Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P5: Spins in Two Dimensions: Graphene, 2DEGs and Quantum WellsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Hanan Dery, University of Rochester Room: 301 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P5.00001: Homoepitaxial graphene tunnel barriers for spin transport Invited Speaker: Adam Friedman Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions (magnetic field, temperature, etc.) usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. We demonstrate successful tunneling, charge, and spin transport with a fluorinated graphene tunnel barrier on a graphene channel. We show that while spin transport stops short of room temperature, spin polarization efficiency values are the highest of any graphene spin devices. We also demonstrate that hydrogenation of graphene can also be used to create a tunnel barrier. We begin with a four-layer stack of graphene and hydrogenate the top few layers to decouple them from the graphene transport channel beneath. We demonstrate successful tunneling by measuring non-linear IV curves and a weakly temperature dependent zero-bias resistance. We demonstrate lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the non-local Hanle effect to be commensurate with previous studies. The measured spin polarization efficiencies for hydrogenated graphene are higher than most oxide tunnel barriers on graphene, but not as high as with fluorinated graphene tunnel barriers. However, here we show that spin transport persists up to room temperature. Our results for the hydrogenated graphene tunnel barriers are compared with fluorinated tunnel barriers and we discuss the possibility that magnetic moments in the graphene tunnel barriers affect the spin transport of our devices. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P5.00002: Tight-binding description of spin-orbit coupling in graphene due to adatoms Susanne Irmer, Denis Kochan, Klaus Zollner, Martin Gmitra, Tobias Frank, Jaroslav Fabian We present realistic effective tight-binding models for proximity spin-orbit coupling in graphene due to adatoms at top, bridge, and hollow positions. The models are built from symmetry arguments and fitted to ab initio calculations for a variety of adsorbants, such as H [1], F [2], Cu, and CH$_{\mathrm{3}}$ [3]. For each of these adatoms we provide magnitudes for orbital couplings to the adsorbants, as well as the intrinsics, Rashba, and pseudospin-inversion asymmetry (PIA) couplings. Our models can be used to study spin relaxation, spin Hall effect, and spin transport using quantum transport models. [1] M. Gmitra, D. Kochan, J. Fabian, Phys. Rev. Lett. 110, 246602 (2013). [2] S. Irmer, T. Frank, S. Putz, M. Gmitra, D. Kochan, J. Fabian, Phys. Rev. B 91, 115141 (2015). [3] K. Zollner, T. Frank, S. Irmer, M. Gmitra, D. Kochan, J. Fabian, arXiv:1507.02820 [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P5.00003: Spintronics with Graphene and van der Waals heterostructures Saroj Dash, M.Venkata Kamalakar, André Dankert Two-dimensional (2D) atomic crystals provide a large class of materials proposed to be important for nanoelectronics and spintronic. Here we present two important advancements in graphene spintronics by employing 2D materials and heterostructures. Graphene is considered to be an ideal material for spin transport due to the high mobility and long spin lifetime of the carriers. We realized spin transport over a long distance of 16 \textmu m and spin lifetimes up to 1.2 ns in large area CVD graphene on SiO$_{\mathrm{2}}$/Si substrate at room temperature [1]. Subsequently, using the h-BN tunnel barrier/graphene van der Waals heterostructure; we observe an enhancement in the tunnel spin polarization [2], and a negative spin signal for thicker h-BN barriers. These findings open a platform for exploring novel spin functionalities in 2D crystal heterostructures and understanding the basic science that control their behavior. [1] M. V. Kamalakar et al., Long Distance Spin Communication in Chemical Vapor Deposited Graphene, Nature Communications 6, 6766 (2015). [2] M. V. Kamalakar et al., Enhanced Tunnel Spin Injection into Graphene using Hexagonal Boron Nitride; Scientific Reports 4, 61446 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P5.00004: Proximity Anisotropic Magnetoresistance in Graphene Jeongsu Lee, Jaroslav Fabian We theoretically investigate charge transport in graphene that is on a ferromagnetic-insulator substrate. The substrate induces spin polarization in graphene---ferromagnetic proximity effect---as demonstrated recently experimentally [1]. We show, using realistic models [2, 3], that the presence of spin-orbit coupling in proximity ferromagnetic graphene leads to anisotropic magnetoresistance whereby graphene's resistance changes with varying magnetic field orientation (both in and out of plane). We evaluate the magnitude as well as the angular dependence of this novel effect using conventional transport models [4] and propose specific experimental schemes to measure it. This work is supported by DFG SFB 689. References [1] Z. Wang, C. Tang, R. Sachs, Y. Barlas, and J. Shi, Phys. Rev. Lett. 114, 016603, (2015) [2] M. Gmitra, D. Kochan, and J. Fabian, Phys. Rev. Lett. 110, 246602 (2013). [3] M. Gmitra, S. Konschuh, C. Ertler, C. Ambrosch-Draxl, and J. Fabian, Phys. Rev. B 80, 235431, (2009) [4] S. Adam, E. H. Hwang, V. M. Galitski, and S. Das Sarma, Proc. Natl. Acad. Sci. U.S.A., 104, 18392, (2007) [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P5.00005: Stoner-like theory of Magnetism in Silicon MOSFETs Denis Golosov We consider quasi-two-dimensional gas of electrons in a typical Si-MOSFET, assuming contact repulsive interaction between electrons. Magnetisation and susceptibility are evaluated within the mean-field approach. The finite thickness of inversion layer results in an interaction-induced electron wave function change, not found in both purely two-dimensional and three-dimensional (bulk) cases. Taking this self-consistent change into account leads to an increased susceptibility and ultimately to a ferromagnetic transition deep in the high-density metallic regime. We further find that in the paramagnetic state, magnetisation increases sublinearly with increasing in-plane magnetic field. In the opposite limit of low carrier densities, the effects of long-range interaction become important and can be included phenomenologically via bandwidth renormalisation. Our treatment then suggests that with decreasing density, the metal-insulator transition is preceded by a ferromagnetic instability. We discuss the validity of our mean-field scheme, and relate the results to the available experimental data. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P5.00006: Rashba scattering in the dilute limit Joel Hutchinson, Joseph Maciejko In two-dimensional (2D) noncentrosymmetric crystals, the spin degeneracy of the electronic band structure may be lifted by Rashba spin-orbit coupling. The resulting spin-split dispersion is responsible for the spin Hall effect and has desirable applications to spintronics. This spin-split dispersion is described in terms of two distinct helicity bands, but below a threshold energy, electrons are confined to one of these. At the bottom of this lower band, the density of states exhibits a van Hove singularity. This is the relevant regime for a dilute spin-orbit coupled 2D electron gas, which has been shown to host a variety of exotic phases in the presence of electron-electron interactions. In this talk we investigate scattering of Rashba electrons off a circular potential barrier in this dilute limit, which is relevant both for impurity scattering in the noninteracting limit as well as for short-range two-particle scattering in the interacting problem. The S matrix and scattering cross section are determined, and it is found that scattering becomes effectively one-dimensional at the band bottom. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P5.00007: Edge spin accumulation in a two-dimensional electron gas with two subbands Alexander Khaetskii, J. Carlos Egues We have studied the edge spin accumulation in 2D electron gas due to the intrinsic mechanism of spin-orbit interaction for the case of a two-subband structure. This study is strongly motivated by recent experiments [1] which observed the spin accumulation near the edges of a high mobility 2D electron system in a bilayer symmetric GaAs structure in contrast to zero effect in a single-layer configuration. Our theoretical explanation is based on the Rashba-like spin-orbit interaction which arises as a result of the coupling between two subband states of opposite parities in a symmetric quantum well [2]. Following the method developed in [3], we have calculated the edge spin density in a quasi-ballistic regime, and explained the experimental results, in particular, a large magnitude of the edge spin density. We showed that one can easily proceed from the regime of strong spin accumulation to the regime of weak one. It opens up a possibility to construct an interesting new spintronic device. [1]. F. Hernandez et al., Phys. Rev. B \textbf{88}, 161305(R) (2013). [2]. E. Bernardes et al., Phys. Rev. Lett. \textbf{99}, 076603 (2007). [3]. A. Khaetskii, Phys. Rev. B \textbf{89}, 195408 (2014). [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P5.00008: Dynamical spin injection into a two-dimensional electron gas in an AlGaAs/GaAs structure Kenro Ohtomo, Yuichiro Ando, Teruya Shinjo, Tetsuya Uemura, Masashi Shiraishi A two-dimensional electron system in a GaAs-based heterostructure is the attractive platform for spintronics since it has high mobility and spin-orbit interaction can be modulated by the gate voltage$^1$. Thus, it is a possible platform to realize electric gate-controlled spin transistor$^2$. However, room-temperature spin transport through GaAs-based heterostructure has yet to be shown. We report first spin transport through the quantum well at GaAs/AlGaAs interface at room temperature. We used spin pumping under ferromagnetic resonance to inject spins from the Ni$_{80}$Fe$_{20}$ to the GaAs/AlGaAs quantum well. Generated spin current propagated through the 1 $\mu$m channel and was detected using spin-charge conversion inverse spin Hall effect in the Pt electrode. In agreement with spin pumping theory, polarity of the spin transport signal was reversed together with magnetization of the Ni$_{80}$Fe$_{20}$. This first demonstration of spin transport through a quantum well at a semiconductor heterostructure interface at room temperature opens a way to realize Datta-Das spin-based transistor.\\ \\ $^1$ J. Nitta, et al., PRL 78, 1335 (1997).\\ $^2$ S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P5.00009: Hole spin coherence in coupled GaAs/AlAs quantum wells Christian Gradl, Michael Kempf, Johannes Holler, Dieter Schuh, Dominique Bougeard, Christian Schueller, Tobias Korn Due to its p-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. Especially for some low-symmetry growth directions, a strong anisotropy of the hole g factor with respect to the in-plane magnetic field direction is theoretically predicted. Therefore, we perform time-resolved Kerr rotation measurements on an undoped [113]-grown double quantum well (QW) structure to resolve the spin dynamics of hole ensembles at low temperatures. Our gated system consists of two QWs with different well widths, which we use for the spatial separation of the optically excited electron-hole pairs. Thus, we are able to create hole ensembles with spin lifetimes of several hundreds of picoseconds in the broader QW without any doping. This allows the observation of a strong hole g factor anisotropy by varying the magnetic field direction in the QW plane. The experimental g factor values are in very good agreement with theoretical predictions. Furthermore, we observe an unexpected additional non-precessing component in the Kerr signal for certain in-plane magnetic field directions. This might have its origin in a precession axis that is tilted relative to the magnetic field due to the crystal structure of this low-symmetry growth direction. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P5.00010: Investigation and direct mapping of the persistent spin helix in confined structures Markus Schwemmer, Matthias Weingartner, Roland V\"{o}lkl, Martin Oltscher, Dieter Schuh, Dominique Bougeard, Tobias Korn, Christian Sch\"{u}ller The spin-orbit field in GaAs-based quantum well (QW) structures typically consists of two different contributions: Dresselhaus and Rashba field. The geometry of the Dresselhaus field, which arises due to the bulk inversion asymmetry, is mostly determined by the growth direction of the quantum well. The Rashba field instead is caused by a structure inversion asymmetry, which can be controlled, e.g. by the modulation doping. For the specific case of a (001)-grown GaAs quantum well with equal strength of Dresselhaus and Rashba fields, the effective spin-orbit field is oriented along the in-plane [110] direction for all \textit{k} values and the spin splitting for this direction vanishes. For optically injected spins, which are initially oriented perpendicular to the QW plane, a persistent spin helix (PSH) state forms. We use a femtosecond pulsed TiSa-Laser system combined with a magneto-optical Kerr effect microscope for time- and space-resolved mapping of the PSH. With this technique, we investigate the PSH behavior in confined structures, e.g., thin channels along the helix direction. Hence we find that lateral confinement increases the effective PSH lifetime drastically. In more complex structures, we observe that PSH formation is even stable under a forced direction change. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P5.00011: Role of contact resistance in the effective spin relaxation rate in graphene spin valves Gordon Stecklein, Yoska Anugrah, Jing Li, Steven J. Koester, Paul Crowell Recent experiments (Maassen et al., PRB $\textbf{86}$ 235408 (2012), Idzuchi et al., PRB $\textbf{91}$ 241407(R) (2015)) have identified the role of finite contact resistances in determining the spin lifetime in graphene based on Hanle measurements of lateral spin valves. We have investigated this effect in spin valves fabricated using Co/AlOx tunnel barriers and graphene grown by chemical vapor deposition. By carrying out non-local spin valve and Hanle measurements over a wide range of gate voltages, we observe a variation in the spin signal that can be explained by the role of the contacts. Using the measured interface resistance, we quantify the degree of contact-induced spin sinking as the ratio of the contact resistance to the channel spin resistance and show that the variation in spin signal is explained by variation in this spin sinking parameter. By properly accounting for the effect of the contact resistance, we measure a spin lifetime that varies between 150-500 picoseconds. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P5.00012: Boundary conditions for transition-metal dichalcogenide monolayers in the continuum model Csaba G. P\'eterfalvi, Andor Korm\'anyos, Guido Burkard We derive the boundary conditions for MoS$_2$ and similar transition-metal dichalcogenide honeycomb (2H polytype) monolayers with the same type of $\mathbf{k}\!\cdot\!\mathbf{p}$ Hamiltonian within the continuum model around the K points. [1] In an effective 2-band description, the electron-hole symmetry breaking quadratic terms are also taken into account. We model the effect of the edges with a linear edge constraint method that has been applied previously to graphene. Focusing mainly on zigzag edges, we find that different reconstruction geometries with different edge-atoms can generally be described with one scalar parameter varying between 0 and $2\pi$. We analyze the edge states and their dispersion relation in MoS$_2$ in particular, and we find good agreement with the results of previous density functional theory calculations for various edge types. \\ {[1]} Cs.~G.~P\'eterfalvi, A.~Korm\'anyos, G.~Burkard, arXiv:1509.00184 (2015). [Preview Abstract] |
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