We performed a high energy resolution ARPES investigation of over-doped Ba0.1K0.9Fe2As2 with Tc = 9 K. The Fermi surface topology of this material is similar to that of KFe2As2 and differs from that of slightly less doped Ba0.3K0.7Fe2As2, implying that a Lifshitz transition occurred between x = 0.7 and x = 0.9. Albeit for a vertical node found at the tip of the emerging off-M-centered Fermi surface pocket lobes, the superconducting gap structure is similar to that of Ba0.3K0.7Fe2As2, suggesting that the paring interaction is not driven by the Fermi surface topology.PACS numbers: 74.70. Xa, 74.25.Jb, The discovery of high-temperature superconductivity without hole Fermi surface (FS) pocket [1][2][3][4][5][6][7] in AFe 2 Se 2 (A = Tl, K, Cs, Rb) imposed severe constraints to the electron-hole quasi-nesting scenario as the main Cooper pairing force in the Fe-based superconducting (SC) materials [8] and raised fundamental questions related to the importance of their FS topology. To answer these questions, it is necessary to investigate heavily hole-doped compounds. Previous angle-resolved photoemission spectroscopy (ARPES) studies of fully hole-doped KFe 2 As 2 indicate that the FS near the M(π, 0) point is formed by small off-M-centered hole FS pocket lobes [9,10] rather than the M-centered ellipsoid-like electron FS pockets commonly observed in the other materials [8]. Interestingly, KFe 2 As 2 has a very low critical temperature (T c ) of only 3 K [11,12], which was early [9] interpreted as, and is still widely considered as, a consequence of the evolution of the FS topology. Despite their incapability to access the band structure at the M point and thus to reveal completely the SC gap structure, laser-ARPES measurements suggest a rather complicated nodal SC gap profile around the Γ point [13], which is consistent with the finite residual thermal conductivity (κ 0 /T ) of this material at low temperature [14,15].While their existence is widely accepted, the origin of the nodes in KFe 2 As 2 and their relationship with the pairing mechanism remain unclear and could possibly involve a fundamental change in the SC order parameter upon doping K from the optimal Ba 0.6 K 0.4 Fe 2 As 2 composition, for which both the ARPES [16][17][18] and thermal conductivity [19] techniques agree on a nodeless SC gap. Indeed, Tafti et al. [20] recently reported a sudden reversal in the pressure (P ) dependence of T c in KFe 2 As 2 without discontinuity in the Hall coefficient R H (P ) and in the electrical resistivity ρ(P ), which was interpreted as an evidence for a change in the order parameter incompatible with a Lifshitz transition (change in the FS topology [21]). Based on a rigid-band shift model, the Lifshitz transition corresponding to the apparition of the small off-M-centered hole FS pocket lobes was estimated to occur within the 0.8 ≤ x ≤ 0.9 doping range [22]. Determining the k-space structure of the SC gap in the vicinity of this transition is of critical importance.In this Letter, we investigate the electronic structure...

We have performed high-resolution angle-resolved photoemission spectroscopy of layered chalcogenide 1T-Fe(x)Ta(1-x)S(2) which undergoes a superconducting transition in the nearly commensurate charge-density-wave phase (melted Mott phase). We found a single electron pocket at the Brillouin-zone center in the melted Mott phase, which is created by the backfolding of bands due to the superlattice potential of charge-density-wave. This electron pocket appears in the x region where the samples show superconductivity, and is destroyed by the Mott- and Anderson-gap opening. Present results suggest that the melted Mott state and the superconductivity coexist in real space, providing a new insight into the interplay between electron correlation, charge order, and superconductivity.

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We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe 1−x Se x (0 x 0.45) to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a welldefined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergence of superconductivity.

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe0.75Ru0.25)2As2 (Tc = 15 K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 meV and 4.5 meV superconducting gaps on the inner and outer Γ-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the Mcentered electron Fermi surface pockets. Our results are consistent with the J1 − J2 model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.PACS numbers: 74.70. Xa, 74.25.Jb, The mechanism for Cooper pairing is the most important issue in high-T c superconductivity. Owing to its momentum resolution capabilities, angle-resolved photoemission spectroscopy (ARPES) is a very powerful tool to investigate the superconducting (SC) gap size and symmetry directly in the momentum space. Previous ARPES experiments revealed Fermi surface (FS) dependent and nearly isotropic SC gaps for both hole-doped [1, 2] and electron-doped [3] 122-ferropnictides. In contrast, a circular horizontal node at k z = π was recently reported on a hole FS pocket with strong three-dimensional (3D) character in isovalent P-substituted BaFe 2 (As 0.7 P 0.3 ) 2 [4]. Whether such feature is unique or common to other isovalently-substituted BaFe 2 As 2 materials is still under intense debate. Ba(Fe 1−x Ru x ) 2 As 2 is another isovalent substituted system with maximum critical temperatures T c around 20 K [5-8]. As with BaFe 2 (As 0.7 P 0.3 ) 2 , previous ARPES studies of the electronic band structure indicate that this system exhibits a pronounced 3D character [9][10][11], thus raising the possibility of SC gap nodes. Indeed, recent measurements of a finite residual thermal conductivity suggest the presence of nodes. Unfortunately, samples of Ba(Fe 1−x Ru x ) 2 As 2 usually show only a partial SC volume fraction detrimental to a determination of the momentum-resolved SC gap by ARPES. * p.richard@iphy.ac.cn † dingh@iphy.ac.cnIn this letter, we report high-energy resolution ARPES results on SC Ba(Fe 0.75 Ru 0.25 ) 2 As 2 (T c = 15 K) recorded in the whole 3D Brillouin zone (BZ) by tuning the incident photon energy (hν). Due to improved sample quality, we observed the opening of a SC gap. We demonstrate that the SC gap size at this doping level is nearly isotropic on each FS, and slightly FS-dependent. Interestingly, the global gap structure in this material can be described by a single gap function derived from a strong coupling approach.Large single crystals of Ba(Fe 0.75 Ru 0.25 ) 2 As 2 were grown by the self-flux method [8]. ARPES measurements were performed at the 1-cubed ARPES end-station of BESSY and at Swiss Light Source beamline SIS using a VG-Scienta R4000 electron analyzer with hν ranging from 22 to 64 eV, and at Tohoku University using a VGScie...

We have performed angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced with boron. We found that the Fermi level is located at a dip in the density of states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic order and may cause the superconductivity. The energy bands near the Fermi level show higher three dimensionality and a weaker electron-correlation effect than those of Fe pnictides. The Fermi surface topology is considerably different from that of Fe pnictides, suggesting the difference in the superconducting mechanism between boride and pnictide superconductors.