Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO(3) consists of nondegenerate t(2g) states with different dimensional characters. While the d(xy) bands have quasi-2D dispersions with weak k(z) dependence, the lifted d(xz)/d(yz) bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the d(xz)/d(yz) Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO(3) in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-E(F) electrons in the surface region.

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 report an angle-resolved photoemission study of the charge stripe ordered La1.6−xNd0.4SrxCuO4 system.A comparative and quantitative line shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal state anti-nodal spectral gap opens upon cooling below 80 K. In this process spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different lineshape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral weight redistribution and gapping originating from charge stripe ordering.

The iron-pnictide superconductors have a layered structure formed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional 1 (3D) electronic structure of these hightemperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k z capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba 0.6 K 0.4 Fe 2 As 2 samples. We found a marked k z dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer ∆ 1 and interlayer ∆ 2 pairing. The anisotropy ratio ∆ 1 /∆ 2 , determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J c /J ab in the parent compound 2 . The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrains the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.Angle-resolved photoemission spectroscopy (ARPES) has played an important role in revealing the electronic structure of the pnictides. These measurements have typically been carried out at a fixed incident photon energy (hν) and varying incident angles that map out the planar band dispersion as a function of k x and k y . Thus far, four FS sheets have been observed with two hole pockets centred around the (0, 0) point and two electron pockets around the M (π,0) point in the unfolded 2D Brillouin zone. Below the superconducting transition temperature T c , nodeless SC gaps open everywhere on the FS sheets 3-6 , pointing to a pairing order parameter with an s-wave symmetry in the a-b plane, in agreement with a number of theoretical results 7-11 . However, there are other experiments that have indicated possible nodes in the superconducting gap of some pnictides, either line nodes in the a-b plane or nodes along the c axis [12][13][14] . It is well known that on tuning the incident photon energy hν, the allowed direct transitions will shift in energy and, consequently, in the momentum perpendicular to the a-b plane (k z ), which enables the determination of the electronic dispersion along the c axis. spectral intensity measured at 10 K plotted on a false-colour scale as a function of the in-plane momentum (k ) and binding energy along -X using 46-eV photons, which corresponds to k z = 0. Two hole-like bands (α (inner) and β (outer)) are observed. b, Second derivative of the spectral intensity plot as shown in a. c, A set of EDCs within the E-k range indicated by the red rectangle in a. The red EDC is at k F of the α band. d, Second derivative plot of the dispersion along Z-R (k z = π) measured using 32-eV photons. Three hole-like bands (α (inner), α (middle) and β (outer)) are observed. 198NATURE PHYSICS |...