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Universal versus Materials-Dependent Two-Gap Behaviour of the High-Tc Cuprate Superconductors: Angle-Resolved Photoemission Study of La2-xSrxCuO4

Description: We have investigated the doping and temperature dependences of the pseudogap and superconducting gap in the single-layer cuprate La{sub 2-x}Sr{sub x}CuO{sub 4} by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ({pi}, 0) of magnitude {Delta}* and the gap around the node characterized by the d-wave order parameter {Delta}{sub 0}. In comparison with Bi2212 having higher T{sub c}'s, {Delta}{sub 0} is smaller, while {Delta}* and T* are similar. This result suggests that {Delta}* and T* are approximately material-independent properties of a single CuO{sub 2} plane, in contrast to the material-dependent {Delta}0, representing the pairing strength.
Date: November 30, 2008
Creator: Yoshida, T.; Hashimoto, Makoto; Ideta, S.; Fujimori, Atsushi; Tanaka, K.; Mannella, Norman et al.
Partner: UNT Libraries Government Documents Department

Unusual layer-dependent charge distribution, collective mode coupling, and superconductivity in multilayer cuprate Ba2Ca3Cu4O8F2

Description: Low energy ultrahigh momentum resolution angle resolved photoemission spectroscopy study on four-layer self-doped high Tc superconductor Ba2Ca3Cu4O8F2 (F0234) revealed fine structure in the band dispersion, identifying the unconventional association of hole and electron doping with the inner and outer CuO2 layers, respectively. For the states originating from two inequivalent CuO2 layers, different energy scales are observed in dispersion kinks associated with the collective mode coupling, with the larger energy scale found in the electron (n-) doped state which also has stronger coupling strength. Given the earlier finding that the superconducting gap is substantially larger along the n-type Fermi surface, our observations connect the mode coupling energy and strength with magnitude of the pairing gap.
Date: January 22, 2009
Creator: Chen, Yulin; Iyo, Akira; Yang, Wanli; Ino, Akihiro; Arita, M.; Johnston, Steve et al.
Partner: UNT Libraries Government Documents Department

Electronic structure of the BaFe2As2 family of iron-pnictide superconductors

Description: We use high-resolution angle-resolved photoemission to study the electronic structure of the BaFe{sub 2}As{sub 2} pnictides. We observe two electron bands and two hole bands near the X point, ({pi},{pi}) of the Brillouin zone, in the paramagnetic state for electron-doped Ba(Co{sub 0.06}Fe{sub 0.94}){sub 2}As{sub 2}, undoped BaFe{sub 2}As{sub 2}, and hole-doped Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}. Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe{sub 2}As{sub 2} family matches reasonably well with the prediction of local-density approximation calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.
Date: June 29, 2009
Creator: Reza, Adriana; Lu, D.H.; Analytis, J.G.; Chu, J.-H.; Mo, S.-K.; He, R.-H. et al.
Partner: UNT Libraries Government Documents Department

Electronic structure of the iron-based superconductor LaOFeP

Description: The recent discovery of superconductivity in the iron oxypnictide family of compounds1?9 has generated intense interest. The layeredcrystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature ofthe ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, havebeen put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach, and an itinerant ground statein the weak-coupling approach. The first approach stresses onsite correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electronphysics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here wereport angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, Tc55.9 K),the first-reported iron-based superconductor. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.
Date: September 4, 2008
Creator: Lu, D. H.; Yi, M.; Mo, S.-K.; Erickson, A. S.; Analytis, J.; Chu, J.-H. et al.
Partner: UNT Libraries Government Documents Department

Spin Spectrometer at the ALS and APS

Description: A spin-resolving photoelectron spectrometer, the"Spin Spectrometer," has been designed and built. It has been utilized at both the Advanced Light Source in Berkeley, CA, and the Advanced Photon Source in Argonne, IL. Technical details and an example of experimental results are presented here.
Date: April 20, 2007
Creator: Laboratory, Lawrence Livermore National; Missouri-Rolla, University of; Technologies, Boyd; Morton, Simon A; Morton, Simon A; Tobin, James G et al.
Partner: UNT Libraries Government Documents Department

Two-photon Photo-emission of Ultrathin Film PTCDA Morphologies on Ag(111)

Description: Morphology- and layer-dependent electronic structure and dynamics at the PTCDA/Ag(111) interface have been studied with angle-resolved two-photon photoemission. In Stranski-Krastanov growth modes, the exposed wetting layer inhibited the evolution of the vacuum level and valence band to bulk values. For layer-by-layer growth, we observed the transition of electron structure from monolayer to bulk values within eight monolayers. Effective masses and lifetimes of the conduction band and the n=1 image potential state were measured to be larger for disordered layers. The effective mass was interpreted in the context of charge mobility measurements.
Date: November 29, 2007
Creator: Yang, Aram; Yang, Aram; Shipman, Steven T.; Garrett-Roe, Sean; Johns, James; Strader, Matt et al.
Partner: UNT Libraries Government Documents Department

Two-photon Photoemission of Organic Semiconductor Molecules on Ag(111)

Description: Angle- and time-resolved two-photon photoemission (2PPE) was used to study systems of organic semiconductors on Ag(111). The 2PPE studies focused on electronic behavior specific to interfaces and ultrathin films. Electron time dynamics and band dispersions were characterized for ultrathin films of a prototypical n-type planar aromatic hydrocarbon, PTCDA, and representatives from a family of p-type oligothiophenes.In PTCDA, electronic behavior was correlated with film morphology and growth modes. Within a fewmonolayers of the interface, image potential states and a LUMO+1 state were detected. The degree to which the LUMO+1 state exhibited a band mass less than a free electron mass depended on the crystallinity of the layer. Similarly, image potential states were measured to have free electron-like effective masses on ordered surfaces, and the effective masses increased with disorder within the thin film. Electron lifetimes were correlated with film growth modes, such that the lifetimes of electrons excited into systems created by layer-by-layer, amorphous film growth increased by orders of magnitude by only a few monolayers from the surface. Conversely, the decay dynamics of electrons in Stranski-Krastanov systems were limited by interaction with the exposed wetting layer, which limited the barrier to decay back into the metal.Oligothiophenes including monothiophene, quaterthiophene, and sexithiophene were deposited on Ag(111), and their electronic energy levels and effective masses were studied as a function of oligothiophene length. The energy gap between HOMO and LUMO decreased with increasing chain length, but effective mass was found to depend on domains from high- or low-temperature growth conditions rather than chain length. In addition, the geometry of the molecule on the surface, e.g., tilted or planar, substantially affected the electronic structure.
Date: May 15, 2008
Creator: Yang, Aram & Yang, Aram
Partner: UNT Libraries Government Documents Department

A combined droplet train and ambient pressure photoemission spectrometer for the investigation of liquid/vapor interfaces

Description: We describe a combined ambient pressure photoelectron spectroscopy/droplet train apparatus for investigating the nature and heterogeneous chemistry of liquid/vapor interfaces. In this instrument a liquid droplet train with typical droplet diameters from 50...150 {micro}m is produced by a vibrating orifice aerosol generator (VOAG). The droplets are irradiated by soft X-rays (100...1500 eV) in front of the entrance aperture of a differentially pumped electrostatic lens system that transfers the emitted electrons into a conventional hemispherical electron analyzer. The photoemission experiments are performed at background pressures of up to several Torr, which allows the study of environmentally important liquid/vapor interfaces, in particular aqueous solutions, under equilibrium conditions. The exposure time of the droplet surface to the background gases prior to the XPS measurement can be varied, which will allow future kinetic measurements of gas uptake on liquid surfaces. As an example, a measurement of the surface composition of a {chi} = 0.21 aqueous methanol solution is presented. The concentration of methanol at the vapor/liquid interface is enhanced by a factor of about 3 over the bulk value, while the expected bulk value is recovered at depths larger than about 1.5 nm.
Date: May 1, 2008
Creator: Starr, David E.; Wong, Ed K.; Worsnop, Douglas R.; Wilson, Kevin R. & Bluhm, Hendrik
Partner: UNT Libraries Government Documents Department

High resolution angle-resolved photoemission study of hightemperature superconductors: charge-ordering, bilayer splitting andelectron-phonon coupling

Description: The latest development of angle-resolved photoemission spectroscopy (ARPES) technique has seen extremely high energy resolution and momentum resolution, as well as multiple angle detection. These advancements have led to new findings through efficient Fermi surface mapping, fine electronic structure resolving, and direct determination of electron self-energy. In this paper, we will highlight some recent high resolution ARPES work on high temperature superconductors. These include: (1) charge-ordering and evolution of electronic structure with doping; (2) bilayer splitting and Fermi surface topology of Bi2212; and (3) strong electron phonon coupling and electron electron interaction in high temperature superconductors.
Date: July 7, 2003
Creator: Zhou, Xingjiang; Hussain, Zahid & Shen, Zhi-xun
Partner: UNT Libraries Government Documents Department

Final Report for Department of Energy Grant DE-FG02-97ER45666 ?Interface Diffusion and Deep Level Formation of SiC and Other Wide Gap Materials?

Description: This final report describes the research effort focusing on the nature of charge transfer and localized electronic structure at semiconductor interfaces, one of the most fundamental issues in the solid state. The basic charge exchange between two materials in general is directly connected with the systematic atomic bonding changes and redistribution that occurs at the nanoscale interface. Our programhas extended our understanding of the atomic-scale nature of electrostatic barrier formation, heterojunction band offsets, and the optical and electronic features of impurity confinement in a set of model materials systems, including nanometer-scale wide band gap semiconductor and insulator structures. This fundamentally new class of materials investigation utilizes a powerful and unique combination of techniques that are revealing the atomic-scale movement, chemical bonding, and resultant charge transfer across well-defined model interfaces.
Date: March 24, 2006
Creator: Brillson, Leonard J.
Partner: UNT Libraries Government Documents Department

Non-free-electron momentum- and thickness-dependent evolution ofquantum well states in the Cu/Co/Cu(001) system

Description: We present systematic k{sub {parallel}}-dependent measurements of the Fermi surface and underlying band structure of quantum well states in Cu/Co/Cu(001). Compared to bands from normal emission, we find a complicated evolution of ''split'' QW states as a function of the thicknesses of both the copper overlayer and the cobalt barrier layer. Self-consistent calculations show that the penetration of the quantum well states into the cobalt barrier layer is significant and leads to the observed very non-free-electron behavior of these states.
Date: May 21, 2005
Creator: Rotenberg, Eli; Wu, Y.Z.; An, Joonhee M.; Van Hove, Michel A.; Canning, Andrew; Wang, Lin-Wang et al.
Partner: UNT Libraries Government Documents Department

Electron-Phonon Coupling in High-Temperature CuprateSuperconductors as Revealed by Angle-resolved PhotoemissonSpectroscopy

Description: We present an application programming interface (API) used to simplify application-level access to both data and semantic range query machinery where both data and search indices are stored and accessed using the HDF5 data model. While the APIs for HDF5 data access and FastBit indexing/query are rather complex, our API simplifies use of these powerful software technologies.
Date: May 1, 2005
Creator: Zhou, X.J.; Hussain, Z. & Shen, Z.-X.
Partner: UNT Libraries Government Documents Department

Determination of Band Curvatures by Angle-resolved Two-photonPhotoemission in thin films of C(60) on Ag(111)

Description: The thickness-dependent interfacial band structure wasdetermined for thin films of Co(6) on Ag(111) by angle-resolvedtwo-photon photoemission spectroscopy.
Date: March 1, 2005
Creator: Shipman, Steven T.; Szymanski, Paul; Garrett-Roe, Sean; Yang,Aram; Strader, Matthew L. & Harris, Charles B.
Partner: UNT Libraries Government Documents Department

Progress on PEEM3 - An Aberration Corrected X-Ray PhotoemissionElectron Microscope at the ALS

Description: A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed and built at the Advanced Light Source (ALS). An electron mirror combined with a much-simplified magnetic dipole separator is to be used to provide simultaneous correction of spherical and chromatic aberrations. It is installed on an elliptically polarized undulator (EPU) beamline, and will be operated with very high spatial resolution and high flux to study the composition, structure, electric and magnetic properties of complex materials. The instrument has been designed and is described. The instrumental hardware is being deployed in 2 phases. The first phase is the deployment of a standard PEEM type microscope consisting of the standard linear array of electrostatic electron lenses. The second phase will be the installation of the aberration corrected upgrade to improve resolution and throughput. This paper describes progress as the instrument enters the commissioning part of the first phase.
Date: May 20, 2006
Creator: MacDowell, Alastair A.; Feng, J.; DeMello, A.; Doran, A.; Duarte,R.; Forest, E. et al.
Partner: UNT Libraries Government Documents Department

The evolution of Ga and As core levels in the formation of Fe/GaAs (001):A high resolution soft x-ray photoelectron spectroscopic study

Description: A high resolution soft x-ray photoelectron spectroscopic study of Ga and As 3d core levels has been conducted for Fe/GaAs (001) as a function of Fe thickness. This work has provided unambiguous evidence of substrate disrupting chemical reactions induced by the Fe overlayer--a quantitative analysis of the acquired spectra indicates significantly differing behavior of Ga and As during Fe growth, and our observations have been compared with existing theoretical models. Our results demonstrate that the outdiffusing Ga and As remain largely confined to the interface region, forming a thin intermixed layer. Whereas at low coverages Fe has little influence on the underlying GaAs substrate, the onset of substrate disruption when the Fe thickness reaches 3.5 Angstrom results in major changes in the energy distribution curves (EDCs) of both As and Ga 3d cores. Our quantitative analysis suggests the presence of two additional As environments of metallic character: one bound to the interfacial region and another which, as confirmed by in situ oxidation experiments, surface segregates and persists over a wide range of overlayer thickness. Analysis of the corresponding Ga 3d EDCs found not two, but three additional environments--also metallic in nature. Two of the three are interface resident whereas the third undergoes outdiffusion at low Fe coverages. Based on the variations of the integrated intensities of each component, we present a schematic of the proposed chemical makeup of the Fe/GaAs (001) system.
Date: July 14, 2008
Creator: Thompson, Jamie; Neal, James; Shen, Tiehan; Morton, Simon; Tobin, James; Waddill, George Dan et al.
Partner: UNT Libraries Government Documents Department

A universal high energy anomaly in angle resolved photoemissionspectra of high temperature superconductors -- possible evidence ofspinon and holon branches

Description: A universal high energy anomaly in the single particlespectral function is reported in three different families of hightemperature superconductors by using angle-resolved photoemissionspectroscopy. As we follow the dispersing peak of the spectral functionfrom the Fermi energy to the valence band complex, we find dispersionanomalies marked by two distinctive high energy scales, E_1 approx 0.38eV and E_2 approx 0.8 eV. E_1 marks the energy above which the dispersionsplits into two branches. One is a continuation of the near parabolicdispersion, albeit with reduced spectral weight, and reaches the bottomof the band at the Gamma point at approx 0.5 eV. The other is given by apeak in the momentum space, nearly independent of energy between E_1 andE_2. Above E_2, a band-like dispersion re-emerges. We conjecture thatthese two energies mark the disintegration of the low energyquasiparticles into a spinon and holon branch in the high T_c cuprates.
Date: December 19, 2006
Creator: Graf, J.; Gweon, G.-H.; McElroy, K.; Zhou, S.Y.; Jozwiak, C.; Rotenberg, E. et al.
Partner: UNT Libraries Government Documents Department

Tandem-ESQ for Accelerator-Based BNCT

Description: A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed and built at the Advanced Light Source (ALS). An electron mirror combined with a much-simplified magnetic dipole separator is to be used to provide simultaneous correction of spherical and chromatic aberrations. It is installed on an elliptically polarized undulator (EPU) beamline, and will be operated with very high spatial resolution and high flux to study the composition, structure, electric and magnetic properties of complex materials. The instrument has been designed and is described. The instrumental hardware is being deployed in 2 phases. The first phase is the deployment of a standard PEEM type microscope consisting of the standard linear array of electrostatic electron lenses. The second phase will be the installation of the aberration corrected upgrade to improve resolution and throughput. This paper describes progress as the instrument enters the commissioning part of the first phase.
Date: June 1, 2006
Creator: Kreiner, A.J.; Kwan, J.W.; Burlon, A.A.; Di Paolo, H.; Henestroza, E.; Minsky, D.M. et al.
Partner: UNT Libraries Government Documents Department

Growth and structure of water on SiO2 films on Si investigated byKelvin probe microscopy and in situ X-ray Spectroscopies

Description: The growth of water on thin SiO{sub 2} films on Si wafers at vapor pressures between 1.5 and 4 torr and temperatures between -10 and 21 C has been studied in situ using Kelvin Probe Microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH) water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 C and ice-like below 0 C.
Date: June 14, 2007
Creator: Verdaguer, A.; Weis, C.; Oncins, G.; Ketteler, G.; Bluhm, H. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Exploring nanomagnetism with soft x-ray microscopy

Description: Magnetic soft X-ray microscopy images magnetism in nanoscale systems with a spatial resolution down to 15nm provided by state-of-the-art Fresnel zone plate optics. X-ray magnetic circular dichroism (X-MCD) is used as element-specific magnetic contrast mechanism similar to photoemission electron microscopy (PEEM), however, with volume sensitivity and the ability to record the images in varying applied magnetic fields which allows to study magnetization reversal processes at fundamental length scales. Utilizing a stroboscopic pump-probe scheme one can investigate fast spin dynamics with a time resolution down to 70 ps which gives access to precessional and relaxation phenomena as well as spin torque driven domain wall dynamics in nanoscale systems. Current developments in zone plate optics aim for a spatial resolution towards 10nm and at next generation X-ray sources a time resolution in the fsec regime can be envisioned.
Date: October 30, 2006
Creator: Fischer, P.; Kim, D.-H.; Mesler, B.L.; Chao, W.; Sakdinawat,A.E. & Anderson, E.H.
Partner: UNT Libraries Government Documents Department

Highly polarized emission in spin resolved photoelectron spectroscopy of alpha-Fe(001)/GaAs(001)

Description: Highly spin-polarized sources of electrons, Integrated into device design, remain of great interest to the spintronic and magneto-electronic device community Here, the growth of Fe upon GaAs(001) has been studied with photoelectron spectroscopy (PES), including Spin Resolved PES. Despite evidence of atomic level disorder such as intermixing, an over-layer with the spectroscopic signature of alpha-Fe(001), with a bcc real space ordering, Is obtained The results will be discussed in light of the possibility of using such films as a spin-polarized source in device applications.
Date: May 19, 2009
Creator: Tobin, James; Yu, Sung Woo; Morton, Simon; Waddill, George; Thompson, Jamie; Neal, James et al.
Partner: UNT Libraries Government Documents Department

An Alternative Model for Electron Correlation in Pu

Description: Using a density functional theory based approach that treats the 5f electrons relativistically, a Pu electronic structure with zero net magnetic moment is obtained, where the 5f orbital and 5f spin moments cancel each other. By combining the spin and orbital specific densities of states with state, spin and polarization specific transition moments, it is possible to reconstruct the experimentally observed photoemission spectra from Pu. Extrapolating to a spin-resolving Fano configuration, it is shown how this would resolve the extant controversy over Pu electronic structure.
Date: October 23, 2007
Creator: Yu, S; Tobin, J & Soderlind, P
Partner: UNT Libraries Government Documents Department

The work function of sub-monolayer cesium-covered gold: A photoelectronspectroscopy study

Description: Using visible and X-ray photoelectron spectroscopy we measured the work function of a Au(111) surface at a well-defined sub-monolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61 {+-} 0.08 eV consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.
Date: June 13, 2008
Creator: LaRue, J.L.; White, J.D.; Nahler, N.H.; Liu, Z.; Sun, Y.; Pianetta, P.A. et al.
Partner: UNT Libraries Government Documents Department

The Origin of the Monochromatic Photoemission Peak in Diamondoid Monolayers

Description: Recent photoemission experiments have discovered a highly monochromatized secondary electron peak emitted from diamondoid self-assembled monolayers on metal substrates. New experimental data and simulation results are presented to show that a combination of negative electron affinity and strong electron-phonon scattering is responsible for this behavior. The simulation results are generated using a simple Monte Carlo transport algorithm. The simulated spectra contain the main spectral features of the measured ones.
Date: October 31, 2008
Creator: Clay, William A.; Liu, Zhi; Yang, Wanli; Fabbri, Jason D.; Dahl, Jeremy E.; Carlson, Robert M.K. et al.
Partner: UNT Libraries Government Documents Department

The Effectiveness of HCl and HF Cleaning of Si0.85Ge0.15 Surface

Description: The cleaning of Si{sub 0.85}Ge{sub 0.15} surfaces using HCl and HF solutions is studied using synchrotron radiation photoelectron spectroscopy. The HF solution is found to be effective in removing both the Si oxide and the Ge oxide while the HCl solution can only remove part of the Ge oxide. For samples treated with HF, four spectral components are needed to fit the Ge 3d photoemission spectra. One is the bulk component and the other three are attributed to the surface Ge atoms with mono-hydride, di-hydride and tri-hydride terminations, respectively.
Date: July 11, 2008
Creator: Sun, Y.; Liu, Z.; Sun, S. & Pianetta, P.
Partner: UNT Libraries Government Documents Department