The tip of a cryogenic scanning tunnelling microscope is used to controllably contact single atoms adsorbed on metal surfaces. The transition between tunnelling and contact is gradual for silver, while contact to adsorbed gold atoms is abrupt. The single-atom junctions are stable and enable spectroscopic measurements of, e.g., the Abrikosov–Suhl resonance of single Kondo impurities.
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Kröger et al. (2009) studied this question.
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