The diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128 °C is measured using a novel atom-tracking technique that can resolve every diffusion event. The atom tracker employs lateral-positioning feedback to lock the scanning tunneling microscope (STM) probe tip into position above selected atoms with subangstrom precision. Once locked the STM tracks the position of the atoms as they migrate over the crystal surface. By tracking individual atoms directly, the ability of the instrument to measure dynamic events is increased by a factor of $\sim 1000$ over conventional STM imaging techniques.

• Received 18 September 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.459