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EVIDENCE FOR THE ITINERANT ELECTRON MODEL OF FERROMAGNETISM AND FOR SURFACE PHOTOEMISSION FROM ANGLE-RESOLVED PHOTOEMISSION STUDIES OF IRON

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

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

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 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

On the electronic configuration in Pu: spectroscopy and theory

Description: Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy-loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory have been used to study the electron configuration in Pu. These methods suggest a 5f{sup n} configuration for Pu of 5 {le} n < 6, with n {ne} 6, contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission and x-ray absorption spectra. Instead, these spectra support the traditional conjecture of a 5f{sup 5} configuration in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.
Date: October 11, 2006
Creator: Tobin, J G; Soderlind, P; Landa, A; Moore, K T; Schwartz, A J; Chung, B W et al.
Partner: UNT Libraries Government Documents Department

Synchrotron Radiation Photoemission Spectroscopic Study of Band Offsets and Interface Self-cleaning by Atomic Layer Deposited HfO2 on In0.53Ga0.47As and In0.52Al0.48As

Description: The Synchrotron Radiation Photoemission Spectroscopic (SRPES) study was conducted to (a) investigate the surface chemistry of In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.48}As post chemical and thermal treatments, (b) construct band diagram and (c) investigate the interface property of HfO{sub 2}/In{sub 0.53}Ga{sub 0.47}As and HfO{sub 2}/In{sub 0.52}Al{sub 0.48}As. Dilute HCl and HF etch remove native oxides on In{sub 0.53}Ga{sub 0.47}As and In{sub 0.52}Al{sub 0.47}As, whereas in-situ vacuum annealing removes surface arsenic pile-up. After the atomic layer deposition of HfO{sub 2}, native oxides were considerably reduced compared to that in as-received epi-layers, strongly suggesting the self-clean mechanism. Valence and conduction band offsets are measured to be 3.37 {+-} 0.1eV, 1.80 {+-} 0.3eV for In{sub 0.53}Ga{sub 0.47}As and 3.00 {+-} 0.1eV, 1.47 {+-} 0.3eV for In{sub 0.52}Al{sub 0.47}As, respectively.
Date: October 31, 2008
Creator: Kobayashi, Masaharu; /SLAC, SSRL; Chen, P.T.; Sun, Y.; Goel, N.; Majhi, P. et al.
Partner: UNT Libraries Government Documents Department

Time-Resolved Single-State Measurements of the Electronic Structure of Isochoric Heated Copper

Description: Time-resolved x-ray photoelectron spectroscopy is used to probe the non-steady-state evolution of the valence band electronic structure of laser heated ultra-thin (50 nm) Cu. Single-shot x-ray laser induced time-of-flight photoelectron spectroscopy with picosecond time resolution is used in conjunction with optical measurements of the disassembly dynamics that have shown the existence of a metastable liquid phase in fs-laser heated Cu foils persisting 4-5 ps. This metastable phase is studied using a 527 nm wavelength 400 fs laser pulse containing 0.1-2.5 mJ laser energy focused in a large 500 x 700 {micro}m{sup 2} spot to create heated conditions of 0.07-1.8 x 10{sup 12} W cm{sup -2} intensity. Valence band photoemission spectra showing the changing occupancy of the Cu 3d level with heating are presented. These are the first picosecond x-ray laser time-resolved photoemission spectra of laser-heated ultra-thin Cu foil showing changes in electronic structure. The ultrafast nature of this technique lends itself to true single-state measurements of shocked and heated materials.
Date: October 22, 2004
Creator: Nelson, A J; Dunn, J; Widmann, K; Ao, T; Ping, Y; Hunter, J et al.
Partner: UNT Libraries Government Documents Department

Anomalous momentum dependence of the quasiparticle scattering ratein overdoped Bi2Sr2CaCu2O8

Description: The question of the anisotropy of the electron scattering in high temperature superconductors is investigated using high resolution angle-resolved photoemission data from Pb-doped Bi2Sr2CaCu2O8(Bi2212) with suppressed superstructure. The scattering rate of low energy electrons along two bilayer split pieces of the Fermi surface is measured (via the quasiparticle peak width), and no increase of scattering towards the antinode (Pi,0) region is observed, contradicting the expectation from Q=(Pi, Pi) scattering. The results put a limit on the effects of Q=(Pi, Pi) scattering on the electronic structure of this overdoped superconductor with still very high Tc.
Date: October 14, 2002
Creator: Bogdanov, P. V.; Lanzara, A.; Zhou, X. J.; Yang, W. L.; Eisaki, H.; Hussain, Z. et al.
Partner: UNT Libraries Government Documents Department

Investigation of the Quantrum Structure of Surfaces with Far UV Excitation Spectrosscopies

Description: Clean surfaces and those with adsorbates have been investigated to obtain electronic and atomic structure data. The various modes of synchrotron radiation (SR) photoemission spectroscopy (PES) have been performed mostly beam line 7.0.1 at the Advanced Light Source (ALS) in Berkeley, CA and some at the Synchrotron Radiation Center (SRC) in Madison, WI. These were done on silicon, carbon, and group III nitrides.
Date: October 17, 2008
Creator: Lapeyre, Gerlad
Partner: UNT Libraries Government Documents Department

Spin-Orbit Effects in Spin-Resolved L2,3 Core Level Photoemission of 3d Ferromagnetic Thin Films

Description: We present spin-resolved 2p core level photoemission for the 3d transition metal films of Fe and Co grown on Cu(100). We observe clear spin asymmetry in the main 2p core level photoemission peaks of Fe and Co films consistent with trends in the bulk magnetic moments. The spin polarization can be strongly enhanced, by variation of the experimental geometry, when the photoemission is undertaken with circularly polarized light, indicating that spin-orbit interaction can have a profound in spin polarized photoemission. Further spin polarized photoemission studies using variable circularly polarized light at high photon energies, high flux are indicated, underscoring the value of synchrotron measurements at facilities with increased beam stability.
Date: October 2, 2007
Creator: Komesu, T; Waddill, G D; Yu, S W; Butterfield, M & Tobin, J G
Partner: UNT Libraries Government Documents Department

Standing-wave excited soft x-ray photoemission microscopy: application to Co microdot magnetic arrays

Description: We demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM), with application to a square array of circular magnetic Co microdots. The method is based on excitation with soft x-ray standing-waves generated by Bragg reflection from a multilayer mirror substrate. Standing wave is moved vertically through sample simply by varying the photon energy around the Bragg condition. Depth-resolved PEEM images were obtained for all of the observed elements. Photoemission intensities as functions of photon energy were compared to x-ray optical calculations in order to quantitatively derive the depth-resolved film structure of the sample.
Date: October 29, 2010
Creator: Gray, Alexander; Kronast, Florian; Papp, Christian; Yang, See-Hun; Cramm, Stefan; Krug, Ingo P. et al.
Partner: UNT Libraries Government Documents Department

Bulk band gaps in divalent hexaborides: A soft x-ray emission study

Description: Boron K-edge soft x-ray emission and absorption are used to address the fundamental question of whether divalent hexaborides are intrinsic semimetals or defect-doped bandgap insulators. These bulk sensitive measurements, complementary and consistent with surface-sensitive angle-resolved photoemission experiments, confirm the existence of a bulk band gap and the location of the chemical potential at the bottom of the conduction band.
Date: October 3, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Allen, James W.; Bianchi, Andrea D. & Fisk, Zachary
Partner: UNT Libraries Government Documents Department

A polarization study of strained GaAs photocathode structures

Description: The polarized electron source at SLAC has performed extremely well during recent years supplying electrons having a spin polarization of 78% (85%) for high (low) current operation with beam current limited primarily by experimental requirements. However, there is room for improvement in the electron polarization. The less-than-ideal polarizations are a result of both imperfections and depolarizing mechanisms within the photocathode. The structure of the photocathode used at SLAC in the polarized electron source is a single-strained emitting layer structure grown atop a GaAs substrate. Here, the properties of several types of strained GaAs and GaAsP photocathodes have been studied using x-ray diffraction and photoemission.
Date: October 1, 1996
Creator: Mulhollan, G.; Garwin, E.L.; Maruyama, T.; Tang, H.; Mair, R. & Prepost, R.
Partner: UNT Libraries Government Documents Department

Recent progress in photo-injectors

Description: In photoinjector electron guns, electrons are emitted from a photocathode by a short laser pulse and then accelerated by intense RF fields in a resonant cavity. Photoinjectors are very versatile tools. Normally we think of them in terms of the production of high electron density in 6-D phase space, for reasons such as injection to laser accelerators, generation of x-rays by Compton scattering and short wavelength FELs. Another example for the use of photo-injectors is the production of a high charge in a short time, for wake- field acceleration, two-beam accelerators and high-power, long-wavelength FELs. There are other potential uses, such as the generation of polarized electrons, compact accelerators for industrial applications and more. Photoinjectors are in operation in many electron accelerator facilities and a large number of new guns are under construction. The purpose of this work is to present some trend setting recent results that have been obtained in some of these laboratories. In particular the subjects of high density in 6-D phase space, new diagnostic tools, photocathode advances and high-charge production will be discussed.
Date: October 1, 1996
Creator: Ben-Zvi, I.
Partner: UNT Libraries Government Documents Department

Table top, pulsed, relativistic electron gun with GV/m gradient

Description: We present the design and performance characteristics of a compact high voltage pulser with 150 ps rise time, 0.2 to 2 ns adjustable flat top and up to 1 MV amplitude on a 80 Ohm load or up to 0.5 W on a 20 Ohm load, at 1 Hz repetition rate. Combination of a laser triggered SF{sub 6} and a liquid cap is used to form the fast rising pulse and maintain a low jitter between the laser, external trigger, and the high voltage pulsed output. The dark current and breakdown studies with this pulse applied between the electrodes of a diode indicate that fields up to 1 GV/m could be supported by stainless steel and copper cathodes without breaking down. The dark current from a conditioned cathode in a background pressure of 10{sup -7} Torr is below the detection limit of 0.5 mA of our system. Photoemission studies had been conducted with 300 kV applied between copper cathode and stainless steel anode separated by 2 mm. KrF laser of 5 eV photon energy and 20 ns FWHM was used to irradiate the cathode. In these preliminary measurements, 3 nC charge and corresponding quantum efficiency of 3.5 x 10{sup -4} have been obtained. Future plans include increasing the gradient to GV/m range, decreasing the laser pulse duration to ps and subps range and increasing the electron energy to a few MeV.
Date: October 1, 1996
Creator: Srinivasan-Rao, T. & Smedley, J.
Partner: UNT Libraries Government Documents Department

HIGH RESOLUTION PHOTOEMISSION STUDIES OF COMPLEX MATERIALS.

Description: Recent instrumentation developments in photoemission are providing new insights into the physics of complex materials. With increased energy and momentum resolution, it has become possible to examine in detail different contributions to the self-energy or inverse lifetime of the photohole created in the photoexcitation process. Employing momentum distribution and energy distribution curves, a detailed study of the optimally doped cuprate, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}, shows that the material behaves like a non-Fermi liquid with no evidence for the quasi-particles characteristic of a Fermi liquid.
Date: October 13, 1999
Creator: JOHNSON,P.D.; VALLA,T.; FEDOROV,A.; REISFELD,G. & HULBERT,S.L.
Partner: UNT Libraries Government Documents Department

HIGH RESOLUTION PHOTOEMISSION STUDIES OF COMPLEX MATERIALS.

Description: Recent instrumentation developments in photoemission are providing new insights into the physics of complex materials. With increased energy and momentum resolution, it has become possible to examine in detail different contributions to the self-energy or inverse lifetime of the photohole created in the photoexcitation process. Employing momentum distribution and energy distribution curves, a detailed study of the optimally doped cuprate, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}, shows that the material behaves like a non-Fermi liquid with no evidence for the quasi-particles characteristic of a Fermi liquid.
Date: October 13, 1999
Creator: JOHNSON,P.D.; VALLA,T.; FEDOROV,A.; REISFELD,G. & HULBERT,S.L.
Partner: UNT Libraries Government Documents Department

Photocharge transport and recombination measurements in amorphous silicon films and solar cells by photoconductive frequency mixing. Annual subcontract report, May 13, 1994--May 12, 1995

Description: The continuous decay of electron drift mobility in intrinsic a-Si:H and a-SiC:H upon light soaking was investigated by the photomixing technique. The photoconductivity, lifetime and drift mobility in intrinsic hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon carbide (a-SiC:H) while light-soaking were determined using a photomixing technique. In addition to the decay of the photoconductivity and electron lifetime, continuous decay of the electron drift mobility was found during the light soaking process, which reveals a new phenomenon associated with the Staebler-Wronski effect. The drift mobility decreased by a factor of 2 for 20 hour light soaking at 2.5 sun intensity. Experimental data were fitted to a stretched exponential law. Different stretched-exponential parameters for photoconductivity, lifetime and drift mobility were obtained, which indicates the production of defects with different generation kinetics upon light soaking.
Date: October 1, 1995
Creator: Braunstein, R.; Yang, Y. & Dong, S.
Partner: UNT Libraries Government Documents Department

First results from the high-brightness x-ray spectroscopy beamline 9. 3.1 at ALS

Description: Beamline 9.3.1 at the Advanced Light Source (ALS) is a windowless beamline, covering the 1-6 keV photon-energy range. This beamline is designed to achieve the goal of high brightness at the sample for use in the X-ray Atomic and Molecular Spectroscopy (XAMS) science, surface and interface science, biology, and x-ray optical development programs at ALS. X-ray absorption and time of flight photoemission measurements in 2 - 5 keV photon energy along with the flux, resolution, spot size and stability of the beamline will be discussed. Prospects for future XAMS measurements will also be presented.
Date: October 1, 1995
Creator: Ng, W.; Jones, G. & Perera, R.C.C.
Partner: UNT Libraries Government Documents Department

HIGH RESOLUTION PHOTOEMISSION STUDIES OF COMPLEX MATERIALS.

Description: Recent instrumentation developments in photoemission are providing new insights into the physics of complex materials. With increased energy and momentum resolution, it has become possible to examine in detail different contributions to the self-energy or inverse lifetime of the photohole created in the photoexcitation process, Employing momentum distribution and energy distribution curves, a detailed study of the optimally doped cuprate, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub g+{delta}}, shows that the material behaves like a non-Fermi liquid with no evidence for the quasi-particles characteristic of a Fermi liquid.
Date: October 13, 1999
Creator: JOHNSON,P.D.
Partner: UNT Libraries Government Documents Department

Electron transfer and physical and chemical processes at low temperatures

Description: We summarize some phenomena that occur at temperatures of the order of 15K, and are dominated by quantum mechanical tunneling. Although electron tunneling dominates many conduction processes at low temperatures, we discuss evidence that phenomena like oxidation, as well as the solution of alkali metals in ammonia, can also be dominated by electron tunneling. Both phenomena demonstrate that the chemical potential of a metastable system can equilibrate at low temperatures by electron tunneling. The case of alkali metal clusters covered with ammonia is contrasted to covering the clusters with Xe. In this case changes in the activated conduction are observed which are consistent with the dielectric constant of the rare gas.
Date: October 1, 1995
Creator: Strongin, M.; Xia, B. & Jacobsen, F.M.
Partner: UNT Libraries Government Documents Department

DEVELOPMENT OF NEXT-GENERATION DETECTORS AND INSTRUMENTATION FOR PHOTOELECTRON SPECTROSCOPY, DIFFRACTION AND HOLOGRAPHY

Description: We have developed a new multichannel detector for use in photoelectron spectroscopy (as well as other types of high-count-rate spectroscopy) that will operate at rates of up to 1 GHz. Such detectors are crucial to the full utilization of the high-brightness radiation generated by third-generation synchrotron radiation sources. In addition, new software and hardware has been developed to permit rapidly and accurately scanning photoelectron spectra that will be accumulated in as little as a 200 micros. A versatile next-generation sample goniometer permitting equally rapid scanning of specimen angles or photon energies for angle-resolved photoemission studies, photoelectron diffraction, and photoelectron holography measurements, and cooling to below 10K has also been designed and constructed. These capabilities have been incorporated into a unique photoelectron spectrometer/diffractometer at the Advanced Light Source of the Lawrence Berkeley National Laboratory; this experimental system includes ultrahigh energy resolution, in situ rotation, variable polarization, and optional spin detection. This overall system is now being used in studies of a variety of problems including magnetic metals and oxides; metal/metal, metal/metal oxide, and metal-oxide/metal-oxide multilayers; and systems exhibiting giant and colossal magnetoresistance.
Date: October 16, 2005
Creator: Charles S. Fadley, Principal Investigator
Partner: UNT Libraries Government Documents Department

A revolutionary rotatable electron energy analyzer for advanced high-resolution spin-polarized photoemission studies. Final Report

Description: This report details the construction and testing of a unique analyzer for spin-polarized photoemission studies of magnetic materials. This report details the progress of this project for the period from 9/1/96 through 8/31/99. Progress can be divided into two distinct areas. These are the fabrication, construction, and initial testing of the instrumentation, and the concurrent program of preliminary investigations into materials and experiments appropriate for future studies using the instrumentation developed. The analyzer complete with special input electron optics and Mott detector has been assembled in a special design UHV chamber equipped with all the capabilities needed to perform the described programs of research. These include a sophisticated five motorized axis sample manipulator with low and high temperature capability and rapid temperature cycling (acquired in collaboration with Dr. J.G. Tobin of LLNL), vacuum leak detection and gauging, in situ thin film growth instrumentation, and sample cleaning and magnetizing capabilities, The initial testing of the analyzer has been completed with successful data acquisition using both the multichannel detector mode, and spin-dependent using the Mott detector channeltrons. The data collected using the Mott detector were not truly spin dependent (see below), but demonstrate the operation of the lens and detector design. Acquisition of truly spin-dependent data await use of the EPU. Preliminary indications suggest that the analyzer performs at or above the original design parameters. In the second area of progress, we have conducted a number of preliminary studies toward the ends of identifying appropriate initial systems for investigation, and to further explore new experiments that the new instrumentation will help to pioneer. More detailed descriptions of all of these advances are given.
Date: October 1, 1999
Creator: Waddill, G. D. & Willis, R. F.
Partner: UNT Libraries Government Documents Department

Surface-Sensitive, Element-Specific Magnetometry with X-Ray Linear Dichroism

Description: Here it is shown that the magnetic linear dichroism in x-ray photoemission (XMLD) signal can be used to measure the element specific magnetic moments in ultra thin alloy films. Comparison with recent SQUID data provides a quantitative check that demonstrates that the total magnetization derived from summing the constituent elemental moments is correct.
Date: October 6, 1999
Creator: Schumann, F.O.; Willis, R.F. & Tobin, J.G.
Partner: UNT Libraries Government Documents Department