The discovery of ferromagnetism in Mn doped

We utilize a single-atom substitution technique with spectroscopic imaging in a scanning tunneling microscope to visualize the anisotropic spatial structure of magnetic and nonmagnetic transition metal acceptor states in the GaAs ͑110͒ surface. The character of the defect states play a critical role in the properties of the semiconductor, the localization of the states influencing such things as the onset of the metal-insulator transition and in dilute magnetic semiconductors the mechanism and strength of magnetic interactions that lead to the emergence of ferromagnetism. We study these states in the GaAs surface finding remarkable similarities between the shape of the acceptor-state wave function for Mn, Fe, Co, and Zn dopants, which is determined by the GaAs host and is generally reproduced by tight-binding calculations of Mn in bulk GaAs ͓J.-M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 ͑2004͔͒. The similarities originate from the antibonding nature of the acceptor states that arise from the hybridization of the impurity d levels with the host. A second deeper in-gap state is also observed for Fe and Co that can be explained by the symmetry breaking of the surface.

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A scanning-tunneling-microscopy-substitution technique is used to incorporate single Mn atoms into Ga sites in GaAs ͑110͒ surfaces. The electronic states near a single Mn in the acceptor configuration produce a strong in-gap resonance associated with the acceptor level. The isolated Mn acceptor is probed in both p-type and n-type environments to access the neutral and ionized acceptor configurations. The Mn acceptor at the surface substitution site shares bulk characteristics that compare well with tight-binding calculations. The anisotropic structure of the Mn hole state plays an important role in hole-mediated interactions between the Mn acceptors. Isolated Mn pairs show a strong interaction dependence on crystal orientation and spacing. Certain pair orientations produce a strong splitting of the acceptor level into two levels with bondinglike and antibondinglike symmetries. A tight-binding model relates the acceptor level splitting and the spin-spin interaction energy J.

The following are summaries of three of the papers presented at a Joint Meeting of the Scottish and North East Regions and the Electroanalytical Group held on April 9th, 1974, and reported in the May issue of Proceedings (p. 107).