Nb/Cu superlattices with modulation wavelength in the range 1 to 30 nm are synthesized by magnetron sputtering. Both, X-ray diffraction and ion mill Auger electron spectroscopy techniques are used to determine the structure of the superlattices. The X-ray experiments reveal two anomalous expansions in the average lattice spacing at modulation wavelengths λ = 2 and 8 nm. Ultrasonic measurements of the surface-acoustic-wave (SAW) velocities in the superlattices show that surface phonons soften at the same two modulation wavelengths. A preliminary explanation of the experimental results is presented.
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Hu et al. (1988) studied this question.
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