentrations correspond to the superconducting energy gaps (Figure 3). Therefore, an electronic phase diagram of the S phase with temperature and carrier concentration changes can be constructed. At a carrier concentration of 0.089 electrons/Fe there is an insulator-superconductor transition (Figure 4). The insulator-superconductor transitions found in the S-phase of a single-layer FeSe/SrTiO3 thin film are related to the energy band structure (Fig. 1), the photoelectron spectrum and the energy gap (Fig. 2), and the electron phase diagram (Fig. 4). Zhou Xingjiang's research group observed the insulator-superconductor transition in the copper-oxygen compound La-Bi 2201 [Nature Communications 4 (2013) 2459], showing many similarities. For the first time in this work, an insulator-superconductor transition with an increase in carrier concentration was found in an iron-based superconductor, and it was found that this evolution was extremely similar to the insulator-superconductor transition in a copper oxide superconductor, and an iron-based superconductor was established. Closely related to copper oxide superconductors. Insulator-superconductor transitions have been observed in monolayer FeSe/SrTiO3 thin films, which are related to their two-dimensional characteristics and the enhancement of substrate-to-electron correlation, indicating the importance of electron correlation in iron-based superconductors. The results of relevant studies were published in the recent Proceedings of the National Academy of the Sciences of the United States of America [PNAS 111, 18501 (2014)]. The research was funded by the National Natural Science Foundation of China, the "973" project of the Ministry of Science and Technology and the Chinese Academy of Sciences. Customized Handle,Minimalist Walnut Door Handle,Aluminum Alloy Door Handle,Stainless Steel Door Handle Guangzhou Junpai Hardware Co., Ltd , https://www.junpaihardware.com