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We have studied the electronic structure of EuFe2As2-xPx using high-resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to chemical pressure and to superconductivity, we observe a nonrigid-band-like change of the electronic structure along the center of the Brillouin zone (BZ) in the form of an orbital and kz-dependent increase or decrease in the size of the hole pockets near the Γ-Z line. The diameter of the Fermi surface cylinders at the BZ corner which form electron pockets, increases at K and changes in a nonmonotonous way at X. This is in stark contrast to p- and n-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band-like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome. [ABSTRACT FROM AUTHOR]

Angle-resolved photoemission spectroscopy is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictide systems. We find the scattering rate for any given band to depend linearly on energy but to be independent of the control parameter. We demonstrate that the linear energy dependence gives rise to a weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. The resulting small effective Fermi energy favors a BCS [J. Bardeen et al., Phys. Rev. 108, 1175 (1957)] -Bose-Einstein [S. N. Bose, Z. Phys. 26, 178 (1924)] crossover state in the superconducting phase. [ABSTRACT FROM AUTHOR]

The high index (441) surface of bismuth has been studied using scanning tunneling microscopy (STM), angle resolved photoemission spectroscopy (APRES), and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localized states are observed, both of which are linearly dispersing in one in-plane direction (kx), and dispersionless in the orthogonal in-plane direction (ky), and both of which have a Dirac-like crossing at kx=0. Spin ARPES reveals a strong in-plane polarization, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its possible origin as an edge state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one-dimensional quantum spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk. [ABSTRACT FROM AUTHOR]

We analyze the strong hexagonal warping of the Dirac cone of Bi2Te3 by angle-resolved photoemission. Along ΓM, the dispersion deviates from a linear behavior meaning that the Dirac cone is warped outwards and not inwards. We show that this introduces an anisotropy in the lifetime broadening of the topological surface state which is larger along ΓK. The result is not consistent with an explanation by nesting properties. Based on the theoretically predicted modifications of the ground-state spin texture of a strongly warped Dirac cone, we propose spin-dependent scattering processes as explanation for the anisotropic scattering rates. These results could help paving the way for optimizing future spintronic devices using topological insulators and controlling surface-scattering processes via external gate voltages. [ABSTRACT FROM AUTHOR]

YbCo2 Si2 is considered to serve as a stable-valent, isoelectronic reference for the extensively studied heavyfermion system YbRh2Si2 which is situated very close to an antiferromagnetic quantum critical point (QCP). The investigation of the Fermi surface (FS) topology of YbCo2Si2 and its comparison to YbRh2Si2 could help to unravel the strongly disputed nature of this quantum phase transition, whether it originates from a "local" or "itinerant" QCP. Here we study the electronic structure and FS of YbCo2Si2 by means of angle-resolved photoelectron spectroscopy (ARPES) and compare it to ab initio band structure calculations and FS modeling. Our approach allows the electronic structure at the surface and in the bulk to be disentangled. Identifying the bulk contribution, we demonstrate that YbCo2 Si2 exhibits a "small" FS, confirming the formation of a "large" FS in YbRh2Si2. This favors an itinerant QCP, instead of the widely discussed local scenario. Our study demonstrates that ARPES is a reliable tool for the study of bulk electronic states in intermetallic Kondo lattice systems despite the complexity induced by their three-dimensional character and the presence of pronounced surface states. [ABSTRACT FROM AUTHOR]

The electronic structure of the iron chalcogenide superconductor FeSe1–x was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe1–x, DFT calculations, and existing data on FeSetTe1–x. The bands undergo a pronounced orbital-dependent renormalization, different from what was observed for FeSexTe1–x and any other pnictides. [ABSTRACT FROM AUTHOR]

We performed a high-energy resolution angle-resolved photoemission spectroscopy investigation of overdoped 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 Ba0.1K0.9Fe2As2, 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 Ba0.1K0.9Fe2As2, suggesting that the pairing interaction is not driven by the Fermi surface topology, but nevertheless reveals the possible importance of the Fermi surface topology in shaping the superconducting gap structure [ABSTRACT FROM AUTHOR]

In this study, we investigate the electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in normal and superconducting states by means of photoemission and μSR spectroscopies as well as band-structure calculations. We demonstrate that the unusual behavior of these materials is the result of separation into metallic (~12%) and insulating (~88%) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe slabs. Our results thus imply that the antiferromagnetic insulating phase is just a by-product of Rb intercalation and its magnetic properties have no direct relation to the superconductivity. Instead, we find that also in this class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a Van Hove singularity to the Fermi level. [ABSTRACT FROM AUTHOR]

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 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 M-centered 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. [ABSTRACT FROM AUTHOR]

Electronic structure of newly synthesized single crystals of calcium iron arsenide doped with sodium with Tc ranging from 33 to 14 K has been determined by angle-resolved photoemission spectroscopy (ARPES). The measured band dispersion is in general agreement with theoretical calculations, nonetheless implies absence of Fermi-surface nesting at an antiferromagnetic vector. A clearly developing below Tc strongly band-dependant superconducting gap has been revealed for samples with various doping levels. The BCS ratio for optimal doping, 2Δ/kBTc=5.5, is substantially smaller than the numbers observed for related compounds. [ABSTRACT FROM AUTHOR]

Angle-resolved photoelectron spectroscopy, supplemented by theoretical calculations has been applied to study the electronic structure of heavy-fermion material CeFePO, a homologue to the Fe-based high-temperature superconductors, and CeFeAs0.7P0.3O, where the applied chemical pressure results in a ferromagnetic order of the 4f moments. A comparative analysis reveals characteristic differences in the Fe-derived band structure for these materials, implying a rather different hybridization of valence electrons to the localized 4f orbitals. In particular, our results suggest that the ferromagnetism of Ce moments in CeFeAs0.7P0.3O is mediated mainly by Fe 3dxz/yz orbitals, while the Kondo screening in CeFePO is instead due to a strong interaction of Fe 3d3z2-r2 orbitals. [ABSTRACT FROM AUTHOR]