Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators
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Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators M. R. Mahani1 A. Pertsova1, C.M. Canali1, M. F. Islam1 and A.H. MacDonald2 1 Department of Physics and Electrical Engineering, Linnaeus University, Norra vägen 49, 391 82, Kalmar, Sweden. 2 Department of Physics, University of Texas at Austin, U.S.A. ABSTRACT We present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant. In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap. We present explicit results for BiSe antisite defects and compare with STM experiments. INTRODUCTION The study of the spin of individual transition-metal dopants in semiconductor hosts and novel quantum materials, such as topological insulators, is an emergent field in nanoscience known as magnetic solotronics [1]. The capability of controlling, monitoring and manipulating single-spins in these materials is opening exciting prospects for novel spintronics [2, 3] and quantum computation devices at the atomic scale [4]. Among the most important experimental methods, advances in different STM-based techniques permit the investigation of substitutional dopants placed near the surface of a host material with unprecedented accuracy and degree of detail [5, 6]. It is therefore possible for theory to investigate these complex systems and elaborate descriptions which can be compared with experiments under highly controlled conditions. In this paper we consider two examples of this theoretical effort, based on a multiband tight-binding model supplemented by ab-initio methods. In the first case, we examine magnetic ions in GaAs. When substituted for Ga atoms, the magnetic dopants act as spin center strongly coupled to the associated itinerant hole-acceptor states. We investigate in particular the nature of the midgap electronic states for the case of Mn and Fe on the (110) surface. The second example deals with Bi2Se3 which is one of the most studied three-dimensional topological insulators [710]. For this case we examine individual bismuth dopant substituting selenium atoms in the surface layer. We show that our tight-binding model is able to capture the salient features of the local de
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