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We report on the individual observation of substitutional P donors on the reconstructed Si (111) 2 × 1 surface by means of scanning tunneling microscopy and tunneling spectroscopy. Depending on the scanning voltage, the dopants give rise to different features superimposed on the background lattice. This is explained by the influence of the charged P atom on the electronic surface-states resulting in a local band shift. The spatial arrangement of the donors obeys a statistical distribution, excluding significant clustering even at a P concentration of 6 × 1019 cm−3 which is far above the metal-insulator transition.

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Multilayers of Nb and Cu with a fractal or periodic stacking sequence were prepared by electron-beam evaporation onto sapphire substrates. Their superconductivity was investigated by measurements of T c and the temperature and angular dependence of the upper critical field B c2 . For low temperatures TӶT c , all samples show the characteristic behavior of two-dimensional superconductivity independent of the stacking sequence, whereas for temperatures near T c the type of layering determines the effective dimensionality, resulting in a ''multicrossover'' behavior in fractal samples.