Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials III |
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Sponsoring Units: GMAG DMP Chair: Joshua Folk, University of British Columbia Room: 108 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J6.00001: Spin Hall Effect Induced by Resonant Skew Scattering in Graphene Tatiana Rappoport, Aires Ferreira, Miguel Cazalilla, Antonio Castro Neto The spin Hall effect is the appearance of a transverse spin current in a non-magnetic conductor by pure electrical control; it can originate from the spin-dependent skew scattering of electrons by impurities in the presence of SOI. We consider a monolayer of graphene decorated by a small density of impurities generating a spin-orbit interaction in their surroundings. We show that large spin hall effect develops through skew scattering and it is strongly enhanced in the presence of resonant scattering. Our single impurity scattering calculations show that impurities with either intrinsic or Rashba spin-orbit coupling in a graphene sheet originate robust SHE. Also, the solution of the transport equations for a random distribution of impurities suggests that the spin Hall effect is robust with respect to thermal fluctuations and disorder averaging. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J6.00002: Alternative generation of spin current in graphene Jung-Woo Yoo, Mi-Jin Jin, Jungmin Park, Vijayakumar Modepalli, Jun-Hyeon Jo The manipulation of spin current which can be achieved in various device configurations has been under intense research in recent years. The spin current is typically obtained by injecting electrons from the ferromagnetic electrodes. In this study, we employed alternative methods for the generation of spin current in graphene. The first method we studied is using spin Hall effect. In the spin Hall effect, the charge current generates spin current due to a relativistic spin-orbit coupling. Generally the spin-orbit coupling in graphene is extremely weak to produce substantial spin current. We employed physical doping of heavy atoms on top of the graphene layer for the spin Hall induced spin current in graphene. The second alternative method we investigated is seebeck spin tunneling. The ferromagnetic electrode together with thin tunnel barrier (1-3nm of Al$_{2}$O$_{3}$ layer) was employed to introduce thermally induced spin imbalance in graphene. The gate dependence of generated spin current reflects unique electronic structure of graphene. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J6.00003: Transport Studies of Exchange Interaction at Magnetic-Insulator/Graphene and Magnetic-Insulator/Topological-Insulator Interfaces Ching-Tzu Chen, Sunwoo Lee, Davide Cutaia, Peng Wei, Jagadeesh Moodera, Joel Chudow, Daniel Worledge, Anthony Richardella, Nitin Samarth Spintronics, where carrier spin instead of charge serves as the state variable, is a promising candidate for post-CMOS low-voltage logic. An essential component of the spin-FET class of spintronic devices is the electrical modulation of spin. To realize this functionality, we explore the interfacial exchange interaction of quasi-2D systems in proximity to a magnetic insulator (MI). We study the magneto-transport of graphene/MI heterostructures as the model system. In this talk, we will discuss several schemes for probing the interfacial exchange. We demonstrate that the H-bar configuration exhibits strong enhancement in non-local resistance as a result of the exchange interaction. We will also present the magneto-transport results of MI multilayers on topological insulators as another platform for building low-voltage spintronic devices. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J6.00004: Tuning the Kondo effect in graphene in the presence of point defects Jinhai Mao, Yuhang Jiang, Guohong Li, Eva Y. Andrei Removing a single carbon atom from the honeycomb lattice in graphene produces a localized state orthogonal to the undisturbed lattice states. According to theory this will give rise to a local magnetic moment when occupied by an electron, but its fate in the presence of conduction electrons is not known. Will it be screened by many body interactions below a critical Kondo temperature to form a singlet state, or will it remain unscreened? Recent studies using transport or magnetic measurements on graphene in the presence of point disorder have reached opposite conclusions 1,2. We addressed this question by combining STM and transport measurements to study the effect of interactions between the electrons in graphene and local magnetic moments as a function of carrier density and dielectric environment. At high density we observe a clear signature of Kondo screening in the form of a Fano resonance in the density of state that is pinned to the Fermi energy and splits in a magnetic field. We further find that the Kondo temperature strongly depends on the carrier density and that it can be tuned in or out with a gate voltage. 1. Chen, J. H., et.al, Nature Physics 7, 535--538 (2011) 2. Nair, R. R., et. al, Nature Physics 8, 199--202(2012) [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J6.00005: Kondo-like magnetism induced by single vacancies in graphene Chi-Cheng Lee, Yukiko Yamada-Takamura, Taisuke Ozaki A new phase for graphene with a single carbon vacancy was found by our first-principles calculation. Single vacancies can be developed by irradiation experiments in graphene and were found to be magnetic.[1,2] The measured Kondo effect also triggered extensive studies.[3] The current understanding of the ground state best supported by density functional theory is that a Stoner instability gives rise to ferromagnetism of $\pi$ electrons aligned with the localized moment of a $\sigma$ dangling bond. The induced $\pi$ magnetic moments were suggested to vanish at low vacancy concentrations. However, the observed Kondo effect suggests that $\pi$ electrons around the vacancy should antiferromagnetically couple to the local moment and carry non-vanishing moments. Here we propose that a phase possessing both significant out-of-plane displacements and $\pi$ bands with antiferromagnetic coupling to the localized $\sigma$ moment is the ground state.[4] With the features we provide, it is possible for spin-resolved STM, STS, and ARPES measurements to verify the proposed phase. [1] M. M. Ugeda et al., Phys. Rev. Lett. 104, 096804 (2010). [2] R. R. Nair et al., Nature Phys. 8, 199 (2012). [3] J.-H. Chen et al., Nature Phys. 7, 535 (2011). [4] C.-C. Lee et al., http://arxiv.org/abs/1311.0609. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J6.00006: Kondo effect in graphene with Rashba spin-orbit interaction Nancy Sandler, Diego Mastrogiuseppe, Arturo Wong, Kevin Ingersent, Sergio Ulloa We study the Kondo screening of a magnetic impurity in monolayer graphene in the presence of Rashba spin-orbit interaction. The host density of states (DOS), with two split bands and particle-hole symmetry, results in a complex hybridization function that suggests interesting phenomena as a function of the chemical potential and the Rashba strenght. Although the Rashba coupling produced by depositing graphene in a conventional substrate is weak, a strong increase of this interaction was shown to occur by intercalation of Au on a Ni substrate [1] or by hydrogenation of the sample [2]. An effective single channel Anderson model sets the ground to analyze the properties of the system, which are obtained by numerical renormalization group calculations. We find a Kosterlitz-Thouless quantum phase transition (QPT) separating free moment and strong-coupling phases at half-filling, whenever the Rashba coupling is present. Tuning the chemical potential close to sharp features of the hybridization function results in an interesting interference of the Kondo peak and a virtual bound state resonance that appears due to a jump in the DOS. All these features would be visible in STM experiments, providing a realistic system in which to study QPTs. [1] Nat. Commun. 3, 1232; [2] Nat. Phys 9, 284. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J6.00007: Probing magnetic properties of sidewall epitaxial graphene nanoribbons Owen Vail, John Hankinson, Chao Huan, Wenlong Yu, Rui Dong, James Palmer, Ming Ruan, Edward Conrad, Claire Berger, Walter deHeer, Zhigang Jiang Epitaxial graphene nanoribbons grown on sidewall SiC have recently emerged as a novel material system enabling single channel room temperature ballistic transport over micrometer distance. In this work, we study the tunnel magnetoresistance (TMR) of sidewall-ribbon-based magnetic tunnel junctions as a function of temperature and magnetic field (both amplitude and tilting angle). We show that the measured TMR exhibits a spin-switch behavior at temperatures below 30 K, indicating that the sidewall ribbons are magnetic and possess a spin component either parallel or antiparallel to the magnetization direction of the magnetic contact. Furthermore, we find that the TMR signal switches the sign at certain negative bias voltages, which has important implications in device applications. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J6.00008: First principles electronic transport simulations of spin coherence length in disordered graphene nanoribbons due to spin-orbit interaction Alexandre R. Rocha, Wudmir Rojas, Stefano Sanvito, Adalberto Fazzio, Antonio J.R. da Silva Graphene presents high hopes for next-generation electronic applications. In particular, due to the small spin-orbit coupling in carbon one might envision using the electron's spin - instead of its charge - for information processing. In what has been dubbed Spintronics one of the main challenges, as one strives to obtain spin-based devices, is to obtain long spin coherence times (small spin relaxation) during electronic transport. Albeit the spin-coherence length in pristine graphene is deemed to be very large, the presence of defects and impurities can lead to spin-flips due to spin-orbit interactions. The presence of a large number of impurities randomly distributed in the system can, consequently, lead to the loss of spin-coherence. In this talk we will discuss spin-flip processes in disordered graphene nanoribbons containing a number of metal impurties. This will be achieved via a combination of Density Functional Theory - including Spin Orbit effects - with a recursive Green's function method to simulate the electronic transport of disordered systems. This way one is able to atomistically infer the spin-coherence length in graphene nanoribbons in the presence of defects or impurities. As a point in case I will show results for Ni adatoms. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J6.00009: Spatially Separated Spin Carriers in Spin-Semiconducting Graphene Nanoribbons Zhengfei Wang, Shuo Jin, Feng Liu A graphene nanoribbon with sawtooth edges has a ferromagnetic ground state. Using first-principles and tight-binding model calculations, we show that, under a transverse electrical field, the sawtooth graphene nanoribbons become a spin semiconductor whose charge carriers are not only spin polarized in energy space but also spatially separated at different edges. Low-energy excitation produces spin-up electrons localized at one edge and spin-down holes at the opposite edge, and the excitation energy of spin carries can be tuned by the electric field to reach a new state of spin gapless semiconductor. Also, the spin semiconducting states are shown to be robust against at least 10{\%} edge disorder. These features demonstrate a good tunability of spin carriers for spintronics applications. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J6.00010: Large Magnetoresistance in Nanostructured Armchair Graphene Nanoribbon Junctions Suchun Li, Young-Woo Son, Su Ying Quek The prospect of all-carbon nanoelectronics has motivated significant interest in the transport of electrons through graphene and graphene nanoribbon (GNR) based junctions.... The weak intrinsic spin-orbit coupling in graphene also makes graphene an attractive candidate for replacing conventional materials in spintronics applications. Several interesting spin transport properties, such as giant magnetoresistance and half-metallicity, have been predicted. Most of these predictions have centered on GNRs with zigzag atomic edges (ZGNRs). On the other hand, significant progress has been made in the controlled atomic-scale synthesis of GNRs with \textit{armchair} edges (AGNRs), all with specific widths.... Yet, to date, little is known about the potential of such well-defined AGNRs in electronics or spintronics. In this work, we use first principles transport calculations to predict the electron and spin transport properties of nanostructured AGNR junctions. We predict a large magnetoresistance of $\sim$ 900{\%}, related to resonant transmission channels close to the Fermi energy. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J6.00011: Charge transport properties of boron/nitrogen binary doped graphene nanoribbons: An ab initio study Seong Sik Kim, Han Seul Kim, Hyo Seok Kim, Yong-Hoon Kim Opening a bandgap by forming graphene nanoribbons (GNRs) and tailoring their properties via doping is a promising direction to achieve graphene-based advanced electronic devices. Applying a first-principles computational approach combining density functional theory (DFT) and DFT-based non-equilibrium Green's function (NEGF) calculation, we herein study the structural, electronic, and charge transport properties of boron-nitrogen binary edge doped GNRs and show that it can achieve novel doping effects that are absent for the single B or N doping. For the armchair GNRs, we find that the B-N edge co-doping almost perfectly recovers the conductance of pristine GNRs. For the zigzag GNRs, it is found to support spatially and energetically spin-polarized currents in the absence of magnetic electrodes or external gate fields: The spin-up (spin-down) currents along the B-N undoped edge and in the valence (conduction) band edge region. This may lead to a novel scheme of graphene band engineering and benefit the design of graphene-based spintronic devices.\\[4pt] This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J6.00012: Confinement effect on spin-polarized edge states in graphene nanostructures Carlos Ramos-Castillo, Romeo de Coss One of the most intriguing phenomena in condensed matter physics is the existence of edge states on the boundary of a 2D system. In graphene, the edge states have distinct properties from the bulk states and play important roles in the physicochemical properties of the material. In this work, we show ab-initio results of spin-polarized electronic edge states in graphene quantum dots of different sizes and shape. We found a critical size at which the singlet nonmagnetic ground state becomes singlet open-shell with antiferromagnetic order. We found that the critical size is strongly influenced by the shape of the quantum dot. We discuss this behavior based on energetics and electronic structure of the system under study. The calculations are base on the Density functional Theory (DFT). The Linear Combination of Atomic Orbital (LCAO) method for bases functions it was used. For exchange-correlation functional has been used the Generalized Gradient Approximation (GGA). [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J6.00013: Unravelling the zero-field-splitting parameters in Pt-rich polymers with tuned spin-orbit coupling Peter Peroncik, Ryan McLaughlin, Dali Sun, Z. Valy Vardeny Recently pi-conjugated polymers that contain heavy metal Platinum (Pt-polymers, Scientific Reports 3, 2653, 2013) have attracted substantial interest due to their strong and tunable spin-orbit coupling (SOC). The magnetic field effect (MFE), such as magneto-photoluminescence (MPL) is considered to be a viable approach to address the SOC strength in the organics. Alas conventional MFE up to several hundred Gauss is unable to overcome the relative large spin splitting energies in Pt-polymers due to their strong SOC. To overcome this difficulty we study the MPL response in two Pt-polymers at high magnetic field (up to several Telsa). We found that the MPL response is dominated by triplet excitons that are generated in record time, and from the MPL(B) response width we could obtained the triplet zero-field splitting (ZFS) parameters. We found that the ZFS parameters in the Pt-polymers are proportional to the intrachain Pt atom concentration. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J6.00014: Influence of the phonon-mediated spin-orbit coupling in graphene decorated with adatoms Jhih-Shih You, Daw-Wei Wang, Miguel A. Cazalilla Graphene covered with certain heavy adatoms has been predicted( Conan Weeks et al., Phys. Rev. X 1, 021001 (2011)) to become a topological insulator by virtue of a proximity-effect induced spin-orbit coupling. In addition, the adatoms also induce a coupling between the electron spin and the phonons. Using group theory and tight-binding models, we systematically investigate the coupling between the electron spin and in-plane lattice phonons. We discuss the consequences of this coupling for the dynamics of electrons on the graphene $\pi$ band. [Preview Abstract] |
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