347 Matching Results

Search Results

Advanced search parameters have been applied.

Bosonic mechanism for high-temperature

Description: The temperature dependent photoemission data of D. S. Dessau et al. show strong modulations in the superconducting state when compared to the normal state. These are similar to but larger than those seen historically in standard tunneling experiments in lower temperature superconductors. We have analyzed the Dessau data using Nambu-Eliashberg theory assuming some (as yet unknown) boson exchange as the primary mechanism for the superconductivity. The derived {alpha}{sup 2}F's, {lambda}'s and {mu}*'s show features which resemble those derived from inversions of other low-temperature superconductors, albeit that {lambda} here is about 8.67 and {mu}* is approximately 0.15. Several bosonic mechanisms are considered. 29 refs., 3 figs.
Date: January 1, 1991
Creator: Mueller, F.M. (Los Alamos National Lab., NM (USA)); Arnold, G.B. (Notre Dame Univ., IN (USA). Dept. of Physics) & Swihart, J.C. (Indiana Univ., Bloomington, IN (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

Photoelectron diffraction of magnetic ultrathin films: Fe/Cu(001)

Description: The preliminary results of an ongoing investigation of Fe/Cu(001) are presented here. Energy dependent photoelectron diffraction, including the spin-dependent variant using the multiplet split Fe3s state, is being used to investigate the nanoscale structures formed by near-monolayer deposits of Fe onto Cu(001). Core-level photoemission from the Fe3p and Fe3s states has been generated using synchrotron radiation as the tunable excitation source. Tentatively, a comparison of the experimental Fe3p cross section measurements with multiple scattering calculations indicates that the Fe is in a fourfold hollow site with a spacing of 3.6{Angstrom} between it and the atom directly beneath it, in the third layer. This is consistent with an FCC structure. The possibility of utilizing spin-dependent photoelectron diffraction to investigate magnetic ultrathin films will be demonstrated, using our preliminary spectra of the multiplet-split Fe3s os near-monolayer Fe/Cu(001). 18 refs., 10 figs.
Date: January 3, 1991
Creator: Tobin, J.G. (Lawrence Livermore National Lab., CA (USA)); Wagner, M.K. (Wisconsin Univ., Madison, WI (USA). Dept. of Chemistry); Guo, X.Q. & Tong, S.Y. (Wisconsin Univ., Milwaukee, WI (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

A photoemission investigation of compound semiconductor monodisperse clusters

Description: We have used synchrotron radiation photoemission to probe the valence and core level electronic structure of compound-semiconductor monodisperse clusters (nanocrystals). These clusters exhibited a 10% or less variation relative to the mean diameter and were attached to the metal substrates via alkane chains. Direct evidence of gap broadening due to size variation in CdS clusters was observed. The novel utilization of alkane chain attachment is the key to eliminating the otherwise debilitating problem of sample charging, as occurs with powders. The quality of sample preparation was confirmed by other methods such as transmission electron microscopy, Raman scattering and x-ray diffraction. This work provides a direct link between photoemission studies of expitaxial ultra-thin films of compound semiconductors, the photon-spectroscopy measurements of cluster powders and the existing theories of quantum confinement in reduced dimensionality structures. 5 refs., 2 figs.
Date: December 7, 1990
Creator: Tobin, J.G. (Lawrence Livermore National Lab., CA (USA)); Colvin, V.L. & Alivisatos, A.P. (California Univ., Berkeley, CA (USA))
Partner: UNT Libraries Government Documents Department

High current photoemission with 10 picosecond uv pulses

Description: The quantum efficiency and the optical damage threshold of various metals were explored with 10 ps, 266 nm, UV laser pulses. Efficiencies for Cu, Y, and Sm were: 1.4, 5, and 7 {times} 10{sup {minus}4}, with damage thresholds about 100, 10, and 30 mJ/cm{sup 2}. This would permit over 1 {mu}C/cm{sup 2} or current densities exceeding 100 kA/cm{sup 2}. High charge and current densities of up to 66 kA/cm{sup 2} were obtained on 0.25 mm diam cathodes, and 21 kA/cm{sup 2} on a 3 mm diam yttrium cathode. The maximum currents were limited by space charge and the dc field. The experiments with small area illumination indicate that the emitted electrons spread transversely due to Coulomb repulsion and their initial transverse velocity. This increases the effective area above the cathode, reduces the space charge effect and increases emission density on the cathode. The quantum efficiency can be increased substantially by enhancing the field on the surface by either a suitable electrode geometry or microstructures on it. 14 refs., 12 figs., 3 tabs.
Date: June 1, 1990
Creator: Fischer, J.; Srinivasan-Rao, T. & Tsang, T.
Partner: UNT Libraries Government Documents Department

Ultra-high-vacuum double-axis goniometer for use with an electron spectrometer

Description: A double-axis goniometer designed for moving and indexing an electon spectrometer for angle-resolved photoemission studies is described. A feature of the design is that the two rotations operate independently and either can be carried out with the analyzer at any position. The goniometer is designed to be installed in a 14'' spool piece which makes it possible to add it to an existing 14'' uhv system.
Date: January 1, 1981
Creator: Shleifer, M. & Williams, G.P.
Partner: UNT Libraries Government Documents Department

Design of a transuranic VUV spectrometer

Description: Most of our present knowledge about the physics of 5f systems derives from studies of uranium and its compounds. This is particularly true in the case of photoemission measurements where the intense radioactivity has prevented studies at synchrotron sources. The development of a transuranic VUV spectrometer capable of safe operation at a synchrotron source would represent a giant step in 5f research. This paper describes the conceptual design of such a spectrometer.
Date: January 1, 1987
Creator: Arko, A.J.
Partner: UNT Libraries Government Documents Department

Concentration-dependent surface-state shifts: Au on Cu(001)

Description: High-resolution angle-resolved photoemission has been used to investigate the behavior of the Cu(001) {bar M}{sub 2} surface state as a function of Au coverage and substrate temperature. Binding-energy shifts of this state are dependent on the concentration of Au atoms substituted into the top surface layer of the Cu(001) substrate. The results demonstrate the use of surface-localized Tamm states as a specific probe of the chemical environment in the surface plane. 19 refs., 3 figs.
Date: April 1, 1991
Creator: Hansen, J.C. (Eastman Kodak Research Labs., Rochester, NY (United States)); Wagner, M.K. (Wisconsin Univ., Madison, WI (United States). Dept. of Chemistry) & Tobin, J.G. (Lawrence Livermore National Lab., CA (United States))
Partner: UNT Libraries Government Documents Department

Mechanism of the optogalvanic effect in a hollow-cathode discharge

Description: There are two significantly different mechanisms proposed for the origin of the optogalvanic effect in a hollow-cathode discharge: (1) laser excitation of atoms to higher electronic states leads to an increased cross section for electron impact ionization, with the result that the excited atom becomes ionized and the conductivity of the discharge increases; and (2) laser excitation of atoms to higher electronic states perturbs the equilibrium established between the electron temperature and the atomic excitation temperature. Superelastic collisions between the electrons and the laser-excited atoms restore the equilibrium, with the excess energy ending up in an increased electron temperature and therefore an increased conductivity of the discharge. Both mechanisms undoubtedly proceed simultaneously and what needs to be determined is their relative importance at different discharge conditions and different excitation conditions. This is important because laser isotope enrichment schemes have been proposed using selective excitation in a hollow-cathode discharge. In order for these schemes to work, (1) must be the predominant mechanism. We have measured the optogalvanic signal, concentration of uranium atoms, impedance of the discharge, and electron temperature as a function of the discharge current in a neon-filled uranium hollow-cathode discharge. The hollow cathode operating characteristics are used as input parameters in a simple discharge model. Predictions of electron density, changes in electron temperature, and discharge impedance compare well with experimental observations. Our model and experimental observations yield a qualitative understanding of the optogalvanic effect in a hollow-cathode discharge and estimate the relative importance of the two optogalvanic mechanisms.
Date: June 17, 1983
Creator: Keller, R.A.; Warner, B.E.; Zalewski, E.F.; Dyer, P.; Engleman, R. Jr. & Palmer, B.A.
Partner: UNT Libraries Government Documents Department

Photoemission using femtosecond laser pulses

Description: Successful operation of short wavelength FEL requires an electron bunch of current >100 A and normalized emittance < 1 mm-mrad. Recent experiments show that RF guns with photocathodes as the electron source may be the ideal candidate for achieving these parameters. To reduce the emittance growth due to space charge and RF dynamics effects, the gun may have to operate at high field gradient (hence at high RF frequency) and a spot size small compared to the aperture. This may necessitate the laser pulse duration to be in the subpicosecond regime to reduce the energy spread. We will present the behavior of metal photocathodes upon irradiation with femtosecond laser beams, comparison of linear and nonlinear photoemission, and scalability to high currents. Theoretical estimate of the intrinsic emittance at the photocathode in the presence of the anomalous heating of the electrons, and the tolerance on the surface roughness of the cathode material will be discussed.
Date: October 1, 1991
Creator: Srinivasan-Rao, T.; Tsang, T. & Fischer, J.
Partner: UNT Libraries Government Documents Department

Re-analysis of photoemission data for CuO: Revision of the configuration-energy scheme for cuprate materials

Description: The author has recently carried out a very careful analysis of photoemission and inverse photoemission (BIS) data for CuO, probably the most refined and thorough such analysis for any transition-metal compound to date. This has led to a configuration-energy scheme for cuprate materials which is quite different from a generally-accepted one, due largely to Sawatzky and co-workers. For example, it is commonly believed that the doping holes in La{sub 2-x}Sr{sub x}CuO{sub 4} are hosted almost entirely within oxygen 2p orbitals. He finds instead that these holes are shared roughly equally by 2p and 3d orbitals. In this report he shall sketch the motivations for this study the main results, and some of the lessons and surprises encountered along the way. 22 refs., 1 fig.
Date: January 1, 1991
Creator: Brandow, B.H.
Partner: UNT Libraries Government Documents Department

Synchrotron radiation photoemission studies of core level excitation effects

Description: The recent availability of intense polarized radiation tunable over the range 20-150 eV enables several core levels to be excited. Several remarkable effects have recently been reported when core excitation thresholds have been reached. For example, an increase in yield of 50% was observed in GaSe at the Se 3d threshold (h omega approx. = 57 eV). A considerable increase in photoemission intensity at 6 eV binding energy has been reported by Guillot et al. for Ni at the 3p threshold. A very careful study was made using the CIS, CFS, and EDC modes of photoemission spectroscopy of core threshold effects in Ni, Pt, and GaSe. A modulation of the Ni Fermi edge emission as a function of photon energy is found which is attributed to a configuration interaction between 3p/sup 5/3d/sup 10/4s and 3p/sup 6/3d/sup 8/4s epsilon f. Similar effects occur in Pt at the 4f threshold involving the configurations 4f/sup 13/5d/sup 10/6s and 4f/sup 14/5d/sup 8/6s. Further, the lineshapes of the Pt and Ni upper valence band CIS's resemble those obtained by electron energy loss (ELS) and photoabsorption, all of which can be fitted theoretically to a Fano model. Emission from levels lying well below the Fermi level often has superimposed contributions due to incoherent effects which complicate the interpretation. In cases in which core levels are excited well above threshold, the CIS technique may be used to examinefinal states. Of particular interest is the possible consequent determination of adsorbate site geometry. Data are presented for 2p emission from a c(2x2) Na overlayer on Ni (001).
Date: January 1, 1979
Creator: Williams, G.P.; Lapeyre, G.J.; Anderson, J.; Cerrina, F.; Dietz, R.E. & Yafet, Y.
Partner: UNT Libraries Government Documents Department

Photoemission spectroscopy of single crystal HTSC materials: A Fermi liquid electronic structure

Description: Photoemission spectra from HTSC materials (primarily 123-type), cleaved and measured at 20K, reveal a rich DOS structure which compares favorably with a calculated band structure, except for a residual 0.5 eV shift which may reflect some correlation effects. Band dispersion is observed throughout the valence bands, with clear evidence for a 0.2 eV wide band dispersing through E/sub F/. The orbital character at E/sub F/ is a mix of Cu-3d and O-2p. There is unambiguous evidence for a large BCS-like gap (2..delta.. greater than or equal to 4kT/sub c/). 25 refs., 5 figs.
Date: January 1, 1989
Creator: Arko, A.J.; List, R.S.; Bartlett, R.J.; Cheong, S.W.; Olson, C.G.; Yang, A.B. et al.
Partner: UNT Libraries Government Documents Department

Electron spectroscopy studies in heavy fermions

Description: Photoemission experiments (whereby an electron absorbs a packet of light energy and is able to escape from the host material due to its increased energy) can measure directly the energy distribution of electrons in various materials. Our measurements on a recently-discovered class of metallic materials called ''heavy fermions'' show that the electrons that actually carry the electric current in these metals exist only within an extremely narrow range of energies. This range, which we will call the bandwidth, is narrower than that found in ordinary metals like copper by at least a factor of 10. Indeed it is surprising that they can carry electric current at all since such narrow energy ranges (or band widths) are characteristic of electrons confined to their host atoms, as in a non-metal, rather than of electrons that are free to wander through a metal. 8 refs.
Date: February 1, 1986
Creator: Arko, A.J.
Partner: UNT Libraries Government Documents Department

Effects of variation of the laser pulse width on the laser induced photoelectrons

Description: The effects of the variation of the Laser pulse width on the laser induced photoelectrons, from the BNL gun is presented, including the variation in the emittance of the beam, which is crucial to the operation of a Free Electron Laser (FEL). The radio-frequency (RF) electron gun with a photocathode in the end wall of an RF cavity is excited by a laser pulse that is synchronized with the RF, is powered by a conventional type SLAC klystron, and is designed to produce a low emittance beam of 4.65 MeV at the gun exit with the field gradient of 100 MV/m on the Cathode.
Date: January 1, 1991
Creator: Parsa, Z.
Partner: UNT Libraries Government Documents Department

(Electronic and structural properties of individual nanometer-size supported metallic clusters)

Description: The research supported by this Department of Energy contract has primarily been devoted to the study of the electronic properties of surfaces with sub-micron size. In previous years, we have studied the photoexcitation of electrons from field emission tips by a focussed Argon-ion laser beam tuned to operate at specific photon energy. The photoexcited electrons escape into the vacuum by tunneling through a surface potential barrier which is distorted by the application of a strong electric field. The interest in these experiments lies in a better understanding of the photoexcitation process at low photon energies. The techniques that have been developed directly measure the excited state energy distribution of electrons emitted through the surface potential barrier. The basic information gained from this research is relevant to opto-electronic devices which rely on photoexcitation of electrons in the presence of strong interfacial electric fields for their operation. In addition, the work is very relevant to recent developments in vacuum microelectronic devices that are based on electron emission from arrays of sharp tips.
Date: November 1, 1991
Creator: Reifenberger, R.
Partner: UNT Libraries Government Documents Department

Strain enhanced electron spin polarization observed in photoemision from InGaAs

Description: Electron spin polarization in excess of 70% has been observed in photoemission from a 0.1 {mu}m-thick epitaxial layer of In{sub x}Ga{sub 1-x}As with x {approx} 0.13 grown on a GaAs substrate. Under these conditions, the epitaxial layer is expected to be highly strained by the 0.9% lattice mismatch, as confirmed by X-ray diffractometer measurements of the lattice parameter. The electron polarization and the quantum efficiency have been measured as a function of the excitation photon energy from 1.25 to 2.0 eV. A significant enhancement of the electron polarization occurs in the vicinity of 1.33 eV where the expected strain-induced level splitting permits optical excitation of a single band transition. Measurements made on a control sample of 1.14 {mu}m thickness, significantly larger than the critical thickness for pseudomorphic strain, show no polarization enhancement. These measurements represent the first observation of strain-enhanced electron spin polarization for photoemitted electrons. 7 refs., 3 figs.
Date: May 1, 1991
Creator: Maruyama, T.; Garwin, E.L. (Stanford Linear Accelerator Center, Menlo Park, CA (USA)); Prepost, R.; Zapalac, G.H. (Wisconsin Univ., Madison, WI (USA). Dept. of Physics); Smith, J.S. & Walker, J.D. (California Univ., Berkeley, CA (USA))
Partner: UNT Libraries Government Documents Department

A method of producing high quality oxide and related films on surfaces

Description: Aluminum oxide or aluminum nitride films were deposited on MBE grown GaAs(100) using a novel cryogenic-based reactive thin film deposition technique. The process involves the condensation of molecular oxygen, ammonia or other gases normally used for reactive thin film deposition on the substrate before the metal is deposited. The metal vapor is deposited into this layer and reacts with the molecular solid form the desired compound or a precursor than can be thermally decomposed to generate the desired compound. The films produced by this method are free of impurities and the low temperatures can be used to control the film and interfacial structure. The process can be easily integrated with existing MBE-systems and on going research using the same apparatus suggests than photon or electron irradiation could also be used to promote the reactions needed to give the intended material.
Date: January 1, 1991
Creator: Ruckman, M.W.; Strongin, M. (Brookhaven National Lab., Upton, NY (United States)) & Gao, Yongli (Rochester Univ., NY (United States). Dept. of Physics and Astronomy)
Partner: UNT Libraries Government Documents Department

Dimensionality and its effects upon the valence electronic structure of ordered metallic systems

Description: The system c(10x2)Ag/Cu(001) was investigated with Angle-Resolved Photoemission (ARP), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). LEED and AES provided the calibration of a quartz microbalance used to measure the amount of silver evaporated onto the copper single crystal and also established the monolayer geometrical structure at one monolayer exposure. An off-normal ARP bandmapping study performed with polarized HeI and NeI radiation demonstrated the electronically two-dimensional nature of the silver d-bands at coverages of near one monolayer. The states at the surface Brillouin Zone center were assigned upon the basis of their polarization dependences and a structural model of hexagonal symmetry. A normal emission ARP experiment was performed at the Stanford Synchrotron Radiation Laboratory (SSRL) over the photon energy range of 6 to 32 eV. Data from it documented the evolution of the valence electronic structure of the silver overlayer from a two-dimensional hexagonal valence to a three-dimensional behavior converging towards that of bulk Ag(111). A structural study was attempted using the ARP technique of Normal Emission Photoelectron Diffraction over the photon energy range of 3.4 to 3.7 keV at SSRL, the results of which are inconclusive.
Date: July 1, 1983
Creator: Tobin, J.G.
Partner: UNT Libraries Government Documents Department

Bonding, Shake-up energies and shake-up intensities in Cr(NO)/sub 4/

Description: The results of SCF-X..cap alpha..-multiple scattering calculations of the electronic structure of Cr(NO)/sub 4/ in its neutral ground state and in several positive-ion states are reported. This allows a description of the bonding of NO to the Cr atom, explains how the screening of core holes (created in a XPS measurement) is affected and yields the N/sub ls/ shake-up spectrum. There is good agreement with the experimental valence region photo-emission spectra peak positions and with the skake-up peak positions and intensities.
Date: January 1, 1979
Creator: Loubriel, G.
Partner: UNT Libraries Government Documents Department

High-brightness photoemitter development for electron accelerator injectors

Description: Free-electron-laser (FEL) oscillators require a train of high-brightness bunches. Conventional subharmonic bunchers are currently used with rf linacs to generate pulse trains, but the resulting dilution of the transverse phase space and lower beam brightness are unacceptable for high-performance FELs. Recent developments suggest that photoemitters of high quantum efficiency combined with rapid acceleration can produce pulse trains of higher brightness than has been achieved before.
Date: January 1, 1985
Creator: Fraser, J.S.; Sheffield, R.L. & Gray, E.R.
Partner: UNT Libraries Government Documents Department

Determination of electronic states in crystalline semiconductors and metals by angle-resolved photoemission

Description: An important part of the theoretical description of the solid state is band structure, which relies on the existence of dispersion relations connecting the electronic energy and wavevector in materials with translational symmetry. These relations determine the electronic behavior of such materials. The elaboration of accurate band structures, therefore, is of considerable fundamental and practical importance. Angle-resolved photoemission (ARP) spectroscopy provides the only presently available method for the detailed experimental investigation of band structures. This work is concerned with its application to both semiconducting and metallic single crystals.
Date: August 1, 1979
Creator: Mills, K.A.
Partner: UNT Libraries Government Documents Department

Electronic band structure and photoemission: A review and projection

Description: A brief review of electronic-structure calculations in solids, as a means of interpreting photoemission spectra, is presented. The calculations are, in general, of three types: ordinary one-electron-like band structures, which apply to bulk solids and are the basis of all other calculations; surface modified calculations, which take into account, self-consistently if at all possible, the presence of a vacuum-solid interface and of the electronic modifications caused thereby; and many-body calculations, which go beyond average-field approximations and consider dynamic rearrangement effects caused by electron-electron correlations during the photoemission process. 44 refs.
Date: September 1, 1987
Creator: Falicov, L.M.
Partner: UNT Libraries Government Documents Department

Fermi surface of YBCO by DHVA

Description: These proceedings demonstrate how far scientist have come in the last four years of high temperature superconductivity. Knowledge of the energy bands and Fermi surfaces from experiment has come rather late. Photoemission, first showed proof of the validity of the energy band calculations. Positron annihilation, presented by West, after a rough start, is now giving evidence of the Fermi surface. Both of these techniques involve electronic excitations and hence, although they show the Fermi surface, do not put as severe a constraint on various models for superconductivity as does the de Haas-van Alphen (dHvA) effect. This is a true measurement of the electronic ground state in an applied magnetic field where the frequency of oscillatory magnetization yields extremal cross-sectional areas of the Fermi surface. The authors have already reported some of their Fermi surface work at two conferences but present here discussion of several more important aspects of the work. 11 refs., 2 figs.
Date: January 1, 1991
Creator: Smith, J.L.; Fowler, C.M.; Freeman, B.L.; Hults, W.L.; King, J.C. & Mueller, F.M.
Partner: UNT Libraries Government Documents Department

Superconductivity from valence fluctuations with finite u

Description: The finite-U paring mechanism of Newns is found to be opposed by a magnetic tendency arising from Gutzwiller renormalization of the hybridization. This competition restricts superconductivity and also reproduces the parabolic rise and fall of T/sub c/ in La/sub 2/minus//chi//Sr/sub /chi//CuO/sub 4/ with increasing x. 9 refs.
Date: January 1, 1989
Creator: Brandow, B.H.
Partner: UNT Libraries Government Documents Department