Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W15: Focus Session: Spins in Semiconductors - Spin-Orbit Effects and Confinement |
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Sponsoring Units: DMP GMAG FIAP Chair: Giovanni Vignale, University of Missouri Room: D171 |
Thursday, March 24, 2011 11:15AM - 11:51AM |
W15.00001: Chern Number Spins of Mn Acceptor Magnets in GaAs Invited Speaker: State-of-the-art STM techniques have made it possible to substitute transition metal impurities for individual atoms in semiconductor crystals and have provided detailed information on the nature of the bound acceptor or donor states. Individual coupled acceptor (or donor)-impurity centers represent a new class of nanomagnets which we refer to as \textit{acceptor or donor magnets}. Here we determine the effective total spin $J $of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. When individual Mn atoms are close to the sample surface, the total spin changes from $J=1$ to $J =2,$ due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total $J $depends on pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variation as a function of local moment orientation can profoundly influence the quantum-spin dynamics of these magnetic entities. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W15.00002: Effect of magnetic field on the local density of states of Mn acceptor magnets in GaAs M.R. K. Mahani, C.M. Canali, A.H. MacDonald Advances in atomic manipulation, real-space imagining and spectroscopic power of STM techniques have recently made it possible to investigate the local electronic properties of a few substitutional Mn impurities inserted in the GaAs surfaces [1]. Theoretical work [2] predicts that the local density of states in the vicinity of the Mn impurities should depend strongly on the direction of the Mn magnetic moment. In contrast, recent STM experiments [3] from several groups find a negligible dependence of the tunneling LDOS on the magnetic field direction for applied fields up to 7 T. Based on tight- binding calculations we interpret these findings by arguing that large LDOS signals require large angle moment rotations, and that the strength of the magnetic field used in present experiments is not strong enough to substantially modify the magnetic anisotropy landscape of Mn impurities near the GaAs surface.\\[4pt] [1] D. Kitchen et al., Nature, 442, 436 (2006); J. K. Garleff et al., Phys. Rev. B 82, 035303 (2010).\\[0pt] [2] T. O. Strandberg, C. M. Canali, and A. H. MacDonald, Phys. Rev. B 80, 024425 (2009). [3] P. M. Koenraad, Private Communication. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W15.00003: Effects of Anisotropy in Magnetic Quantum Dots Rafal Oszwaldowski, Igor Zutic, Andre Petukhov Magnetic ordering in semiconductor Quantum Dots (QDs) doped with Mn is mediated by the confined carriers (typically holes), which interact with Mn through exchange interaction. The ordering can be affected by the QD flat shape [1], and, by the resultant anisotropic g-factor of holes [2]. A reduction of the in-plane symmetry of the QD by an external potential may influence the magnetic alignment as well [3]. We study the magnetic ordering at different degrees of anisotropy. The ordering arises in absence of external magnetic field, e.g., through formation of magnetic polarons. A typical number of Mn in a QD is large, so we replace their spins by classical magnetic moments. We emphasize the limit of full isotropy (electrons) and extreme anisotropy (holes in a flat QD). Supported by DOE-BST, ONR, AFOSR, and NSF-ECCS CAREER. \\[4pt] [1] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B \textbf{82}, 195320 (2010)\\[0pt] [2] P. Dorozhkin, et al., Phys. Rev. B \textbf{68}, 195313 (2003).\\[0pt] [3] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. \textbf{101}, 207202 (2008) [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W15.00004: Incorporation of Mn into Ge Quantum Dots: Growth Strategies to Control Structure and Magnetism Christopher Nolph, Petra Reinke Manganese doped, magnetic germanium quantum dots are important building blocks for the future of spintronic devices. Our goal is to understand and control how the manipulation of the Mn-environment within the Si(100)-Ge wetting layer-Ge QD systems influence the magnetic properties. We investigate several pathways for Mn-doping of Ge QDs which suppress detrimental germanide formation. The first pathway uses a surface-driven approach: Mn is deposited on the Ge QD surface, forms well-defined clusters on the QD and dissolve during annealing. The second pathway uses co-deposition of Ge and Mn (i) throughout the entire QD growth process, and (ii) only during the formation of the wetting layer. The highest concentration of Mn is about 20{\%}, and the Ge QD growth is only marginally perturbed, albeit germanides begin to form. All processes are observed with scanning tunneling microscopy, which yields morphological and electronic structure information of the reaction sequence. A comprehensive model of all processes will be presented. Preliminary magnetism results, obtained with a vibrating sample magnetometer, indicate a ferromagnetic material with a Curie temperature up to 100K. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W15.00005: Control and properties of magnetic nanostructures in nitride semiconductors Invited Speaker: We review [1] studies of MOVPE (Ga,Fe)N [2-7] and (Ga,Mn)N [8,9], combining magnetic [2-4,6,8,9], magnetooptical [3], and XANES [4,5,8] investigations with a comprehensive structural and chemical characterization by SIMS, TEM, EDS [2,4,6,8,9], synchrotron-XRD [4,6,8], EXAFS [5,8], and PEEM [7]. We show that the Fe ions aggregate into Fe$_{x}$N nanocrystals either by crystallographic or by chemical phase separation, controlled by the growth conditions and by co-doping. Depending on the degree of nitridation, these nanocrystals are either ferromagnetic or antiferromagnetic. \\[4pt] [1] A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528 (2010);\\[0pt] [2] A. Bonanni et al., Phys. Rev. B 75, 125210 (2007);\\[0pt] [3] W. Pacuski et al., Phys. Rev. Lett. 100, 037204 (2008);\\[0pt] [4] A. Bonanni et al., Phys. Rev. Lett. 101, 135502 (2008);\\[0pt] [5] M. Rovezzi et al., Phys. Rev. B 79, 195209 (2009);\\[0pt] [6] A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010);\\[0pt] [7] I. Kowalik et al., arXiv:1011.0847;\\[0pt] [8] W. Stefanowicz et al., Phys. Rev. B 81, 235210 (2010);\\[0pt] [9] A. Bonanni et al. arXiv~: 1008.2083. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W15.00006: From simplicity to complexity: Can transcendental equation and transfer matrix enlighten us about the nature of Rashba physics? Chih-Piao Chuu, Roland Winkler, Qian Niu We present an analytic model of Rashba spin-splitting of conduction electrons in asymmetric quantum wells based on transcendental equation and transfer matrix approaches. The sources of asymmetries of quantum wells, such as interface discontinuity, conduction and valence band profiles, external field or presence of a potential gradient, related to Rashba spin-splitting will be discussed. We will use type I and type II semiconductor heterostructure quantum wells for demonstration. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W15.00007: Signatures of the crystal symmetry after Dyakonov-Perel spin relaxation of photoexcited hot electrons in semiconductor heterostructures Lan Qing, Hanan Dery We reveal unique manifestations of the intimate relation between the crystal structure of zincblende semiconductors and their spin-orbit coupling. We show that reflection of photoexcited hot electrons is capable of tipping the direction of the optically injected net spin vector away from the propagation axis of the exciting circularly polarized beam. The effect is robust even in case of complete electron reflection from a non magnetic target (e.g., GaAs/AlAs). The tipping angle of the net spin vector after spin relaxation is determined by the effective Dyakonov-Perel magnetic field as well as by the momentum alignment and spin-momentum correlation of the initial photoexcited electron population. All of these crystal-structure dependent mechanisms contribute due to the reflection plane induced symmetry breaking. We perform Monte Carlo simulations to calculate the tipping angle and also provide qualitative derivations of the effect. Results are shown for non-magnetic semiconductor heterostructures and for hybrid semiconductor/ferromagnet systems. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W15.00008: The spin dynamics in the strong spin-orbit coupling regime-A collective Rabi oscillation Xin Liu, Xiong-Jun Liu, Jairo Sinova We study the spin evolution in a high-mobility two dimensional electron gas (2DEG) with generic spin-orbit interactions(SOI). A fully understanding of the D'yakonov-Perel's(DP) mechanism is presented by using the microscopic linear response theory from the diffusive to the ballistic regime. We derive a set of spin dynamical equations which capture the characters of the purely exponential and damped oscillatory spin evolution modes in different spin-orbit coupling(SOC) regimes. It is shown that the oscillatory spin dynamics appear when the electron life time is larger than the half of the spin precession time due to the SOI. The Rabi oscillation between two spin bands is the physical origination of the damped oscillatory modes. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W15.00009: Rasbha Spin-Orbit Interaction in Digital Alloys Joseph Pingenot, Kieran Mullen The Rashba spin-orbit interaction couples the electron spatial wavefunction to its spin through breaking the inversion symmetry of a structure. From work by Lange[1], the Rashba spin-orbit effect can be divided into a structural component, originating primarily in the valence band offsets within the nanostructure, and an electric field component, originating primarily in the internal self-consistent electric field and in an applied electric field. A common growth technique, digital alloying, breaks the well into discrete steps and then varies the material composition for each step by constructing a well within the step such that the average material across the step corresponds to the desired percentage, e.g. a Ga$_{0.5}$In$_{0.5}$As well would be pure GaAs through half the step, followed by pure InAs for the rest of the step. With digital alloying, the electron wavefunction is approximately the same as is obtained by using a real alloy. The Rashba spin-orbit couplings, however, are considerably smaller for the digital alloy than the real alloy. This comes about because the digital alloy has a series of positive and negative interfaces at each step, whereas the real alloy has a series of identical steps. We calculate the Rashba coefficient for a variety of realistic well geometries, for both digitial and continuous alloying, and discuss how the consequences for experiment. [1] Jens Lange, {\it Quantentransport in Halbleiter-Heterostrukturen} (1996). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W15.00010: Superconductivity in the repulsive Rashba model Luyang Wang, Oskar Vafek We study the superconducting instability of a two dimensional Rashba spin-orbit coupled system with a weak repulsive interaction by a two step renormalization group (RG) method introduced by Raghu et.al. (PRB 81, 224505 (2010)). We present the superconducting transition temperature Tc in terms of the correlation functions of the non-interacting system. The RG flows in the Cooper channel break down below some scale, which we identify with Tc and verify that the Tc is independent of the intermediate cutoff. Finally, we present results of Tc as a function of spin-orbit coupling strength. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W15.00011: Kohn-Luttinger superconductivity of two-dimensional electrons in the presence of Rashba spin-orbit coupling Ali Ashrafi, Dmitrii Maslov We consider a two-dimensional(2D) system of fermions with weak short-range repulsive interac- tion in the presence of Rashba Spin-Orbit coupling(SOC). We show that although Kohn-Luttinger instability in 2D in the absence of SOC occurs only to third order in the interaction, it occurs to second order in the presence of SOC. The critical temperature of the p-wave transition is calculated. [Preview Abstract] |
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