Direct observation of the symmetry of core states of a single Fe impurity in GaAs

We report the observation of the two mid-gap core d-states of differing symmetry for a single Fe atom embedded in GaAs by scanning tunneling microscopy. By voltage control we can manipulate the charge state (Fe3$+$ or Fe2$+)$ of the embedded Fe atom. For Fe in the Fe3$+$ state two different deep core d-states with E and T2 symmetry are distinguished by the strength of their hybridization with the surrounding host electronic structure. The mid-gap state of Fe that does not hybridize via sigma-bonding is strongly localized to the Fe atom, whereas the mid-gap state that does hybridize via sigma-bonding is extended, and comparable in size to other acceptor states measured previously. Tight-binding calculations of these mid-gap states agree with the measurements, and illustrate that such measurements can determine the degree of hybridization via pi-bonding of impurity d-states. In addition to fundamental probing of mid-gap d-state electronic structure, measurements of such intrinsically spin-orbit-entangled, single-dopant states may be of use for high-speed electrical control of single spins.