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ALSNews           Vol. 102           May 13, 1998

Table of Contents


    1. SITE-SELECTIVE SPECTROSCOPY OF STRONTIUM RUTHENATE WITH SOFT X-RAY EMISSION
    2. REVISED BEAMLINE DESIGN GUIDE OFF THE PRESSES, ON THE WEB
    3. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS - DUE JUNE 1
    4. OPERATIONS UPDATE

1. SITE-SELECTIVE SPECTROSCOPY OF STRONTIUM RUTHENATE WITH SOFT X-RAY EMISSION
(contact: kurmaev@ifmlrs.uran.ru)

One of the most burning questions in contemporary solid-state science is the nature of the mechanism driving superconductivity in the high-temperature superconductors (HTSCs), first discovered more than a decade ago. Some of these ceramic-oxide materials with complex compositions and structures retain their superconductivity at temperatures up to 125K. The Nobel-prize-winning model (Bardeen-Cooper-Schrieffer theory) developed for conventional metallic superconductors is not applicable, but no one yet knows exactly what the alternative is. Clues come not only from direct examination of the materials themselves but also from studies of related materials with similar composition and structure.

A multi-institutional group working at the ALS and at the Photon Factory in Japan has taken a step in this direction by demonstrating the ability to selectively examine the electronic structure of bonding electrons associated with oxygen in two different sites in the crystal structures of the strontium ruthenate compounds Sr[2]RuO[4] and Sr[2]RuO[4.25]. (All characters enclosed in square brackets in this article should be read as subscripts.) For one oxygen site, the experimenters were able to demonstrate hybridization (mixing) between electronic states of oxygen and ruthenium. The nature of the hybridization is different from that occurring in HTSCs and results in a different type of chemical bond.

The salient features in the strontium ruthenate crystal structure are planes comprising only ruthenium and oxygen atoms. Although there are interactions between planes, they are much weaker than those between electrons in the plane, so that the systems are quasi-two-dimensional. There are also oxygen atoms outside the planes (apical oxygens). Owing to the different chemical environments for oxygen in these two types of sites {O(1) in the planes and O(2) outside}, the photon energies to excite oxygen 1s electrons into 2p states are slightly different (chemical shift). The group was able to distinguish oxygen atoms in the two inequivalent sites spectroscopically by exploiting the chemical shift to selectively excite oxygen in the O(1) and O(2) sites. Their experiments combined soft x-ray absorption (total-fluorescence-yield method) and soft x-ray emission.

The chemical shift manifested itself as an excitation-energy dependence of the O K-alpha x-ray emission spectrum (2p -> 1s transition) of Sr[2]RuO[4] near the threshold for exciting x-ray emission; that is, the shape of the emission spectrum changed as the exciting photon energy was stepped at intervals above the threshold. These spectra nicely matched the atom-decomposed partial density of electron states for the two oxygen sites calculated by theorists. Comparing the spectra with angle-resolved photoemission spectra of Sr[2]RuO[4], which can distinguish ruthenium electron states of different symmetry {4d(t[2g]) and 4d(e[g])}, and with ruthenium N[2,3] emission (4d -> 4p transition) spectra, the group determined that the O(1) 2p states are mixed with d(t[2g]) ruthenium states, thereby forming hybridized pi bonds.

Strontium ruthenate is a low-temperature superconductor (critical temperature Tc = 0.93 K) that is structurally similar to the cuprate HTSCs, such as Sr[x]La[2-x]CuO[4] and YBa[2]Cu[3]O[7-d], in which Cu-O planes replace the Ru-O planes. Not only may measurements such as these shed light on HTSCs, but similar experiments apply equally well to a far larger class of so-called "complex materials," again often oxides with elaborate compositions and structures and an immense variety of entrancing and possibly technologically useful properties. For example, colossal magnetoresistance (CMR) compounds exhibit up to 100,000 times larger changes in electrical resistance in an applied magnetic field than occurs in the merely giant magnetoresistance (GMR) materials now coming into commercial use as read heads in high-density magnetic data-storage systems.

Research conducted by E. Z. Kurmaev, D. A. Zatsepin, and N. Ovechkina (Russian Academy of Sciences, Ural Division, Yekaterinburg); S. Stadler and D. L. Ederer (Tulane University); Y. Harada, S. Shin, M. Kasai, and Y. Tokura (University of Tokyo); M. M. Grush and T. A. Callcott (University of Tennessee, Knoxville); R. C. C. Perera (Berkeley Lab); T. Takahashi (Tokuku University, Japan); and K. Chandrasekaran, R. Vijayaraghavan, and U. V. Varadaraju (Indian Institute of Technology) using the soft x-ray fluorescence endstation at Beamline 8.0.1. Some measurements were also made at the Photon Factory, KEK Laboratory, Japan.

Funding: Russian State Program on Superconductivity, Russian Science Foundation for Fundamental Research, NATO, National Science Foundation, U. S. Department of Energy, and Louisiana Education Quality Special Fund.

2. REVISED BEAMLINE DESIGN GUIDE OFF THE PRESSES, ON THE WEB
(contact: ejmoxon@lbl.gov)

A revised and improved "ALS Beamline Design Guide: Information for Beamline Designers, Revision 2," is now available. This manual (formerly known as "ALS Beamline Design Requirements: A Guide for Beamline Designers") is a guide for all those involved in the design and construction of beamlines and endstations acceptable for use at the ALS. It describes the beamline review process, including required documentation, and gives guidelines and policies related to equipment and vacuum protection and personnel safety. There is also a suggested timeline for building a beamline, as well as a chapter devoted to all the procedures required for the timely completion of beamline construction. An extensive, updated list of contacts for additional information and technical assistance is again included in the guide.

A limited number of copies have been printed and sent to a list of Berkeley Lab staff involved in beamline design and construction. If you believe you should be added to this list, send an email to the ALS User Office (alsuser@lbl.gov) with "Please add to BDG list" in the subject line, and include your name and mail stop in the body of the message. For other readers, the new revision is available on the Web in Portable Document Format (PDF). Recipients of printed copies of the Beamline Design Guide will be notified of any significant changes to the guide and asked to print the updated pages and add them to their binders.

3. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS - DUE JUNE 1

The ALS is now accepting proposals from scientists who wish to conduct research at the facility as independent investigators between October 1998 and March 1999 inclusive. The deadline for proposals is June 1, 1998. There will be no automatic rollover of proposals from the previous proposal cycle (June-September 1998). Scientists wishing to renew a previous proposal should notify the ALS User Administrator, Ruth Pepe (contact information below), before the June 1 deadline.

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web. Information on the proposal process is available at the same location. Data sheets on beamlines at the ALS (available in Portable Document Format on the Web) provide information that may be useful to prospective ALS users.

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

The ALS is in a planned shutdown period for maintenance and installations. (See ALSNews Vol. 99 for details of shutdown activities.) Shutdown scheduling meetings will be held on Thursdays at 1:00 in the Building 6 conference room. Weekly operations scheduling meetings, held on Fridays at 3:30 p.m. in the Building 6 conference room, will resume on May 29. User operations are scheduled to resume June 3.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). 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.