Search

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.

Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(±)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A=K,Cs) with a superconducting transition temperature (Tc) of about 30 K. We found A(x)Fe(2)Se(2) (A=K,Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of ~10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(±) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

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 electronic structure of Na2Ti2Sb2O, a parent compound of the newly discovered titanium-based oxypnictide superconductors, is studied by photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES). The obtained band structure and Fermi surface agree well with the band structure calculation of Na2Ti2Sb2O in the non-magnetic state, which indicating that there is no magnetic order in Na2Ti2Sb2O and the electronic correlation is weak. Polarization dependent ARPES results suggest the multi-band and multi-orbital nature of Na2Ti2Sb2O. Photon energy dependent ARPES results suggest that the electronic structure of Na2Ti2Sb2O is rather two-dimensional. Moreover, we find a density wave energy gap forms below the transition temperature and reaches 65 meV at 7 K, indicating that Na2Ti2Sb2O is likely a weakly correlated CDW material in the strong electron-phonon interaction regime., Comment: 10 pages, 4 figures

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique that can directly probe the electronic structure of materials in the momentum space. With ultra-high energy and momentum resolutions, ARPES can help to understand how materials behave. Especially, for high-T C superconductors, the experimental results from ARPES, including the Fermi surface, band dispersion, energy kink, superconducting gap distribution, etc., serve as solid foundations for our understanding on the superconductivity.

${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_{2}$ 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.

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.

Eu(Fe(0.79)Ru(0.21))2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. We studied the low-lying electronic structure of Eu(Fe(0.79)Ru(0.21))2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the dz2 -originated ζ band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. Since BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2 are nodal superconductors and their hPn's are less than 1.33 Å, while the hPn of Eu(Fe(0.79)Ru(0.21))2As2 is larger than 1.33 Å, our results provide more evidence for a direct relationship between nodes, dz2 orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.