### Session N19: Semiconductor Spin Transport

Sponsoring Units: GMAG DMP
Chair: Jim Chelikowsky, University of Texas
Room: Baltimore Convention Center 316

 Wednesday, March 15, 2006 8:00AM - 8:12AM N19.00001: High field magnetoresistance in $p$-(In,Mn)As/$n$-InAs heterojunctions Bruce Wessels , Steven May The high field magnetoresistive properties of a $p$-In$_{0.96}$Mn$_{0.04}$As/$n$-InAs junction have been measured. The heterojunctions were formed by epitaxially depositing an InMnAs thin film on an InAs substrate using metal-organic vapor phase epitaxy. Under forward bias, a large, nonsaturating magnetoresistance is observed at temperatures from 25 to 295 K in fields up to 9 T. At room temperature, the magnetoresistance increases linearly with magnetic field from 1.5 to 9 T and is greater than 700 {\%} at 9 T. The magnetoresistance can be simulated using a modified diode equation, including a field-dependent series magnetoresistance. Wednesday, March 15, 2006 8:12AM - 8:24AM N19.00002: Room Temperature Tunnel Magnetoresistance and Spin Polarized Tunneling Studies with Organic Semiconductor Barrier T. S. Santos , P. Migdal , I. C. Lekshmi , J. S. Moodera , J. S. Lee Organic semiconductors, $\pi$-conjugated, with a weak spin-orbit interaction, show promise for spin-conserved transport applications.[1,2] An organic spin-valve utilizing the molecular organic semiconductor tris (8-hydroxyquinolinato)aluminum (Alq$_{3}$), demonstrated giant magnetoresistance at LHe temperatures.[1] The Alq$_{3}$ films in this spin-valve were $>$130nm, and the spin diffusion length was 45nm. Our current study demonstrates spin polarized tunneling through an ultra-thin layer of Alq$_{3}$ in a magnetic tunnel junction. Significant tunnel magnetoresistance was measured in a MTJ structure at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the I-V behavior and temperature and bias dependence of the TMR, show good quality of the organic tunnel barrier. Spin polarization of the tunnel current from Co, Fe and NiFe electrodes through the Alq$_{3}$ layer was directly measured using a superconducting Al electrode as the spin detector. This demonstration of spin-conserved transport through an organic semiconductor at room temperature shows the potential of this material for further study. Supported by KIST- MIT Program and NSF. 1) Z. H. Xiong, \emph{et al}, $Nature$ \textbf{427} 821 (2004).\\ 2) V. Dediu, \emph{et al}, \emph{Solid State Commun.} \textbf{122} 181 (2002). Wednesday, March 15, 2006 8:24AM - 8:36AM N19.00003: Diluted Magnetic Double Barrier Resonant Tunneling Structures for Novel Magnetically-Defined Quantum Dot and Nano Structures Zaili Fang , A.M. Chang , X.Y. Liu , J.K. Furdyna The further development of Spintronics requires the direct control of the spin degree of freedom. A milestone on the path towards this accomplishment has been recently achieved by the demonstration of the successful operation of a magnetic resonant tunneling diode \footnote{A. Slobodskyy \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 246601 (2003)}. We will report our results on manganese doped double barrier tunneling structures (II-IV group) with varying doping, confining potential and well width. These structures, in which both the barrier and well are doped with manganese, show a Zeeman splitting tendency in the vertical transport through the barrier in magnetic field. This tendency can be exploited as spin filter for spintronic applications. Based on this spin voltage transport property of diluted magnetic semiconductor heterostructures and the interaction of superconductors and semiconductors, we propose a novel spin aligned vertical quantum dot device. Progress on the fabrication of this device will also be reported. Wednesday, March 15, 2006 8:36AM - 8:48AM N19.00004: Spin dependent tunnelling in indirect double-barrier structures Titus Sandu Spin-dependent tunelling and polarization in GaAs/AlAs/GaAs based resonant tunelling diode are studied by a tight-binding model. We compare the GaAs/AlAs/GaAs case with similar structures where the barriers are direct and show the advantages of a GaAs/AlAs/GaAs configuration. Wednesday, March 15, 2006 8:48AM - 9:00AM N19.00005: Magnetic Resonances in the circular polarization of light emitted by Fe/InAs QD spin LEDs M. Yasar , S. Delikanli , R. Mallory , A. Petrou , G. Kioseoglou , A.T. Hanbicki , C.H. Li , B.T. Jonker The circular polarization $P_{circ}$ of the light emitted from InAs QD LEDs was studied as function of applied magnetic field $B$ in the 5-75 K temperature range. The quantum dots are incorporated at the center of a GaAs quantum well of width $L_{W}$ . At$T$ = 5 K we observed two distinct resonances in the $P_{circ}$ versus $B$ plot. For $L_{W}$ = 430 {\AA} the resonances occur at $B$ = 4.6 T (strong) and $B$ = 2.3 T (weak). The strength of the resonances depends critically on bias voltage $V$ (very pronounced at low $V$values ) The resonances become weaker with increasing temperature and disappear completely by $T$ = 60 K. No resonances were observed in LEDs in which the QDs were not incorporated in a quantum well. We propose a model that takes into account the confinement conduction subbands of the GaAs quantum well and the dependence of their energies on magnetic field. Acknowledgements: This work is supported by the DARPA SpinS Project, ONR, and NSF Wednesday, March 15, 2006 9:00AM - 9:12AM N19.00006: Tailoring magnetic anisotropy in ferromagnetic metal / semiconductor contacts for spin injection C. H. Li , G. Kioseoglou , A. T. Hanbicki , T. J. Zega , R. M. Stroud , B. T. Jonker Robust spin injection across an Fe/AlGaAs interface has recently been demonstrated, producing an electron spin polarization $>$32{\%} in a GaAs QW. In an effort to incorporate a spin injecting metal contact with perpendicular remanence and to explore interface effects on spin injection, we have grown MnGa thin films epitaxially on GaAs(001) LED structures by MBE. Streaky RHEED patterns indicate single crystalline films. Although lattice matched to GaAs, TEM shows that while MnGa crystallizes nicely away from the interface, defects exist at the interface. The insertion of a thin ($\sim$ 5ML) Fe seed layer between MnGa and AlGaAs promotes the initial nucleation of MnGa and provides a means to control the structure of the spin-injecting interface, while the magnetic behavior is determined by the MnGa. Samples are processed to form surface emitting LEDs, and the EL is dominated by QW excitonic emission. A 0.5{\%} remanent circular polarization is observed, which tracks the MnGa magnetization obtained by independent SQUID measurements. Comparison between MnGa spin-LEDs with and without the Fe seed layer (including interface properties), and magneto absorption effects in these heterostructures, will be discussed. Wednesday, March 15, 2006 9:12AM - 9:24AM N19.00007: Quasi-one-dimensional spin polarized states in T-shaped nanostructures Pawel Redlinski , Boldizsar Janko We present results of theoretical and numerical investigations of T-shaped semiconductor nano-wire structures. Such structures have been synthesized in Molecular Beam Epitaxy laboratories by using traditional (GaAs) and magnetic (GaMnAs) semiconductors. The wire is formed in a three-stage MBE process and in its final form, the structure looks as if one quantum well (QW) called Stem well grew perpendicularly into the second QW, the so-called Arm well. The quasi 1D states are formed at the intersection of the two QWs. Typically in the T-shaped structures the thickness of the wire is of order of few nm. For such thickness there is only one conduction channel and the energy states of quasi-particles are indexed by a single quantum number, the 1D linear momentum $k$, and by its spin. There are two distinctive features present in this type of systems: strong spin-orbit coupling, and the lack of square symmetry of the wire cross section. These features have profound implications and make this system very important from theoretical point of view. Results, obtained within the \textbf{\textit{k\textbullet p}} formalism, show that at $k\ne$0 the valence band states are non-degenerate with respect to the spin degree of freedom. Both dispersions of spin up and spin down states are well modeled as parabolic bands but with different effective masses. It opens possible to manipulate the spin degrees of freedom in a T-shaped quantum structure. Wednesday, March 15, 2006 9:24AM - 9:36AM N19.00008: Atomistic spin-orbit effects on the electronic structure of T-shaped quantum wires Javier Aizpurua , Garnett W. Bryant The electronic structure and optical properties of GaAs/AlGaAs T-shaped quantum wires are studied by use of an empirical tight-binding method (ETB). This model allows us to study atomistic effects on the electronic structure of wires that have a complicated geometrical cross section. We find that the electronic structure for electrons is similar to that described by effective mass models whereas the electronic structure for holes shows important modifications when spin-orbit coupling is included in the atomistic model. The binding energies of the holes in a T-wire agree with previous effective mass model calculations. However, we find that asymmetries in the spatial distribution of these hole states are induced by atomistic spin-orbit effects. Moreover, the atomistic tight-binding model predicts complex band crossings for hole states that are not predicted by simpler effective mass theories. Consequences for the optical response of T-wires and for excitonic and electron-hole plasma phases in T-wires are discussed. Wednesday, March 15, 2006 9:36AM - 9:48AM N19.00009: Quasi-one-dimensional spin field-effect transistors Jae-Seung Jeong , Hyun-Woo Lee We study a spin field effect-transistor(SFET) with multiple transport modes. Energy dispersion relations and spin profiles of eigen-transport modes are examined numerically and analytically for weak and strong Rashba spin-orbit coupling parameters. Electron transport properties of the multiple-mode SFET are investigated including the Fabry-Perot-type interference due to multiple reflections and the peak splitting by external magnetic fields. Impurity scattering effects are also addressed. Wednesday, March 15, 2006 9:48AM - 10:00AM N19.00010: Lateral diffusive spin transport in layered structures Hanan Dery , Lukasz Cywinski , Lu J. Sham A one dimensional theory of lateral spin-polarized transport is derived from the two dimensional flow in the vertical cross section of a stack of ferromagnetic and paramagnetic layers. This takes into account the influence of the lead on the lateral current underneath, in contrast to the conventional 1D modeling by the collinear configuration of lead/channel/lead. Our theory is convenient and appropriate for the current in plane configuration of an all-metallic spintronics structure as well as for the planar structure of a semiconductor with ferromagnetic contacts. For both systems we predict the optimal contact width for maximal magnetoresistance and propose an electrical measurement of the spin diffusion length for a wide range of materials. This work was supported by NSF DMR-0325599. Wednesday, March 15, 2006 10:00AM - 10:12AM N19.00011: Microscopic calculation of the Gilbert damping in a spin-polarized two-dimensional electron liquid with Rashba spin-orbit interaction Giovanni Vignale , Ewelina Hankiewicz We present a microscopic calculation, based on mode-coupling theory, of (i) the Gilbert damping constants for in-plane and out-of-plane relaxation and (ii)the magnetic anisotropy tensor of the spin-polarized two-dimensional electron liquid in the presence of a spin-orbit interaction of the Rashba form. Wednesday, March 15, 2006 10:12AM - 10:24AM N19.00012: Controlling spin in an electronic interferometer with spin-active interfaces Christoph Bruder , Audrey Cottet , Takis Kontos , Wolfgang Belzig , Christian Sch\"onenberger We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the \textit{spin-dependence} of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire. Wednesday, March 15, 2006 10:24AM - 10:36AM N19.00013: Interferometric detection of spin-polarized transport G. Salis , S. F. Alvarado It is shown that in addition to its sensitivity to spin polarization, the magneto-optic Kerr effect strongly depends on the spatial distribution of spin-polarized charge carriers. Using time-resolved Kerr rotation, the dynamics of spin- polarized electrons can thus be monitored on the nanometer length scale. This is demonstrated experimentally for optically- excited electron spins in the depletion layer of $n$-doped GaAs close to a metallic electrode. The Kerr rotation exhibits fast oscillations that originate from an interference of the light reflected at the electrode with that reflected at the front of the electron distribution moving into the semiconductor. From these oscillations, the dynamics of the electron front is obtained, which is strongly screened by the space-charge field of the excited electron-hole pairs and can be controlled by an electric bias across the Schottky barrier. In addition, the dynamics provides information on the Schottky-barrier height, the depletion-layer thickness and the doping concentration. Wednesday, March 15, 2006 10:36AM - 10:48AM N19.00014: Spin Resolved Current Focusing in InSb Heterostructures Aruna Dedigama , David Deen , Sheena Murphy , Niti Goel , Joel Keay , Michael Santos , Kyoichi Suzuki , Sen Miyashita , Yoshiro Hirayama Spin-resolved current focusing has been observed in InSb/AlInSb structures. While InSb has the most significant Rashba and Dresselhaus effects of any of the III-V semiconductors, Dresselhaus effects are expected to dominate in the symmetrically doped structures used here. The double quantum point contact devices were designed with typical dimensions of 0.5 micron which preserve ballistic transport up to 185K as measured in previous experiments. Focusing peaks were observed near the expected values of perpendicular magnetic field; however the first focusing peak was a doublet. With application of a parallel magnetic field the doublet evolved into a singlet as expected for spin resolved focusing. Wednesday, March 15, 2006 10:48AM - 11:00AM N19.00015: Spintronic Ratchet Sayeef Salahuddin , Supriyo Datta Carefully designed ratchets are of great interest, practically and conceptually, as means to convert fluctuations into useful work. We argue that a recently proposed Spin-Capacitor'' [Appl. Phys. Lett. \textbf{87}, 013115~(2005)] exhibits characteristics that have close resemblance to ratchets. It shows unidirectional current-voltage (I-V) characteristics that depend on the spin excitation spectrum of a neighboring array [http://arxiv.org/abs/cond-mat/0511566]. More interestingly, if the spins in the array are out of equilibrium, useful work can be extracted at the expense of energy/entropy. This is manifested as a \textbf{\textit{non-zero current at zero bias}} and we argue that a recent experiment in an integer quantum hall system [http://link.aps.org/abstract/PRL/v95/e056802] shows evidence for this general principle.