Core-state manipulation of single Fe impurities in GaAs with a scanning tunneling microscope

We demonstrate that a scanning tunneling microscope tip can be used to manipulate the tightly bound core (d-electron) state of single Fe ions embedded in GaAs. Increasing tip-sample voltage removes one d electron from the core of a single Fe, changing the dopant from the (Fe2+)(-) ionized acceptor state to the (Fe3+)(0) isoelectronic state, which alters the spin moment and dramatically modifies the measured local electronic contrast in topographic maps of the surface. Evidence of internal transitions among the d states of the Fe core is also seen in topographic maps where dark anisotropic features emerge from the interference between two paths: the direct tip-sample tunneling and tunneling which excites a d-state core exciton of the Fe dopant.