1. QUANTUM WELL STATES VISUALIZED IN COPPER THIN FILMS
By Art Robinson
Contact: Z.Q. Qiu, qiu@socrates.berkeley.edu

Scientists from the University of California, Berkeley, and Berkeley Lab have combined precision sample-fabrication technology and the spatial resolution achievable with the high brightness of the ALS to make photoemission images of the spatial variation of electron wave functions (quantum well states) in thin copper films. The images qualitatively verify a proposed model in which a longer-wavelength "envelope" function modulates a shorter-wavelength component of the wave function. Quantum well states are believed to underlie the magnetic behavior of the layered metallic nanostructures now under intense development for advanced data storage and memory applications, so understanding them is a major thrust of magnetic-materials research.

In a metal, an electron wave function can be represented by a short-wavelength component that is modulated by an envelope with a longer wavelength. In a large sample, the allowed electron energies fall into a series of bands within which the energy is quasi-continuous, but as the dimensions shrink, the allowed energies become widely separated and few in number. For a film, which is thin in only one direction, the energy is quantized in this way for travel perpendicular to the film surface and is continuous for travel parallel to the surface. In the model for quantum well states, the envelope functions must fit an integer number of half-wavelengths into roughly the film thickness.

The Berkeley group visualized the envelope function by means of photoemission measurements of a copper film, using a finely focused x-ray beam from the ALS to excite photoelectrons. (The results can be seen on the Web at http://www-als.lbl.gov/als/science/sci_archive/copperqws.html.) The intensity in the photoemission spectrum oscillated, with maxima occurring when the photoelectron energies matched those of quantum well states. The researchers used a layer of nickel only one atom thick embedded in the quantum well to probe the wave function. Embedding the nickel between two wedge-shaped copper layers oriented at right angles made it possible either to continuously vary the position of the nickel layer in a film of fixed total thickness or to keep the nickel layer fixed in the middle of a quantum well of continually varying thickness, depending on the direction of travel across the sample surface.

Measuring the photoemission intensity at a fixed electron energy, corresponding to photoelectrons emitted from quantum well states at the Fermi level of copper, while scanning the x-ray beam across the sample, resulted in a pattern of light and dark oscillations. The nickel layer acted to suppress the quantum well states when it was near a node of the envelope function, thereby reducing the intensity of the photoemission. Therefore, scanning in the direction of fixed thickness and variable nickel position produced an image of the envelope with dark areas representing nodes and bright areas representing antinodes. Scanning in the direction corresponding to an increasing well thickness and fixed nickel position gave bright bands as additional quantum well states became allowed in the thicker film.

Tailoring electron wave functions ("wave-function engineering") in magnetic nanostructures with layer thicknesses measured in nanometers may make it possible to control the spin-dependent behavior of electrons. The group intends to test its qualitative analysis with quantitative theoretical calculations. They hope to reach a level of understanding that will turn wave-function engineering in magnetic nanostructures into a practical tool.

Research conducted by R.K. Kawakami, H.J. Choi, E.J. Escorcia-Aparicio, M.O. Bowen, J.-H. Wolfe, E. Arenholz, Z.D. Zhang, and Z.Q. Qiu (University of California, Berkeley) and E. Rotenberg and N.V. Smith (Berkeley Lab), using Beamline 7.0.1.

Funding: the U. S. Department of Energy, Office of Basic Energy Sciences; the National Science Foundation; and the University of California, with additional support from the National Science Foundation of China and the Miller Institute of the University of California.

Publication about this experiment: R.K. Kawakami et al, "Quantum well states in copper thin films," Nature 398, 132 (1999). See also S.D. Bader, "Quantum engineering: Probing magnetism in the well," Nature 398, 104 (1999) and F.J. Himpsel, "Enhanced: Mirrors for electrons," Science 283 (5408), 1655 (1999).

2. ALS TO UNDERGO FIRST REVIEW AS A BERKELEY LAB DIVISION
Contact: RDPepe@lbl.gov

For the first time since becoming a division of Berkeley Lab (see ALSNews Vol. 89, October 29, 1997), the ALS will undergo an annual Berkeley Lab Director's Review. The review will take place on Thursday and Friday, June 10 and 11, in Perseverance Hall (Building 54, room 130). The review committee will consist of experts from industry, academia, other national laboratories, and other synchrotron light sources. ALS management will brief the panel on the growth of the ALS science program, the facility's interactions with its users, accelerator operations, and plans for the future. The committee will meet with the Users' Executive Committee and the Scientific Advisory Committee Chair as well. In addition, more than a dozen users representing the wide range of ALS science will present updates on their research at the ALS. The management and user presentations are open to ALS users, staff, and other interested individuals, although seating is limited. Open sessions include the following:

3. ALS T-SHIRT DESIGN CONTEST IS BACK
Contact: ejmoxon@lbl.gov

The ALS Users' Association T-shirt contest returns this summer by popular demand. ALS users, staff, family, and friends are invited to unleash their creative energy by entering the fourth annual ALS T-Shirt Design Contest, sponsored by the ALS Users' Executive Committee. The winner will have his or her signed artwork featured on T-shirts for participants at this year's ALS Users' Association Meeting (October 18-19).

T-shirt designs should be no larger than 8.5 in. by 11 in. (22 cm by 28 cm). Rough drawings and concepts, as well as more polished artwork, are all acceptable. The words "Advanced Light Source" or "ALS" must appear somewhere in the design.

Send designs by Friday, July 23, to
Elizabeth Moxon
Advanced Light Source
Berkeley Lab, MS 4-230
Berkeley, CA 94720
or fax to 510-495-2111. Enter now, and enter often.

4. OPERATIONS UPDATE

The ALS is in a planned shutdown for installations and maintenance. User operations are scheduled to resume at 8:00 a.m. on June 21. Beam reliability for user shifts during the week before the shutdown (May 24-31) was 97.5%. There were no significant outages.

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.