Quantum Interference Observed in Thin Films
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. Pioneering photoemission experiments at other facilities have established that nanometer-scale layers act as one-dimensional quantum wells, allowing electrons to exist only in discrete energy states. New work at the ALS using wedge-shaped samples has revealed additional, more subtle interference effects between the quantum well states (QWS).
The detection of interference effects between QWS in different layers requires a series of photoemission measurements in which the layers vary in thickness. The use of wedge-shaped samples can accomplish this in a single scan. The small spot size of the ALS beam facilitates this technique because any variations in wedge thickness across the beam spot are negligible.
A copper wedge layered at right angles over a cobalt wedge, as shown in the figure above, allows the simultaneous variation of two thicknesses independently.
Above, the variation in photoemission intensity as a function of both copper and cobalt thicknesses shows interference between QWS in the two layers.
Not only does this work provide additional confirmation of the quantum well states model, it opens the door to future "wave-function engineering"--the manipulation and design of wave functions as needed for nanometer-size magnetic components. 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).
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