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The ${122}^{\ensuremath{\star}}$ series of iron chalcogenide superconductors, for example ${\mathrm{K}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$, 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 ${122}^{\ensuremath{\star}}$ 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 ${122}^{\ensuremath{\star}}$ iron chalcogenides, including the isovalently ``doped'' ${\mathrm{K}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2\ensuremath{-}z}{\mathrm{S}}_{z},{\mathrm{Rb}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2\ensuremath{-}z}{\mathrm{Te}}_{z}$, and ${(\text{Tl},\mathrm{K})}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2\ensuremath{-}z}{\mathrm{S}}_{z}$. 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 ${122}^{\ensuremath{\star}}\text{-series}$ iron chalcogenides can be understood via one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.

${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$ is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM) studies on ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$. Our STM studies find a charge density wave (CDW) order with the wave vector of ($\ensuremath{\pi}, \ensuremath{\pi}$). The electronic structure shows both multiorbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the ($\ensuremath{\pi}, \ensuremath{\pi}$) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_{2}{\mathrm{Sb}}_{2}\mathrm{O}$.

Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in ${\mathrm{BaFe}}_{2}({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}{)}_{2}$ was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap of $\ensuremath{\sim}5$ meV obtained from the low-fluence-data bottleneck model fit is consistent with the angle-resolved photoemission spectroscopy results for the $\ensuremath{\gamma}$- and $\ensuremath{\beta}\ensuremath{-}\mathrm{hole}$ Fermi surfaces. The superconducting state nonthermal optical destruction energy was determined from the fluence dependent data. The planar optical destruction energy density scales well with ${T}_{\mathrm{c}}^{2}$ and is found to be similar in a number of different layered superconductors.

The effect of K, Co, and P dopings on the lattice dynamics in the ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ system is studied by infrared spectroscopy. We focus on the phonon at $\ensuremath{\sim}253\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, the highest energy in-plane infrared-active Fe-As mode in ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$. Our studies show that Co and P dopings lead to a blueshift of this phonon in frequency, which can be simply interpreted by the change in lattice parameters induced by doping. In sharp contrast, an unusual redshift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.

We have studied the low-lying electronic structure of a new ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type superconductor ${\mathrm{KNi}}_{2}{\mathrm{Se}}_{2}$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor ${\mathrm{K}}_{x}\mathrm{Fe}{\phantom{\rule{0.16em}{0ex}}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$. The Fermi surface shows weak variation along the ${k}_{z}$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.

We report the electronic structure reconstruction of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from $\mathrm{Ca}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{)}_{2}{\mathrm{As}}_{2}$ and the annealed ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface still persists in the CT phase of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Furthermore, we found at least three well separated electronlike bands around the zone corner in the CT phase of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, which are more dispersive than the electronlike bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.

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.

${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ is a new layered superconductor. We have studied the low-lying electronic structure of the single-crystalline ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ superconductor, whose superconducting transition temperature is 4.83 K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the $X$ point and shows little warping along the ${k}_{z}$ direction. Our results demonstrate the multiband and two-dimensional nature of the electronic structure. The comparison between the photoemission data and the band calculations gives the renormalization factor of $\ensuremath{\sim}$1.14, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth deficiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the ${\mathrm{BiS}}_{2}$-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling.

The in-plane resistivity anisotropy has been studied with the Montgomery method on the detwinned parent compound of the iron-based superconductor FeTe. The observed resistivity in the antiferromagnetic (AFM) direction is larger than that in the ferromagnetic (FM) direction, which is different from that observed in BaFe${}_{2}$As${}_{2}$ before. We show that the opposite resistivity anisotropy behavior in FeTe could be attributed to the strong Hund's rule coupling effects, which should be understood in a localized picture: Hund's rule coupling makes hopping along the FM direction easier than along the AFM direction in FeTe, similar to the colossal magnetoresistance observed in some manganites.

BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ is a unique iron-based superconductor, where the superconductivity is induced by the isovalent substitution of phosphorus (P) for arsenic (As). Unlike other iron pnictides, the superconducting gap in BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ has been suggested to contain nodal lines by various experiments. The exact nature of the isovalent doping and nodal gap are key open issues in building a comprehensive picture of the iron-based superconductors. With angle-resolved photoemission spectroscopy, we found that the P substitution in BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ alters the electronic structure significantly. With P doping, the hole and electron Fermi surface sheets expand simultaneously and the band velocities are enhanced indicating a suppression of electron correlations. Moreover, the P doping induces strong ${k}_{z}$ dispersion on the ${d}_{xz}$-originated band with significant mixing of the ${d}_{{z}^{2}}$ orbital around Z, while the ${d}_{xy}$-originated band and the electron pockets are relatively intact. These rule out theories suggesting that the nodal gap is due to the vanishing ${d}_{xy}$ hole pocket, while support those considering a ${d}_{{z}^{2}}$-dominated hole Fermi surface around Z being responsible. Our results are thus helpful to explain the nodal superconductivity in BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ and understand the role of lattice parameter or pressure effect in iron-based superconductors.

Although nodeless superconducting gap has been observed on the large Fermi pockets around the zone corner in KxFe2-ySe2, whether its pairing symmetry is s-wave or nodeless d-wave is still under intense debate. Here we report an isotropic superconducting gap distribution on the small electron Fermi pocket around the Z point in KxFe2-ySe2, which favors the s-wave pairing symmetry.

Superconductivity arises from pairing of mobile electrons. The symmetry of the electron-pair wave function contains important clues about the mechanism underlying the pairing. A Chinese-Korean experimental collaboration carries out element-selective substitutional doping of BaFe${}_{2}$As${}_{2}$ and searches for the pairing symmetry of the resulting iron-based superconductors.

The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the C-4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent

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 ir