  Does the size of atoms present a lower limit to the size of electronic
structures that can be fabricated in solids? This limit can be overcome by
using devices that exploit quantum mechanical scattering of electron waves at
atoms arranged in focussing geometries on selected surfaces. Calculations
reveal that features smaller than a hydrogen atom can be obtained. These
structures are potentially useful for device applications and offer a route to
the fabrication of ultrafine and well defined tips for scanning tunneling
microscopy.
