STM-tip induced displacement of Co atoms embedded in a Cu(0 0 1) surface

We present experimental evidence for a controlled movement of single atoms of Co incorporated in a Cu(0 0 1) surface under the influence of a scanning tunnelling microscope (STM) tip. The Co atoms move preferentially along such [1 1 0] nearest neighbour directions in the Cu surface which lie in an acute angle to the vertical sweep direction of the STM tip. The induced motion of the Co atoms is controllable by the tip bias voltage. We propose that a tip mediated surface vacancy exchange process is responsible for the observed motion of the impurity atoms.