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Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving d_{xz}, d_{yz}, and d_{xy} orbitals. The nematic energy scale between d_{xz} and d_{yz} bands reaches a maximum of 50 meV at the Brillouin zone corner. We are also able to track the d_{xz} electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.

The diversities in crystal structures and ways of doping result in extremely diversified phase diagrams for iron-based superconductors. With angle-resolved photoemission spectroscopy, we have systematically studied the effects of chemical substitution on the electronic structure of various series of iron-based superconductors. Beyond the Fermi-surface alteration that has been reported most often in the past, we found two more extraordinary effects of doping: (1) the site and band dependencies of quasiparticle scattering and, more importantly, (2) the ubiquitous and significant change of electronic correlation by both isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found that the electronic correlation could be suppressed by applying either the chemical pressure or doping electrons but not by doping holes. Together with other findings provided here, these results complete the microscopic picture of the electronic effects of dopants, which facilitates a unified understanding of the diversified phase diagrams and resolutions to many open issues of various iron-based superconductors.

The coexisting regime of spin-density wave (SDW) and superconductivity in iron pnictides represents a novel ground state. We have performed high-resolution angle-resolved photoemission measurements on NaFe_{1-x}Co_{x}As (x=0.0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is nonexclusive. Particularly, we find that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in a SDW-free NaFe_{1-x}Co_{x}As (x=0.045), which puts strong constraints on theory.

We present the ultra-low-temperature heat-transport study of iron-based superconductors Ba(Fe_{1-x}Ru_{x})_{2}As_{2} and BaFe_{2}(As_{1-x}P_{x})_{2}. For optimally doped Ba(Fe_{0.64}Ru_{0.36})_{2}As_{2}, a large residual κ_{0}/T at zero field and a sqrt[H] dependence of κ_{0}(H)/T are observed, which provide strong evidences for nodes in the superconducting gap. This result demonstrates one more nodal superconductor in iron pnictides. The similarities between isovalent Fe and P dopings strongly suggest that the nodal superconductivity in Ba(Fe_{0.64}Ru_{0.36})_{2}As_{2} may have the same origin as in BaFe_{2}(As_{0.67}P_{0.33})_{2}. Furthermore, in underdoped Ba(Fe_{0.77}Ru_{0.23})_{2}As_{2} and strongly underdoped BaFe_{2}(As_{0.82}P_{0.18})_{2}, κ_{0}/T manifests similar nodal behavior, which result shows the robustness of nodal superconductivity in the underdoped regime and puts constraint on theoretical models.

The nature of the parent compound of a high-temperature superconductor (HTS) often plays a pivotal role in determining its superconductivity. The parent compounds of the cuprate HTSs are antiferromagnetically ordered Mott insulators, while those of the iron-pnictide HTSs are metals with spin-density-wave order. Here we report the electronic identification of two insulating parental phases and one semiconducting parental phase of the newly discovered family of K_{x}Fe_{2-y}Se_{2} superconductors. The two insulating phases exhibit Mott-insulator-like signatures, and one of the insulating phases is even present in the superconducting and semiconducting K_{x}Fe_{2-y}Se_{2} compounds. However, it is mesoscopically phase-separated from the superconducting or semiconducting phase. Moreover, we find that both the superconducting and semiconducting phases are free of the magnetic and vacancy orders present in the insulating phases, and that the electronic structure of the superconducting phase could be developed by doping the semiconducting phase with electrons. The rich electronic properties discovered in these parental phases of the K_{x}Fe_{2-y}Se_{2} superconductors provide the foundation for studying the anomalous behavior in this new class of iron-based superconductors.