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. 117        December 9, 1998

Table of Contents


    1. X-RAY RAMAN SCATTERING ESTABLISHES HTSC ENERGY SCALES
    2. MEMORIAL FOR TIM RENNER SCHEDULED FOR 12/14/98
    3. NEW SCIENCE HIGHLIGHT POSTED ON THE WEB
    4. 59TH ANNUAL PHYSICAL ELECTRONICS CONFERENCE ANNOUNCED
    5. MEET SCIENTIST JOHN BROWN IN MICROWORLDS
    6. WHO'S IN TOWN: A SAMPLING OF ALS USERS
    7. OPERATIONS UPDATE

1. X-RAY RAMAN SCATTERING ESTABLISHES HTSC ENERGY SCALES
by Art Robinson (Contact: Pieter.Kuiper@itn.hh.se)

One signature of the onset of superconductivity as a material's temperature falls below the transition temperature (Tc) is the formation of electron pairs. A group of Swedish and Dutch scientists working at Beamline 7.0 has used resonant x-ray Raman scattering to investigate electronic transitions between copper 3d states in Sr2CuO2Cl2, an insulating model compound for the copper-based high-temperature superconductors (HTSCs). This compound has a high concentration of copper atoms, making it amenable to detailed study. Contrary to earlier conjecture, the researchers found that the transitions have energies that are too high to be directly involved in the pairing mechanism for high-temperature superconductivity, the source of which is one of the great unsolved problems of condensed-matter physics.

The common feature of HTSCs is their sandwich structure, in which the essential ingredient is a set of parallel copper–oxygen planes. In these planes, each Cu2+ ion is surrounded by four oxygen atoms. The Cu2+ ions have nine 3d electrons in the ten available 3d orbitals, which leaves one 3d orbital unoccupied. One knows from the angular dependence of x-ray absorption that this unoccupied or hole orbital is oriented in the plane with its four lobes pointing toward the four neighboring oxygen atoms, because the electrons are repelled by the negative charge on the oxygen. In a coordinate system with x and y axes pointing from copper toward oxygen, this 3d orbital is called an x^2-y^2 orbital. Owing to quantum-mechanical selection rules, transitions between this and other d states are forbidden by absorption or emission of a single photon and hence are too weak to study by conventional means. However, resonant Raman scattering is a two-photon process that, together with the brightness afforded by the ALS, allows their observation.

The idea is to first excite an electron from the Cu 3p core level into the empty 3d orbital (Cu M3 and M2 resonances near 75 eV) with x-ray photons (an allowed transition) and then observe the emitted x rays. If the same electron falls back into the 3p hole that it left behind, it re-radiates the absorbed photon, adding to a large elastic peak in the emission spectrum due to Rayleigh scattering. But there is also a significant probability that one of the other 3d electrons falls into the core hole, emitting a photon of a different energy (the Raman scattered photon) in the process and leaving a hole orbital with a different orientation. The net effect is a transition between d states and a reorientation of the hole orbital. The energy difference between the elastic and Raman-scattered photons is then the same as that for the transition between d states. By comparing the intensities emitted normal to the copper–oxygen plane and those emitted parallel to the plane, one can deduce the hole's orientation. The researchers found the xy orbital at 1.35 eV, the xz and yz orbitals at 1.7 eV, and the z^2 orbital at 1.5 eV, all of them too far above the ground state to play a direct role in whatever pairing mechanism is operative in the HTSCs.

Research conducted by P. Kuiper (Hogskolan i Halmstad, Sweden); J. Guo, C. Sathe, L. Duda, and J. Nordgren (Uppsala University, Sweden); and F. de Groot and G. Sawatzky (Groningen University, The Netherlands).

Funding: National Research Council, Sweden.

Publication about this experiment: P. Kuiper et al., Phys. Rev. Lett. 80, 5024 (1998).

2. MEMORIAL FOR TIM RENNER SCHEDULED FOR 12/14/98
(Contact: JPMinton@lbl.gov)

We are saddened to report that ALS Scientist Tim Renner passed away on November 17 after a long battle with colon cancer. He was 48. Born in Weatherford, Texas, Renner graduated from the University of Texas at Austin in 1972 and received his doctorate from the University of Chicago in 1978. He joined the ALS Experimental Systems Group in 1994 and over the past four years worked on a new beamline and endstation for conducting spatially resolved photoelectron spectroscopy to analyze defects in semiconductor microcircuits (Beamline 7.3.1.2). Anyone who would like to contribute to an educational fund for Renner's children can write a check to "Ian and Zoe Renner" and send it to

Gayle Lambert
130 Ricardo Avenue
Piedmont, CA 94611

3. NEW SCIENCE HIGHLIGHT POSTED ON THE WEB
(Contact: sacchi@lure.u-psud.fr)

A new Science Highlight about the use of x-ray resonant scattering with circularly polarized light on a magnetized iron/vanadium sample (reported in ALSNews Vol. 116) is now available on the Web. The highlight reports how measurements of scattered x-ray intensity and absorption data obtained at Beamline 6.3.2 were used to reconstruct all the components of the dielectric tensor of iron in the multilayer near the iron L absorption edges. The dielectric tensor quantifies the response of the material to electromagnetic fields. To see the highlight, point your browser to http://www-als.lbl.gov/als/science/sci_archive/xrayscat.html or follow the "Recent Science Highlight" link on the ALS home page.

4. 59TH ANNUAL PHYSICAL ELECTRONICS CONFERENCE ANNOUNCED

The 59th Annual Physical Electronics Conference (PEC) will be held at the Clark Kerr Campus of the University of California, Berkeley, California, USA on July 7-9, 1999. Hosted by the Lawrence Livermore National Laboratory, this conference will provide a forum for the dissemination and thorough discussion of important new research results in the physics and chemistry of surfaces and interfaces. The conference will continue to emphasize basic, fundamental science at the atomic and molecular level and its application to processes of industrial and commercial importance. For more information, see the conference Web site at http://www.pec-conf.org. Information on this and other events is posted on the ALS Web site's Meetings and Workshops page at http://www-als.lbl.gov/als/workshops/.

5. MEET SCIENTIST JOHN BROWN IN MICROWORLDS
by Elizabeth Moxon

Have you ever wondered what kind of people work at the ALS? Or what kind of background and expertise ALS staff bring to their work? Visit our award-winning interactive Web site, MicroWorlds (http://www.lbl.gov/MicroWorlds/), and find out in The Bright and the Busy. Designed and written by student interns, The Bright and the Busy features profiles of staff members and describes their work and what they like to do when they are not under the dome at the ALS. Readers can try solving a puzzler challenge or send questions to the currently featured staff member. New this month is a biography of John Brown, a member of the Center for X-Ray Optics (CXRO) team that uses the powerful x-ray microscope on Beamline 6.1 to probe biological and material samples.

6. 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) will continue to perform ultraviolet photoluminescence studies of wide-band-gap semiconductors.

Beamline 1.4.3: Hoi-Ying Holman and Regine Goth-Goldstein (Berkeley Lab) and Ron Sims and Karl Nieman (Utah Water Resource Laboratory) will study the biodegradation of pyrene from a Superfund site in Montana. Upal Ghosh (Carnegie Mellon University) will perform infrared spectromicroscopy on soil samples. Ted Raab (University of Colorado at Boulder) will continue studying soil/root interactions in the rhizosphere.

Beamline 7.0.1: Phil Woodruff (University of Warwick, UK) will continue to study chemically resolved photoelectron diffraction of molecules on surfaces. Adam Hitchcock (McMaster University) will conduct spectromicroscopy studies of blood-contact polymers. Tim Grundl (University of Wisconsin-Milwaukee) will continue to perform spectromicroscopy studies of clay minerals.

Beamline 7.3.1.1: Andy Smith (Daresbury Laboratory, UK) will study meteorites and other mineral samples using PEEM2.

Beamline 7.3.3: Bob Glaeser (University of California, Berkeley) will explore the use of a low-bandpass monochromator to do protein crystallography on microcrystals. Tom Alber (University of California, Berkeley) will explore the possibility of carrying out MAD protein crystallography on an ALS bend magnet.

Beamline 10.3.1: Dale Sayers (North Carolina State University) will study metal particles in lung tissue. Christoph Flink (Berkeley Lab) will study copper concentration profiles in the near-surface region of silicon substrates.

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

Beam reliability for the last two weeks (November 23-December 6) was 97.4% overall and 97.7% for user shifts. All outages were of short duration.

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.