Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G23: Focus Session: MAG.THY II / Transport & General |
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Sponsoring Units: GMAG DMP DCOMP Chair: Wei Cai, Standford University Room: Baltimore Convention Center 320 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G23.00001: Efficient Ab-initio Calculation of the Anomalous Hall Conductivity of Fe by Wannier Interpolation Xinjie Wang, David Vanderbilt, Jonathan Yates, Ivo Souza Recently, a first-principles calculation of the anomalous Hall conductivity (AHC) of Fe as a Brillouin-zone integral of the Berry curvature was carried out and found to be in reasonable agreement with experimental results.\footnote{Y.~Yao {\it et al.}, Phys. Rev. Lett. {\bf 92}, 037204 (2004).} However, these authors observed extraordinarily strong and rapid variations of the Berry curvature with wavevector $k$ in the vicinity of avoided crossings and near-degeneracies in reciprocal space. A conventional first-principles calculation thus requires an extremely dense k-point mesh and is quite time-consuming. Here, we present an efficient first-principles approach for computing the AHC based on Wannier interpolation. First, a conventional electronic-structure calculation is performed for Fe, with spin-orbit included, on a relatively coarse k-point mesh. Second, maximally-localized Wannier functions are constructed by a post-processing step,\footnote{I.~Souza,N.~Marzari, and D.~Vanderbilt, Phys. Rev. B {\bf 65}, 035109 (2001).} thus transforming the full ab-initio problem into an effective tight- binding form. Finally, the needed quantities such as Berry potentials and curvatures are interpolated onto a fine k-point mesh and used to compute the AHC. Our approach gives good agreement with conventional, less efficient first-priciples calculations. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G23.00002: Spin and charge currents and current rectification in Luttinger liquids B. Braunecker, D. E. Feldman, J. B. Marston Asymmetries in spin and charge transport properties are of great interest for spintronic and electronic applications. We show that externally-driven spin and charge currents in a Luttinger liquid model of a one-dimensional quantum wire are strongly modified by the presence of a localized magnetic or nonmagnetic scatterer. A diode effect appears at low voltages when this scatterer is spatially asymmetric, and a non-monotonous dependence of the current on the voltage is possible.\footnote{D. E. Feldman, S. Scheidl, and V. M. Vinokur, {\it Phys. Rev. Lett.} {\bf 94}, 186809 (2005); B. Braunecker, D. E. Feldman, and J. B. Marston, {\it Phys. Rev. B} {\bf 72}, 125311 (2005)} [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G23.00003: Interplay of Rashba and \textit{sp-d} exchange couplings in magnetic 2DEGs Francisco Mireles, Henrique H. P. Freire, J. Carlos Egues In diluted magnetic semiconductor (DMS) quantum wells the \textit{sp-d} exchange interaction between the itinerant conduction electrons in the well and the localized electrons in the $d$ orbitals of the Mn impurities gives rise to interesting spin-dependent physics [1]. Recently, the interplay of the Rashba spin-orbit and the \textit{sp-d} exchange interactions in Mn-based wells has been recognized via Shubnikov-de-Haas measurements [2]. While the Rashba spin-orbit has been extensively studied in non-magnetic 2DEGs, its role in DMS systems with a competing \textit{sp-d} exchange interaction has not yet been addressed theoretically. In this work we present a\textbf{\textit{ k.p}} derivation of an effective Hamiltonian for a Mn-based quantum well with competing Rashba and \textit{sp-d} interactions, and show numerical results for the magnetoresistance $\rho _{xx}$ of typical magnetic 2DEGs using our effective Hamiltonian model. Our results shows interesting beating patterns of the $\rho _{xx}$ as a function of the temperature and carrier density which suggests a significant interplay between the spin-orbit and \textit{sp-d} exchange interactions, as a recent experiment observes [2]. [1] J. C. Egues, PRL \textbf{78}, 4578 (1998); H. J. P. Freire and J. C. Egues, cond-mat/0412491. [2] Y. S. Gui \textit{et al.} EPL. \textbf{65}, 393 (2004). [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G23.00004: Berry Phase and \textit{Ab initio }Calculation of Anomalous {\&} Spin Hall Effect Invited Speaker: Berry phase in momentum space in spin-orbit coupled bands affects the charge and spin transport of electrons in real space, and produces fascinating new phenomena. Two examples are the anomalous Hall effect and the recently proposed intrinsic spin Hall effect. Avoiding the ambiguity introduced by model Hamiltonians, we evaluate the Berry curvature accurately for real materials by parameter-free \textit{ab initio }methods. We have calculated the anomalous Hall conductivity for ferromagnetic crystals of Fe, Co, Ni, Mn$_{5}$Ge$_{3}$, and CuCr$_{2}$Se$_{4-x}$Br$_{x}$, and investigated the magnetization dependence and sign change, with results in quantitative agreement with experiments [1,2]. Using the \textit{ab initio }technique, we have also evaluated the intrinsic spin Hall conductivity in non-magnetic semiconductors (GaAs, Si, Ge, AlAs) as well as metals (W and Cu) [3,4]. We have systematically studied how its sign and magnitude depend on strain, doping density, and frequency for the semiconductors, and found very large values for the metals. [1] Yugui Yao \textit{et al}., \textit{Phys. Rev. Lett.} 92, 037204 (2004). [2] Changgan Zeng, Yugui Yao, Qian Niu, and Hanno, H. Weitering, (\textit{Phys. Rev. Lett. }in review). [3] Yugui Yao and Zhong Fang, \textit{Phys. Rev. Lett.} 95, 156601 (2005). [4] Guangyu Guo, Yugui Yao, Qian Niu, \textit{Phys. Rev. Lett.} 94, 226601 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G23.00005: Emerging Magnetism at Pt Nanocontacts and Nanowires Erio Tosatti, Andrea Dal Corso, Anna Delin, Alexander Smogunov, Ruben Weht Nanocontacts made of transition metals that are nonmagnetic in bulk may display a possible onset of Hund's rule local nanomagnetism. Zero-temperature DFT electronic structure calculations indicate that magnetism may occur in monatomic nanowires of 4d and 5d transition metals, generally favored by tensile stress, and sometimes also by spin-orbit coupling.[1] In the particular case of a Pt monatomic nanowire, resurgence of orbital magnetism is predicted to conspire with intra-atomic exchange to give rise to a spontaneous magnetization even at zero stress. Here we expect an extraordinarily large magnetic anisotropy, spin and orbital magnetization lying strictly parallel to the nanowire axis. A Pt nanowire-like contact should thus behave as an Ising nanomagnet. Since magnetic exchange splittings generally affect the number of conducting channels crossing the Fermi level, it is likely that the ballistic conductance through a nanowire-like contact[2] will be affected by the emerging nanomagnetism of Pt. [1] A. Delin and E. Tosatti, Phys. Rev. B 68, 144434 (2003); A. Delin, et al., Phys. Rev. Lett. 92, 057201 (2004). [2] A. Smogunov, A. dal Corso and E. Tosatti, Surface Science 507, 609 (2002); ibid. Surf. Sci. 532, 549 (2003); ibid. Surf. Sci. 566, 390 (2004); A. Smogunov et al., Phys. Rev. B 70, 045417 (2004); and to be published. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G23.00006: Atomic and electronic properties of Ni/Al$_{2}$O$_{3}$/Ni magnetic tunnel junction You Lin, Ivan Oleynik Numerous experimental and theoretical investigations show that the spin-polarization of the tunneling current in magnetic tunnel junctions (MTJs) is not an intrinsic property of the ferromagnet alone, but depends on the electronic properties of the insulator and the ferromagnet-insulator interface. We report the results of first-principles density-functional studies of Ni{\_}/Al$_{2}${\_}O$_{3}${\_}/Ni MTJs. The atomic structure has been studied for different interface terminations. The interfacial energetics was quantified based on the work of separation and the most stable interfacial structures have been identified. The electronic and magnetic properties have been thoroughly analyzed in the framework of local densities of states. The possibility of strongly-correlated electronic effects has also been considered by performing additional LDA+U calculations of interfacial electronic structure. We will discuss various factors that might contribute to spin polarization of the tunneling current in the MTJs. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G23.00007: Interface properties of nickel/organics/nickel magnetic tunnel junctions Wesley Fabella, Ivan Oleynik In recent years, substantial experimental efforts were directed towards developing a new class of spintronic devices based on organic polymers, oligomers and small molecules. In particular, recent experiments indicate efficient spin injection and spin transport in ferromagnet/organic systems. However, the electron and spin transport through metal-molecule-metal junctions depends crucially on the properties of ferromagnet/organics interfaces. We performed first-principles density functional theory (DFT) calculations of nickel/organic-molecule/nickel magnetic tunnel junctions. Organic systems include both saturated alkane and unsaturated thiophene molecules. The choice of the system was made based on available experimental information. The atomic structures of metal/molecule interfaces were built and the geometries of molecule/metal interfaces were determined by optimizing the interfacial work of separation. We will discuss the electronic properties of molecule/electrode junctions including distribution of charge densities, local charges, local magnetic moments and local densities of states on individual atoms. The spin-dependent interfacial properties will be discussed in relation to spin-dependent transport in organics-based MTJs. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G23.00008: Nonequilibrium Transport in Quantum Impurity Models:(Bethe-Ansatz for open systems) Pankaj Mehta, Natan Andrei We develop an exact non-perturbative framework to compute steady-state properties of quantum-impurities subject to a finite bias. We show that the steady-state physics of these systems is captured by nonequilibrium scattering eigenstates which satisfy an appropriate Lippman-Schwinger equation. Introducing a generalization of the equilibrium Bethe-Ansatz - the Nonequilibrium Bethe-Ansatz (NEBA), we explicitly construct the scattering eigenstates for the Interacting Resonance Level model and derive exact, nonperturbative results for the steady-state properties of the system. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G23.00009: Ab initio investigation of domain walls in ferromagnetic films and wires Heike C. Herper, Peter Entel Domain walls can be viewed as special type of interfaces. In case of relatively thin domain walls a magnetoresistance effect can be observed due to the domain wall, which has been addressed in a considerable number of experimental and theoretical studies. However, only few ab initio investigations have been done in this field. We have determined the influence of domain walls on the resistance in ferromagnetic Co and Ni layers employing the fully-relativistic, spin-polarized Screened Korringa-Kohn-Rostoker method (SKKR) and the Kubo-Greenwood formula. In order to investigate the magnetoresistance in thin wires we made use of a real-space SKKR and Kubo-Greenwood method. The magnetoresistance and the formation energy of the domain walls have been calculated depending on the thickness and the type of the domain wall. As expected the formation energy decreases with increasing number of FM layers. Throughout this paper all domain walls are assumed to be oriented in the plane of layers. In order to avoid contributions from the anisotropic magnetoresistance the current flows perpendicular to the domain walls, i.e. parallel to the surface normal. The results support the predictions from Zhang and Levy concerning the width dependence of the domain wall resistance. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G23.00010: Orbital magnetic susceptibility in an integrable system M.X. Lou, J.M.A.S.P. Wickramasinghe, R.A. Serota We investigate, analytically and numerically, the orbital magnetism of free electron gas in a rectangular box, which is a model classically integrable system. We find that both the average orbital susceptibility and its fluctuations are determined by the two-level van Vleck susceptibility that involves the last occupied (Fermi) level and the first unoccupied level. This is in line with previous results for disordered (classically chaotic) systems. The mesoscopic fluctuations, however, are much larger in integrable systems. We find that, in units of ${\mu _B^2 } \mathord{\left/ {\vphantom {{\mu _B^2 } \Delta }} \right. \kern-\nulldelimiterspace} \Delta $, where $\Delta $ is the mean level spacing, the average orbital susceptibility is $\sim \left\langle {\left| {\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over {L}} _{if} } \right|^2} \right\rangle \left\langle {(\varepsilon _f -\varepsilon _i )^{-1}} \right\rangle $, where $\left\langle {\left| {\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\frown$}}\over {L}} _{if} } \right|^2} \right\rangle \sim \sqrt N $, $L$ is the orbital angular momentum, $N$ is the number of electrons in the box, and $\varepsilon _f -\varepsilon _i $ is the level spacing (in units of $\Delta )$ between the last occupied level and the first unoccupied level. $\left\langle {\varepsilon _f -\varepsilon _i )^{-1}} \right\rangle $ is principally divergent and hence determined by the relevant cut-off. [Preview Abstract] |
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