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ALSNews           Vol. 80           June 25, 1997

Table of Contents


    1. OPERATIONS UPDATE
    2. EXPERIMENT VERIFIES QUASI-PARTICLE MODEL FOR DIAMOND
    3. ACCELERATOR PHYSICISTS ESTABLISH GOLDEN ORBIT
    4. MAXIMUM SEES ITS FIRST UNDULATOR LIGHT

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 88.7% overall and 88.2% for user shifts. All outages were of short duration.

Operations Summary for June 25 - July 14

Jun 25, 00:00-08:00           User scrubbing & special operations
Jun 25, 08:00- Jun 30, 07:15  1.9-GeV/400-mA/288-bunch user operations
Jun 30, 07:30-24:00           Maintenance & startup
Jul 01, 00:00-24:00           Accelerator physics
Jul 02, 00:00-08:00           User scrubbing & special operations
Jul 02, 08:00- Jul 03, 23:15  1.9-GeV/400-mA/288-bunch user operations
Jul 04, 00:00-24:00           Holiday
Jul 05, 00:00-08:00           Startup
Jul 05, 08:00- Jul 07, 07:15  1.9-GeV/400-mA/288-bunch user operations
Jul 07, 07:30-24:00           Maintenance & startup
Jul 08, 00:00-24:00           Accelerator physics
Jul 09, 00:00-08:00           User scrubbing & special operations
Jul 09, 08:00- Jul 14, 07:15  1.9-GeV/400-mA/288-bunch user operations

2. EXPERIMENT VERIFIES QUASI-PARTICLE MODEL FOR DIAMOND
(contacts: terminello1@llnl.gov, himpsel@comb.physics.wisc.edu)

Researchers using angle-resolved photoelectron spectroscopy on Beamline 8.0.1 have reported the first quantitative experimental verification of the theoretical electronic structure of diamond. This result for diamond, a semiconductor with a relatively simple electronic structure, helps pave the way for work with more complex materials such as high-temperature superconductors. Such work leads in turn toward the design of advanced materials with custom properties.

Electrons in solids may be tightly bound to individual atomic nuclei (core electrons) or may have quantum states extending throughout the volume of a sample (valence electrons). A delocalized state is characterized by its discrete "crystal momentum" vector and its energy. In samples with large numbers of atoms, closely spaced momentum and energy values form continuous structures called energy bands. Conventional band theorists calculate bands for the ground state, in which only the valence states with lowest energy are occupied. They also regard each electron as behaving independently of other electrons. This is called the one-electron approximation.

While useful in many simpler situations, this approximation breaks down in many situations involving the behavior of electrons in solids. This can result in faulty calculations of critical parameters for the solids' large-scale properties. Thus, more complex, "many-electron" theories, and tools that can evaluate them by measuring the momenta and energies of valence states, are necessary.

One such tool is angle-resolved valence-band photoelectron spectroscopy, which uses ultraviolet and soft-x-ray photons to excite electrons from initial valence states to final states in empty higher-energy bands, from which they can travel to the surface and escape. The experimenters worked in constant-final-state mode, measuring only photoelectrons with a fixed kinetic energy (all of which come from final states with the same energy). By varying the photon energy, they could sample different initial valence states. The direction of the photoelectrons provided information about their momentum distributions in directions parallel to the sample surface. Stacking the momentum distributions for each photon energy in a computerized image, they could display the energy-momentum relationship (band structure) in these directions.

It might seem that the exciting photon energy is simply the energy difference between the initial and final states calculated by one-electron band theory, but this is not quite correct. Instead, excitation leaves an empty state or hole in the valence band, which is effectively positively charged relative to the ground state. According to many-electron theories, the charge redistribution around the hole and the excited electron forms a "quasi-particle" which affects the energy of all electronic states and must be considered to make accurate calculations.

The experiments at Beamline 8.0.1 verified the quasi-particle model for diamond. In particular, they showed that the energy difference between the highest- and lowest-energy states in diamond's valence band (called the band width and measured to be 23.0 +/- 0.2 eV) agreed with quasi-particle calculations by two research groups but disagreed with one-electron calculations, which predicted band widths about 1.5 eV smaller than measured.

The band width may be the single most important parameter in the electronic structure of a solid; for example, when the band width is small compared to the energy associated with electrostatic repulsion between neighboring electrons, each electron tends to localize around one atom, because this is the easiest way for the electrons to avoid each other. Some of the most interesting materials today, including high-temperature superconductors, have narrow band widths and electrons that straddle the boundary between localized and delocalized behavior. The theory for such narrow-band materials is not well established, and its development will require reliable experimental methods to measure key parameters.

Collecting the large quantity of data required for accurate band mapping was aided by the large area detector (display or ellipsoidal mirror analyzer) used. This device images photoelectrons emitted within an 84-degree cone from the sample onto a channel-plate detector, preserves angular information, and offers energy selectivity. Thus it simultaneously records isoenergetic electrons emerging in different directions. This approach, combined with the high brightness of the ALS, allowed diffraction patterns to be recorded in only a few seconds each.

This research was conducted by I. Jiménez, L.J. Terminello (co-principal investigator), D.G.J. Sutherland, and J.A. Carlisle (Lawrence Livermore National Laboratory); E.L. Shirley (National Institute of Standards and Technology); and F.J. Himpsel (co-principal investigator, University of Wisconsin at Madison) using the display analyzer at Beamline 8.0.1. Funding was provided by the U. S. Department of Energy (Contract No. W-7405-ENG-48), the National Science Foundation (Award No. DMR-9632527), and the Spanish Ministerio de Educación y Ciencia.

3. ACCELERATOR PHYSICISTS ESTABLISH GOLDEN ORBIT
(contact: R_Keller@lbl.gov)

In order to improve consistency in electron beam position from one operating condition to another, the Accelerator Physics Group has established an absolute orbit for all storage ring conditions. This "golden orbit" is intended to give users a constant source position from week to week, even when the storage ring energy changes.

Before the establishment of the golden orbit, the beam orbit was corrected independently for different operating conditions, allowing the beam to move up to 0.5 mm between 1.5 GeV and 1.9 GeV operation, for example. After the last shutdown, however, improvements in beam stability and the verification of the reliability and precision of the new insertion device beam position monitors (ID-BPMs) made it possible to define an absolute orbit that appears in identical positions in all existing ID-BPMs. This change in orbit policy also depended upon the successful calibration, through "beam-based alignment," of the outer two BPMs of every arc sector. In these locations, the magnetic centers of the QF and QD quadrupole magnets, which straddle the straight sections, determine the ideal beam positions.

Because the new orbit corrections are based on measurements by ID-BPMs, users of insertion device beamlines will have the greatest increase in week-to-week orbit stability. Users of bend magnet beamlines will also see an improvement, however, because of the more consistent beam settings around the ring.

4. MAXIMUM SEES ITS FIRST UNDULATOR LIGHT
(contact: glorusso@grace.lbl.gov)

At 11:00 p.m. on June 13, after two days of concentrated team efforts to get x rays through its main chamber and objective, the Multiple-Application X-Ray Imaging Undulator Microscope (MAXIMUM) saw the bright undulator light of Beamline 12.0.1.2 for the first time. The arrival of x rays on target was marked by fluorescent-screen images, by photoelectrons recorded on the instrument's cylindrical mirror analyzer, and by the loud yells of excited team members.

MAXIMUM is a scanning photoelectron microscope based on Schwarzschild objectives with multilayer coated mirrors tuned for 130 eV. Originally used at the Synchrotron Radiation Center in Wisconsin, it moved in 1995 to bend-magnet Beamline 6.3.2 at the ALS, and now it will receive its brightest light ever at undulator Beamline 12.0.1.2. With undulator light, near-diffraction-limited conditions (spot size < 50 nm and energy resolution < 250 meV) should be attainable. Data acquisition time and signal-to-noise ratios are also expected to improve.

In recent years MAXIMUM has been used to investigate a variety of scientific and technological problems, including valence band discontinuity in semiconductor heterojunctions, formation of silicides in patterned microstructures (see ALSNews Vol. 67, December 11, 1996), and electromigration in metallic interconnects. The emphasis for MAXIMUM's research program in its new home will remain on materials issues of technological importance.

The MAXIMUM core team includes researchers from the Center for X-Ray Optics (Berkeley Lab) and the Center for X-Ray Lithography (University of Wisconsin-Madison). Many ALS staff members assisted in the commissioning of Beamline 12.0.1.2 and of MAXIMUM. Gian Franco Lorusso (University of Wisconsin-Madison, glorusso@grace.lbl.gov) is responsible for operation of MAXIMUM at the ALS.
Funding for MAXIMUM and Beamline 12.0.1.2: National Science Foundation, U.S. Department of Energy.