We have performed high-resolution angle-resolved photoemission spectroscopy on Fe-based superconductor LiFeAs (T(c)=18 K). We reveal multiple nodeless superconducting (SC) gaps with 2Δ/k(B)T(c) ratios varying from 2.8 to 6.4, depending on the Fermi surface (FS). We also succeeded in directly observing a gap anisotropy along the FS with magnitude up to ~30%. The anisotropy is fourfold symmetric with an antiphase between the hole and electron FSs, suggesting complex anisotropic interactions for the SC pairing. The observed momentum dependence of the SC gap offers an excellent opportunity to investigate the underlying pairing mechanism.

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We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor ðTl; RbÞ y Fe 2Àx Se 2 (T c ¼ 32 K). We determined the orbital characters and the k z dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4p z and Fe 3d xy orbitals to the small electron pocket. We argue that the emergence of Se 4p z states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.

We report the observation by angle-resolved photoemission spectroscopy measurements of a highly anisotropic Dirac-cone structure in high quality SrMnBi 2 crystals. We reveal a well-defined sharp quasi-particle, linearly dispersive withforming a hole-like anisotropic Dirac-cone. The density of states for the cone remains linear up to as high as ~650 meV of binding energy. The scattering rate of the quasi-particle (QP) increases linearly as function of binding energy, indicating a non-Fermi-Liquid behavior.Our results suggest the existence of a dilute two-dimensional electron gas system in this three-dimensional material.

The electronic structure of the intercalated iron-based superconductor Ba 2 Ti 2 Fe 2 As 4 O (T c ∼ 21.5 K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacing layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect, i.e. 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti 2 As 2 O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the Fe 3d shell. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors. PACS numbers: 71.27.+a, 74.70.Xa, In iron-based superconductors (IBSCs), the most common ways to suppress long-range antiferromagnetic order and obtain high-T c superconductivity is to introduce carriers [1][2][3] and/or internal strain [4-6] by element substitution. However, an inevitable problem is that element substitution also introduces disorder, and impurity scattering is believed to be detrimental to superconductivity [7], though not as seriously as in cuprate superconductors. It has been revealed that the impurity scattering effects are site dependent and the scattering strength is gradually reduced when the dopants move away from the Fe plane [8,9]. This may partially explain why the maximum T c is much higher and the superconducting dome is much wider in (Ba,K)Fe 2 As 2 as compared with Ba(Fe,Co) 2 As 2 [8]. Therefore, finding an alternative way to dope carriers but without introducing disorder would be a promising path for reaching higher T c superconductivity.A remarkable feature in the IBSCs is that there is an intimate relationship between the electronic correlations and the d-shell occupancy. For hole doping, extremely low coherence temperatures are expected, while electron doping reinforces Fermi-liquid properties [10,11]. Moreover, the orbital polarization can be tuned by the magnitudes of the Coulomb interaction U and the Hund's rule coupling J, leading to a redistribution of electrons among five Fe 3d orbitals [12]. Electronic correlations weaken the hybridization between Fe and ligand atoms, reducing the effective occupancy of the Fe 3d orbitals [13]. However, as the total electron count on the Fe and ligand atoms in crystals, such as BaFe 2 As 2 , is conserved, such charge redistribution between them does not produce any doping effect on the Fermi surfaces (FSs).In this work, we prove that doping can be induced by electronic correlations in the IBSC Ba 2 Ti 2 Fe 2 As 4 O (Ba22241, T c ∼ 21.5 K) due to the intercalation of metallic Ti 2 As 2 O layers. Ba22241 can be regarded as a superlattice consisting of alter...