The record superconducting transition temperature (T(c)) for the iron-based high-temperature superconductors (Fe-HTS) has long been 56 K. Recently, in single-layer FeSe films grown on SrTiO3 substrates, indications of a new record of 65 K have been reported. Using in situ photoemission measurements, we substantiate the presence of spin density waves (SDWs) in FeSe films--a key ingredient of Fe-HTS that was missed in FeSe before--and we find that this weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise pronounced SDWs in single-layer FeSe. Beyond providing a comprehensive understanding of FeSe films and directions to further enhance its T(c), we map out the phase diagram of FeSe as a function of lattice constant, which contains all the essential physics of Fe-HTS. With the simplest structure, cleanest composition and single tuning parameter, monolayer FeSe is an ideal system for testing theories of Fe-HTS.

Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. Here from the ARPES perspective, we briefly review the main results from our group in the recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.

The similarities between the Fe-based superconductors and cuprate superconductors imply a possible unified picture of high temperature superconductivity. However, various chemical doping effects in Fe-based superconductors can lead to qualitatively similar phase diagrams that show diverse and complicated details, which pose great challenges of establishing a unified picture. Studying how chemical doping affects the electronic structure and superconductivity, and finding the real universal control parameter for superconductivity, are very important for establishing a unified picture and revealing the mechanism of high temperature superconductivity. In this article, we review a series of angle resolved photoemission studies on the chemical doping effect in Fe-based superconductors, involving both type I Fe-based superconductors with both electron and hole Fermi pockets, and type Ⅱ Fe-based superconductors with only electron Fermi pockets, and involving chemical doping of hetero-valent doping, isovalent doping, and chemical doping at different sites in unit cell. Comprehensive studies and analysis are conducted from various aspects of doping effects, including Fermi surfaces, impurity scattering, and electron correlation, and their roles in evolving the superconductivity. Electron correlation is found to be a universal electronic parameter behind the diverse phase diagrams of Fe-based superconductors, which naturally explains the qualitatively similar phase diagrams of various Fe-base superconductors despite of doping them in different ways. The electron correlation in Fe-based superconductors is closely related to both the carrier type of dopant and the lattice structure parameters, such as bond length. The different impurity scattering effects and different structures may affect the optimal Tc and thus leading to the diversity and complexity in the phase diagram. Fermi surface topology and its evolution with doping may play a secondary role in determining Tc. In order to enhance the Tc, one needs to optimize a moderate electronic correlation while minimizing the impurity scattering in the Fe-anion layer. Our results explain many puzzles and controversies and provide a new view for understanding the phase diagrams, resistivity behaviors, superconducting properties, etc. Our findings also strongly challenge the weak coupling theories based on the Fermi surface nesting, but favors the strong-coupling pairing scenario, where the competition between the electron kinetic energy and the local correlation interactions is a driving parameter of superconducting phase diagram. Like the t-J model of cuprates, in the picture of local antiferromagnetic exchange pairing, superconductivity appears in Fe-based superconductor when the electron correlation strength is at a moderate level. If the correlation is too weak, the system cannot exhibit superconductivity and remains metallic at low temperature. If the correlation is too strong, magnetic order appears in type I Fe-based superconductor, while type Ⅱ Fe-based superconductor shows a bandwidth-control correlated insulating state. The control parameter of the phase diagram is carrier doping for cuprates, but electron correlation strength for Fe-based superconductors. Our experimental results give a unified understanding of iron-based superconductors as a bandwidth-controlled system.