ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 112          September 30, 1998

Table of Contents


    1. QUANTUM INTERFERENCE OBSERVED IN THIN FILMS
    2. USERS' MEETING ADVANCE REGISTRATION DEADLINE OCTOBER 5
    3. MACROMOLECULAR CRYSTALLOGRAPHY BEAM TIME PROPOSALS DUE OCTOBER 12
    4. LONG-TERM SCHEDULE EXTENDED THROUGH MARCH 1999
    5. NOMINATIONS FOR AWARDS PRESENTED AT USERS' MEETING
    6. ALS VACUUM POLICY NOW AVAILABLE ON WEB
    7. WHO'S IN TOWN: A SAMPLING OF ALS USERS
    8. OPERATIONS UPDATE
    9. ERRATUM

1. QUANTUM INTERFERENCE OBSERVED IN THIN FILMS
By Lori Tamura (Contact: qiu@physics.berkeley.edu)

The pressure on the magnetic data storage industry to pack more information into smaller volumes has pushed the study of thin magnetic films to the quantum level. Investigations into the properties of magnetic materials at the nanostructure scale have revealed that, when two layers of ferromagnetic material are separated by a thin nonmagnetic spacer, the relative spin directions of the ferromagnetic layers oscillate as a function of the spacer thickness. Pioneering photoemission experiments at other facilities have shown that this oscillation is associated with quantum well state effects in the spacer. New work at the ALS using wedge-shaped samples has revealed additional, more subtle interference effects.

According to the quantum well states model, when the thickness of the spacer is reduced to a few nanometers, the continuum of electron states normally available becomes quantized into discrete energy levels. The only energies allowed correspond to those in which an integral number of half-wavelengths of the electron's wave function fit into the quantum well of the spacer. As the width of the well (i.e., the spacer) increases, the allowed energy levels in the spacer quantum well will shift. Whenever a quantum well level passes through the Fermi level, the energy of the system is at a maximum. To lower the system energy, the parallel spins of the ferromagnets realign into an antiparallel configuration to dump away the quantum well state at the Fermi level. Therefore, the coupling between the two ferromagnets oscillates as the spacer thickness increases or decreases.

Changes in the thickness of layers other than the spacer would also affect the coupling strength of the two ferromagnets, because the electron waves reflected from all the layer boundaries interfere with the quantum well states in the spacer. Thus, changing the thickness of the ferromagnetic layer changes the quantum well states in the spacer, which in turn determine the coupling strength. Although quantum well states in the spacer can be detected as peaks in photoemission intensity, detecting interference effects, which are easily overwhelmed by thickness variations, requires a series of highly sensitive measurements with very small thickness increments.

The ALS team overcame these difficulties and confirmed the existence of quantum interference in the spacer through an elegant experiment made possible by the small spot size of the ALS beam. Dispensing with the top ferromagnetic layer to access the information in the spacer, the researchers grew a thin wedge of copper (spacer) on top of a thin wedge of cobalt (ferromagnet) on a substrate of nickel. The wedges were layered at right angles to each other, so that their thicknesses could be varied independently by changing the direction of the scan. The thicknesses of the layers vary by a negligible amount across the beam spot. The resulting photoemission patterns clearly show the periodic modulation of the copper quantum well states as a function of both the copper thickness and the cobalt thickness.

The researchers, led by Z.Q. Qiu of the University of California, Berkeley, are currently extending their investigations into looking at the behavior of coupled quantum wells, obtaining simultaneous images of electronic and magnetic effects, and developing a method for directly probing wave functions.

Research conducted by R.K. Kawakami, E.J. Escorcia-Aparicio, H.J. Choi, and Z.Q. Qiu (University of California, Berkeley); E. Rotenberg and N.V. Smith (ALS); and T.R. Cummins and J.G. Tobin (Lawrence Livermore National Laboratory), using the XPD chamber at Beamline 7.0.1.

Funding: U.S. Department of Energy, Division of Materials Sciences, with additional support from the Hellman Family Faculty Fund at Berkeley.

Publication about this experiment: R. K. Kawakami et al., Phys. Rev. Lett. 80, 1754 (1998).

2. USERS' MEETING ADVANCE REGISTRATION DEADLINE OCTOBER 5
(Contact: alsum@lbl.gov)

All those interested in attending this year's ALS Users' Meeting on October 22-23 are encouraged to send in their registration before October 5. The registration fee is $70.00 for students and $120.00 for all other participants. The registration fee covers attendance at all plenary, poster, and vendor sessions, as well as lunches both days and the Thursday evening banquet. Additional guest banquet tickets may be purchased separately for $30.00. Registration forms, hotel information, and a preliminary program are available on the Web at http://www-als.lbl.gov/als/usermtg/index.html.

3. MACROMOLECULAR CRYSTALLOGRAPHY BEAM TIME PROPOSALS DUE OCTOBER 12

The User Services Office is still accepting proposals from scientists who wish to conduct research as independent investigators at the Macromolecular Crystallography Facility (Beamline 5.0) between January and June 1999. The deadline for submissions is October 12, 1998. There will be no automatic rollover of proposals from the previous proposal cycle (July to December 1998). Scientists wishing to renew a previous proposal should notify the ALS User Administrator, Ruth Pepe (contact information below).

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web (http://www-als.lbl.gov/als/quickguide/independinvest.html). Information on the proposal process is available at the same location. A data sheet on the Macromolecular Crystallography Facility provides information that may be useful to prospective users. It is available on the Web as a PDF file at http://www-als.lbl.gov/als/als_users_bl/5.0-Datasheet.pdf. If you do not have Web access and would like to request a data sheet, send an email request to alsuser@lbl.gov. Beamline information is also available on the Web in HTML format at http://www-als.lbl.gov/als/als_users_bl/bl_table.html.

To request a proposal form by mail, contact:
Ruth Pepe, ALS User Administrator
Tel: (510) 486-5268
Fax: (510) 486-4773
Email: alsuser@lbl.gov

For information on beamlines available to independent investigators, contact:
Gary Krebs, ALS User Services Group Leader
Tel: (510) 486-7727
Fax: (510) 486-4102
Email: g_krebs@lbl.gov

4. LONG-TERM SCHEDULE EXTENDED THROUGH MARCH 1999
(Contact: annette_greiner@lbl.gov)

An extended long-term operating schedule for the ALS is now available on the Web at http://www-als.lbl.gov/als/schedules/current_ltsch.html. The schedule covers operations through March 1999 and is available as a Web graphic, in Portable Document Format, as an Excel file, and as a Word document. The page can be reached through the ALS home page by clicking on "Accelerator Information" and following the link to the "July 1998 through March 1999 Schedule."

5. NOMINATIONS FOR AWARDS PRESENTED AT USERS' MEETING
(Contact: W_Meyer-Ilse@lbl.gov)

Three prizes will be awarded during the ALS Users' Meeting. New this year will be the Shirley Prize for outstanding scientific achievement at the ALS and a Student Poster Prize for the best presentation based on research performed during Ph.D. work at the ALS. The Halbach Prize for outstanding development in instrumentation will also be awarded, as in previous years. Please send nominations for the Shirley Prize and the Halbach Prize to the Users' Executive Committee via email to W_Meyer-Ilse@lbl.gov by October 18. Please include a brief statement to support your nomination.

6. ALS VACUUM POLICY NOW AVAILABLE ON WEB
(Contact: ejmoxon@lbl.gov)

ALS users, beamline designers, and staff now have easy Web access to the ALS Vacuum Policy and Vacuum Guidelines for Beamlines and Experiment Endstations, Revision 2. (http://www-als.lbl.gov/als/quickguide/vacuumpolicy.html) The document, found in the Facilities, Experiment Information and Support section of the Quick Guide for Users, details ALS vacuum policies and specifications for beamlines and endstations.

Supplementary information about vacuum policy and contact information for technical assistance are available in ALS User Advisory No. 9: Vacuum Policy for User Endstations for Protection of Beamline Components and Storage Ring Vacuum. This advisory is available in printed form from the User Services Office (email: alsuser@lbl.gov) and on the Web at http://www-als.lbl.gov/als/user-advis/09-vacuum_policy.html.

7. WHO'S IN TOWN: A SAMPLING OF ALS USERS

To highlight the richness of our user community and help introduce recent arrivals, we offer this listing of some of the experimenters who will be collecting data during the next two weeks at the ALS.

Beamline 1.4.1: Joel Ager, Wei Shan, and Eugene Haller (Berkeley Lab Materials Sciences Division) will be studying the ultraviolet photoluminescence of wide-band-gap semiconductors.

Beamline 1.4.3: Upal Ghosh (Carnegie Mellon University) will be studying contaminants in soil particles. Felicia Hendrickson and Robert Glaeser (Berkeley Lab Life Sciences Division) will be studying conformational changes of protein microcrystals. Hoi-Ying Holman (Berkeley Lab Earth Sciences Division) and Regina Goth-Goldstein (Berkeley Lab Environmental Energy Technology Division) will be studying polyaromatic hydrocarbon contaminant uptake in human cells.

Beamline 7.3.3: Ed Franco (Aracor Ltd.) will be testing the submicron x-ray focusing capabilities of x-ray capillaries. Paul Alivisatos, Keren Jacobs and Chieu-Ju Lee (Univ. of California, Berkeley) will be working on nanocrystals that are within a high-pressure diamond anvil cell.

Beamline 8.0.1: Joachim Stoehr, in collaboration with J.B. Kortright, will use x-ray magneto-optical Kerr effect (XMOKE) studies to examine elemental moments in epitaxial Fe/Cr multilayers as well as to study magnetic structure in spin-valve systems. Dennis Lindle (Univ. of Nevada, Las Vegas) and Michael S. Lubell (City College of New York) will conduct photoemission studies of global change molecules.

Beamline 9.0.2.2: Researchers working with Tom Baer and Rick Lafluer (North Carolina) will be performing pulsed-field-ionization photoelectron coincidence spectroscopy to study the photoionization and photodissociation of various compounds. This new technique allows them to study rotationally resolved, state-selected dissociations of ionic species.

Beamline 9.3.2: Scott Chambers (Pacific Northwest National Laboratory) will model hematite's surface structure with low-energy x-ray photoelectron diffraction. Satish Myneni (Berkeley Lab Earth Sciences Division) will be characterizing aqueous transition-metal organic and inorganic complexes from L-edge spectra.

8. OPERATIONS UPDATE
(Contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 76.3% overall and 81.7% for user shifts. Running time was reduced by outages for repairs to the four half-sine-wave generating chassis in the bump magnet power supply system, for replacing two quadrupole magnet power supplies that failed, and for initial repairs to Beamline 5.0's M1 mirror, which developed an in-vacuum water leak. In addition, a low-conductivity water pressure dip tripped water flow switches in five sectors of the storage ring, causing the electron beam to be lost, and a major power dip affecting most ALS systems required a four-hour recovery.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Weekly operations scheduling meetings are held on Fridays at 3:30 p.m. in the Building 6 conference room. The Accelerator Status Hotline at (510) 486-6766 (ext. 6766 from Lab phones) features a recorded message giving up-to-date information on the operational status of the accelerator.

9. ERRATUM

In last issue's listing of Who's in Town, researcher Laurent Nahon (Univ. de Paris-Sud, France) was incorrectly listed as a user on Beamline 7.3.1.1. ALSNews regrets the error.