Scot A. Kellar

Dual nature of the electronic structure of (La2-x-yNdySrx)CuO4 and La1.85Sr0.15CuO4

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

Combined near edge X-ray absorption fine structure and X-ray photoemission spectroscopies for the study of amorphous carbon thin films

Abstract Near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies were used to study the chemical bonding in hard amorphous carbon (a-C) thin films. The analysis of their XPS spectra obtained at different photon energy excitations showed that the fraction of sp 3 hybridized atoms increased from the surface to the bulk. About 90% of the carbon atoms became sp 2 hybridized over the depth probed by XPS after annealing the film at 850°C. The intensity of the π* resonance in the NEXAFS spectrum of the hard a-C film annealed at 850°C was significantly smaller than the same resonance in a softer film annealed at 350°C. However, the sp 2 concentration of the former film, measured by XPS, was much higher. This demonstrates that the intensity of the π* resonance in the NEXAFS spectra of a-C films is not strictly proportional to the concentration of sp 2 hybridized atoms, but it can be strongly influenced by other effects, such as the localized character of the particular π* bonding present in the analyzed a-C film. A sharp resonance was observed at the same photon energy as the exciton in diamond, at 289.5 eV, in the NEXAFS spectra of the a-C films. This resonance might indicate that a proportion of carbon atoms in the analyzed films occupied similar sites as the carbon atoms in crystalline diamond.

Adsorption Site and Structure Determination of c(2x2) N2/Ni(100) using Angle-resolved Photoemission Extended Fine Structure

Abstract We have determined the atomic spatial structure of c(2 × 2) N 2 Ni (100) with angle-resolved photoemission extended fine structure using the nitrogen 1s core level. The chemically shifted N 1s peak intensities were summed to obtain ARPEFS curves for both nitrogen atoms in the molecule. We used a new, highly optimized program based on the Rehr-Albers scattering matrix formalism to find the adsorption site and to determine the bond lengths quantitatively. The nitrogen molecule stands upright at an atop site, with a NNi bond length of 2.25(1) A, a NN bond length of 1.10(7) A, and a first layer NiNi spacing of 1.76(4) A. The shake-up peak shows an identical ARPEFS diffraction pattern, confirming its intrinsic nature and supporting a previous use of this feature to decompose the peak into contributions from the chemically inequivalent nitrogen atoms. Comparison to a previously published theoretical treatment of NNNi and experimental structures of analogous adsorbate systems demonstrates the importance of adsorbate-adsorbate interactions in weakly chemisorbed systems.

Dual nature of the electronic structure of (La(2--x--y)Nd(y)Sr(x))CuO(4) and La(1.85)Sr(0.15)CuO(4).

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

Dual Nature of the Electronic Structure of the Stripe Phase

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

Dual Nature of the Electronic Structure of (La{sub 2-x-y}Nd{sub y}Sr{sub x})CuO{sub 4} and La{sub 1.85}Sr{sub 0.15}CuO{sub 4}

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

Dual Nature of the Electronic Structure of (Lasb2-x-yNdsbySrsbx)CuOsb4 and Lasb1.85Srsb0.15CuOsb4

High resolution angle-resolved photoemission measurements have been carried out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and (La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic stripes. In addition to the straight segments near (pi, 0) and (0, pi) antinodal regions, we have identified the existence of nodal spectral weight and its associated Fermi surface in the electronic structure of both systems. The ARPES spectra in the nodal region show well-defined Fermi cut-off, indicating a metallic character of this charge-ordered state. This observation of nodal spectral weight, together with the straight segments near antinodal regions, reveals dual nature of the electronic structure of the stripes due to the competition of order and disorder.

Electronlike Fermi surface and remnant ({pi},0) feature in overdoped La{sub 1.78}Sr{sub 0.22}CuO{sub 4}

We have performed an angle-resolved photoemission study of overdoped La{sub 1.78}Sr{sub 0.22}CuO{sub 4}, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}. The data analysis using energy distribution curves, momentum distribution curves, and intensity maps all show evidence of an electronlike Fermi surface, which is well explained by band-structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around ({pi},0), where the band is predicted to lie above E{sub F}.

Electron-like Fermi surface and Remnant (pi,0) features in overdoped La(1.78)Sr(0.22)CuO4

We have performed an angle-resolved photoemission study of overdoped La{sub 1.78}Sr{sub 0.22}CuO{sub 4}, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}. The data analysis using energy distribution curves, momentum distribution curves, and intensity maps all show evidence of an electronlike Fermi surface, which is well explained by band-structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around ({pi},0), where the band is predicted to lie above E{sub F}.

Photoemission study of Pb doped Bi2212-a Fermi surface picture

High resolution angle resolved photoemission data from Pb doped Bi_2Sr_2CaCu_2O_8 (Bi2212) with suppressed superstructure is presented. Improved resolution and very high momentum space sampling at various photon energies reveal the presence of two Fermi surface pieces. One has the hole-like topology, while the other one has its van Hove singularity very close to (pi,0), its topology at some photon energies resembles the electron-like piece. This result provides a unifying picture of the Fermi surface in the Bi2212 compound and reconciles the conflicting reports.