Quantum Well States in Copper Thin Films
"Wave-function engineering" may make it possible to tailor electron wave functions in magnetic nanostructures in order to control the spin-dependent behavior of electrons. Some magnetic nanostructures are already in commercial production, such as high-sensitivity read heads in the newest data-storage disks. High-speed, low-power, non-volatile magnetic random-access memories based on spin-dependent quantum tunneling between layers are under development that would dramatically change the architecture of computer design. As a step toward wave-function engineering, 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.
The researchers used a layer of nickel only one atom thick embedded in the quantum well to probe the wave function as the focused photon beam scanned the sample. A double-wedge sample (left) varied the position of the nickel layer in a well of fixed thickness in the horizontal direction and the total copper thickness in the vertical direction (center). The image of the photoemission intensity at the Fermi level (right) shows both horizontal and vertical variations. The horizontal oscillations in the photoemission intensity map the spatial variation of the quantum well envelope function, whereas the vertical intensity variations show the presence of additional envelope modes as the quantum well thickness increases.
The group intends to test its qualitative analysis with quantitative theoretical calculations. The members 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); E. Rotenberg and N.
V. Smith (ALS), using Beamline 7.0.1.
Funding: U. S. Department of Energy, Office of Basic Energy Sciences;
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).
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