Experimental study of quantum size effects in thin metal films by electron tunneling

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Abstract
Periodic structure in the electron tunneling spectra of Pb, Mg, Au, and Ag has been observed. Such spectra represent in fact a direct observation of size-dependent electronic states in thin metal films. Films with thicknesses from 100 to 1000 Å were studied and the effect on the electronic standing-wave energies was measured. The physical model for these effects and their observability involve the existence of so-called commensurate states. The spacing of the quantized energy levels provides a direct measurement of the electron group velocity while their location in energy determines the position of band edges and other critical energy states in the band structure of the metals. In some cases, the effective mass can also be determined. A qualitative theoretical picture is sufficient to understand all of the sailient features of the observations. A number of experiments including alloying, strain, and electric field modulation are also described.