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Hierarchy of multiple many-body interaction scales in high-temperature superconductors

Description: To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher energy scale, we present improved experimental data from four families of high-T{sub c} superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly ('kink') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO{sub 2} band extending to energies of {approx} 2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.
Date: December 21, 2006
Creator: Hussain, Zahid; Meevasana, W.; Zhou, X.J.; Sahrakorpi, S.; Lee, W.S.; Yang, W.L. et al.
Partner: UNT Libraries Government Documents Department

An investigation of resonant photoemission in Gd with x-ray linear dichroism

Description: The constructive summing of direct and indirect channels above the absorption threshold of a core level can cause a massive increase in the emission cross section, leading to a phenomenon called "resonant photoemission". Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiments and theoretical simulations, we have probed the nature of the resonant photoemission process in Gd metal. It now appears that temporal matching as well as energy matching is a requirement for true resonant photoemission.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Linear dichroism and resonant photoemission in Gd 011

Description: Magnetic Linear Dichroism in Angular Distributions (MLDAD) from Photoelectron Emission was used to probe the nature of Resonant Photoemission. Gd 5p and Gd 4f emission were investigated. Using novel theoretical simulations, we were able to show that temporal matching is a requirement for ``True`` Resonant Photoemission, where the Resonant Photoemission retains the characteristics of Photoelectron Emission.
Date: May 13, 1998
Creator: Mishra, S.R.; Cummins, T.R.; Gammon, W.J.; van der Laan, G.; Goodman, K.W. & Tobin, J.G.
Partner: UNT Libraries Government Documents Department

Quasi-Freestanding multilayer graphene films on the carbon face of SiC

Description: The electronic band structure of as-grown and doped graphene grown on the carbon face of SiC is studied by high-resolution angle-resolved photoemission spectroscopy, where we observe both rotations between adjacent layers and AB-stacking. The band structure of quasi-freestanding AB-bilayers is directly compared with bilayer graphene grown on the Si-face of SiC to study the impact of the substrate on the electronic properties of epitaxial graphene. Our results show that the C-face films are nearly freestanding from an electronic point of view, due to the rotations between graphene layers.
Date: June 30, 2010
Creator: Siegel, D. A.; Hwang, C. G.; Fedorov, A. V. & Lanzara, A.
Partner: UNT Libraries Government Documents Department

High Resolution Upgrade for Core-level Photoemission Spectroscopy

Description: Upgraded the high resolution core-level photoemission beamline U4A at the National Synchrotron Light Source (NSLS) so that it has higher resolution ({approx}30-50 meV) over a broader spectral range (10-200 eV) than is currently available at any other photoemission beamline at NSLS. Such an upgraded beamline will prove to be extremely useful in new studies of bimetallic systems and semiconductor interfaces.
Date: August 31, 1999
Creator: Madey, T. E.
Partner: UNT Libraries Government Documents Department

Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Effects of covalency on the p-shell photoemission of transition metals: MnO

Description: This article discusses the effects of covalency on the p-shell photoemission of transition metals. The treatment of covalency has not been included previously in ab initio theoretical studies of the 2p-shell XPS of transition-metal complexes. In this work, covalent interactions between the metal and ligands are treated on an equal footing with spin-orbit splittings.
Date: April 13, 2006
Creator: Bagus, Paul S. & Ilton, Eugene S.
Partner: UNT College of Arts and Sciences

Probing the band structure of LaTe2 using angle resolved photoemission spectroscopy

Description: With the current interest in the rare-earth tellurides ashigh temperature charge density wave materials, a greater understandingof the physics of these systems is needed, particularly in the case ofthe ditellurides. We report a detailed study of the band structure ofLaTe_2 in the charge density wave state using high-resolution angleresolved photoemission spectroscopy (ARPES). From thiswork we hope toprovide insights into the successes and weaknesses of past theoreticalstudy as well as helping to clear up prior ambiguities by providing anexperimental basis for future work inthe tellurides.
Date: November 1, 2006
Creator: Garcia, Daniel R.; Zhou, Shuyun Y.; Gweon, Gey-Hong; Jung, M.H.; Kwon, Y.S. & Lanzara, Alessandra
Partner: UNT Libraries Government Documents Department


Description: Angle-resolved HeI photoemission spectra of Fe(001) are reported and interpreted within the framework of a direct transition model using Callaway's ferromagnetic band structure. The generally good agreement between predicted and experimental peak positions is taken to be strong support for the itinerant electron theory of ferromagnetism. Spectra taken with nearly grazing incidence p-polarized light emphasize the one-dimensional density of states peaks, supporting Kliewer's theoretical predictions of surface photoemission. The importance of electron refraction is noted, as is the value of interpolation calculations for interpreting ARP spectra.
Date: October 1, 1977
Creator: Kevan, S.D.; Wehner, P.S. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department


Description: The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Date: May 1, 1985
Creator: Tobin, J. G.; Robey, S. W. & Shirley, D. A.
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

In/Si(111): Self-assembled one and two-dimensional electrongases

Description: We present angle-resolved photoemission measurements forultrathin In films on Si(111). Depending on the coverage, this systemself-organizes into a metallic monolayer with either 4x1 or sqrt7 x sqrt3symmetry relative to the substrate. Electronically, they behave likeideal one- and two-dimensional electron gases (1DEG and 2DEG),respectively. The 4x1 system has atomic chains of In whose energy bandsdisperse only parallel to the chains, while for the sqrt7 x sqrt3 system,the dominant reciprocal space features (in both diffraction andbandstructure) resemble a pseudo-square lattice with only weakersecondary features relating to the sqrt7 x sqrt3 periodicity. In bothmaterials the electrons show coupling to the structure. The 1DEG couplesstrongly to phonons of momentum 2kF, leading to an 8x"2" Peierls-likeinsulating ground state. The 2DEG appears to be partially stabilized byelectron gap formation at the sqrt 7 x sqrt3 zone boundary.
Date: January 22, 2001
Creator: Rotenberg, Eli; Yeom, H.W.; Takeda, S.; Matsuda, I.; Horikoshi,K.; Schaefer, J. 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

Photoelectron angular distributions beyond the dipole approximation

Description: This paper reports the measurement of nondipolar asymmetry parameters for the angular distributions of Ar 1s, Kr 2s, and Kr 2p photoemission within 2-3 keV above their respective thresholds. Pronounced asymmetries with respect to direction of photon propagation are present even at low electron kinetic energies. The energy dependence of the asymmetry is Z- and subshell-specific and causes the direction of preferred electron emission to change sign at least once in each case. Towards higher energies, the asymmetry increases and the photoelectron emission pattern is forward-skewed for all 3 cases. Measured asymmetry parameters are in good agreement with recent predictions from nonrelativistic calculations which include interference between electric-dipole and electric-quadrupole transition amplitudes in the photoabsorption process.
Date: August 1996
Creator: Kraessig, B.; Jung, M.; Gemmell, D. S.; Kanter, E. P.; LeBrun, T.; Southworth, S. H. et al.
Partner: UNT Libraries Government Documents Department

Near-Field Spectroscopy of Selectively Oxidized Vertical Cavity Surface Emitting Lasers

Description: Selectively oxidized vertical cavity surface emitting lasers (VCSELS) have been studied by spectrally resolved near field scanning optical microscopy (NSOM). We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and significantly different from those measured in the far field.
Date: December 9, 1999
Partner: UNT Libraries Government Documents Department

Ni spin switching induced by magnetic frustration in FeMn/Ni/Cu(001)

Description: Epitaxially grown FeMn/Ni/Cu(001) films are investigated by Photoemission Electron Microscopy and Magneto-Optic Kerr Effect. We find that as the FeMn overlayer changes from paramagnetic to antiferromagnetic state, it could switch the ferromagnetic Ni spin direction from out-of-plane to in-plane direction of the film. This phenomenon reveals a new mechanism of creating magnetic anisotropy and is attributed to the out-of-plane spin frustration at the FeMn-Ni interface.
Date: March 8, 2009
Creator: Wu, J.; Choi, J.; Scholl, A.; Doran, A.; Arenholz, E.; Hwang, Chanyong et al.
Partner: UNT Libraries Government Documents Department

Revealing Charge Density Wave Formation in the LaTe2 System byAngle Resolved Photoemission Spectroscopy

Description: We present the first direct study of charge density wave(CDW) formation in quasi-2D single layer LaTe2 using high-resolutionangle resolved photoemission spectroscopy (ARPES) and low energy electrondiffraction (LEED). CDW formation is driven by Fermi surface (FS)nesting, however characterized by a surprisingly smaller gap (~;50 meV)than seen in the double layer RTe3 compounds, extending over the entireFS. This establishes LaTe2 as the first reported semiconducting 2D CDWsystem where the CDW phase is FS nesting driven. In addition, the layerdependence of this phase in the tellurides and the possible transitionfrom a stripe to a checkerboard phase is discussed.
Date: November 15, 2006
Creator: Garcia, D. R.; Gweon, G.-H.; Zhou, S. Y.; Graf, J.; Jozwiak, C. M.; Jung, M. H. et al.
Partner: UNT Libraries Government Documents Department

Switching a magnetic vortex by interlayer coupling in epitaxially grown Co/Cu/Py/Cu(001) trilayer disks

Description: Epitaxial Py/Cu/Co/Cu(001) trilayers were patterned into micron sized disks and imaged using element-specific photoemission electron microscopy. By varying the Cu spacer layer thickness, we study how the coupling between the two magnetic layers influences the formation of magnetic vortex states. We find that while the Py and Co disks form magnetic vortex domains when the interlayer coupling is ferromagnetic, the magnetic vortex domains of the Py and Co disks break into anti-parallel aligned multidomains when the interlayer coupling is antiferromagnetic. We explain this result in terms of magnetic flux closure between the Py and Co layers for the antiferromagnetic coupling case.
Date: July 16, 2010
Creator: Wu, J.; Carlton, D.; Oelker, E.; Park, J. S.; Jin, E.; Arenholz, 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

The Electron Emission Characteristics of Aluminum, Molybdenum and Carbon Nanotubes Studied by Field Emission and Photoemission.

Description: The electron emission characteristics of aluminum, molybdenum and carbon nanotubes were studied. The experiments were setup to study the emission behavior as a function of temperature and exposure to oxygen. Changes in the surface work function as a result of thermal annealing were monitored with low energy ultra-violet photoelectron spectroscopy for flat samples while field emission energy distributions were used on tip samples. The change in the field emission from fabricated single tips exposed to oxygen while in operation was measured using simultaneous Fowler-Nordheim plots and electron energy distributions. From the results a mechanism for the degradation in the emission was concluded. Thermal experiments on molybdenum and aluminum showed that these two materials can be reduced at elevated temperatures, while carbon nanotubes on the other hand show effects of oxidation. To purely reduce molybdenum a temperature in excess of 750 ºC is required. This temperature exceeds that allowed by current display device technology. Aluminum on the other hand shows reduction at a much lower temperature of at least 125 ºC; however, its extreme reactivity towards oxygen containing species produces re-oxidation. It is believed that this reduction is due to the outward diffusion of aluminum atoms through the oxide. Carbon nanotubes on the other hand show signs of oxidation as they are heated above 700 ºC. In this case the elevated temperatures cause the opening of the end caps allowing the uptake of water. Oxygen exposure experiments indicate that degradation in field emission is two-fold and is ultimately dependent on the emission current at which the tip is operated. At low emission currents the degradation is exclusively due to oxidation. At high emission currents ion bombardment results in the degradation of the emitter. In between the two extremes, molybdenum tips are capable of stable emission.
Date: December 2002
Creator: Sosa, Edward Delarosa
Partner: UNT Libraries

Absence of X-point band overlap in divalent hexaborides and variability of the surface chemical potential

Description: Angle-resolved photoemission measurements of divalent hexaborides reveals a &gt;1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. While the global ARPES band structure and gap size observed are consistent with the results of bulk-sensitive soft x-ray absorption and emission boron K-edge spectroscopy, the surface-sensitive photoemission measurements also show a variation with cation, surface and time of the position of the surface chemical potential in the band structure.
Date: November 4, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Mo, Sung-Kwan; Allen, James W.; Sarrao, John L.; Bianchi, Adrian D. et al.
Partner: UNT Libraries Government Documents Department