Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions

The discovery of ferromagnetism in Mn doped GaAs [1] has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices [2-4]. A major hurdle for realistic applications of (Ga,Mn)As, or other dilute magnetic semiconductors, remains their below room-temperature ferromagnetic transition temperature. Enhancing ferromagnetism in semiconductors requires understanding the mechanisms for interaction between magnetic dopants, such as Mn, and identifying the circumstances in which ferromagnetic interactions are maximized [5]. Here we report the use of a novel atom-by-atom substitution technique with the scanning tunnelling microscope (STM) to perform the first controlled atomic scale study of the interactions between isolated Mn acceptors mediated by the electronic states of GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn-Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga1-xMnxAs structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples. Our experimental methods also provide a realistic approach to create precise arrangements of single spins as coupled quantum bits for memory or information processing purposes

Positive Evolutionary Selection of an HD Motif on Alzheimer Precursor Protein Orthologues Suggests a Functional Role

HD amino acid duplex has been found in the active center of many different enzymes. The dyad plays remarkably different roles in their catalytic processes that usually involve metal coordination. An HD motif is positioned directly on the amyloid beta fragment (Aβ) and on the carboxy-terminal region of the extracellular domain (CAED) of the human amyloid precursor protein (APP) and a taxonomically well defined group of APP orthologues (APPOs). In human Aβ HD is part of a presumed, RGD-like integrin-binding motif RHD; however, neither RHD nor RXD demonstrates reasonable conservation in APPOs. The sequences of CAEDs and the position of the HD are not particularly conserved either, yet we show with a novel statistical method using evolutionary modeling that the presence of HD on CAEDs cannot be the result of neutral evolutionary forces (p<0.0001). The motif is positively selected along the evolutionary process in the majority of APPOs, despite the fact that HD motif is underrepresented in the proteomes of all species of the animal kingdom. Position migration can be explained by high probability occurrence of multiple copies of HD on intermediate sequences, from which only one is kept by selective evolutionary forces, in a similar way as in the case of the “transcription binding site turnover.” CAED of all APP orthologues and homologues are predicted to bind metal ions including Amyloid-like protein 1 (APLP1) and Amyloid-like protein 2 (APLP2). Our results suggest that HDs on the CAEDs are most probably key components of metal-binding domains, which facilitate and/or regulate inter- or intra-molecular interactions in a metal ion-dependent or metal ion concentration-dependent manner. The involvement of naturally occurring mutations of HD (Tottori (D7N) and English (H6R) mutations) in early onset Alzheimer's disease gives additional support to our finding that HD has an evolutionary preserved function on APPOs

Growth and characterization of In1-xMnxAs diluted magnetic semiconductors quantum dots

The growth of low-temperature In1-xMnxAs quantum dots (QD) on low-temperature GaAs(001) was investigated. Three different growth conditions with variable As-2 flux, Mn:In flux ratio and QD layer thickness were studied by atomic force microscopy and transmission electron microscopy. All three conditions generated In1-xMnxAs QD with a cubic crystal structure. It is shown that an excess of As-2 combined with a flux ratio of Mn:In = 0.10 and a limited deposited thickness of 5.4 ML In0.59Mn0.41As are the best conditions to obtain symmetric QD coherent to the substrate. Photoluminescence analysis performed on the latter samples showed a narrow band near 1100 nm, indicating 3D-confinement of the dots. The magnetic properties of a multilayer of 5.4ML In0.59Mn0.41As QD were analyzed by magneto-optical Kerr effect at 1.6K. No magnetization could be detected, whereas measurements performed on a multilayer of 2.4 ML In0.59Mn0.41As quantum wells indicated ferromagnetism. The absence of ferromagnetism for the QD is assigned to the superparamagnetism of the islands and the low Mn:In ratio used for the growth. It is suggested that these growth conditions are generating QD with a lower chance for interdot exchange interaction within a layer. (c) 2005 Elsevier B.V. All rights reserved

Spatial Structure of Mn-Mn Acceptor Pairs in GaAs

The local density of states of Mn-Mn pairs in GaAs is mapped with cross-sectional scanning tunnelingmicroscopy and compared with theoretical calculations based on envelope-function and tight-bindingmodels. These measurements and calculations show that the crosslike shape of the Mn-acceptor wavefunction in GaAs persists even at very short Mn-Mn spatial separations. The resilience of the Mn-acceptorwave function to high doping levels suggests that ferromagnetism in GaMnAs is strongly influenced byimpurity-band formation. The envelope-function and tight-binding models predict similarly anisotropicoverlaps of the Mn wave functions for Mn-Mn pairs. This anisotropy implies differing Curie temperaturesfor Mn -doped layers grown on differently oriented substrate