Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J17: Transport in Graphene and its Heterostructures |
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Sponsoring Units: DCMP Chair: Jeanie Lau, University of California, Riverside Room: 102AB |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J17.00001: Theoretical study of Coulomb drag in graphene Derek Ho, Shaffique Adam In this theoretical work we examine the problem of Coulomb drag in graphene. We consider the common situation of heterostructures comprising graphene double layers separated by a thin sheet of hexagonal boron nitride in both zero and finite magnetic fields. We study three distinct physical mechanisms, namely the interaction induced energy-transfer between the layers, momentum transfer between the layers and disorder-induced puddles of electrons and holes that shift the system locally away from the double neutrality point. We argue that all three mechanisms are necessary to account for the available experimental data. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J17.00002: Magnetotransport in graphene and other two dimensional materials Shaffique Adam, Indra Yudhistira In this work we address theoretically the classical and quantum magnetotransport in graphene [1] and other two dimensional materials [2].~ We demonstrate that at room temperature, the largest contribution to the magnetoresistance arises from the disorder-induced carrier density inhomogeneity that gives a quadratic magnetoresistance at low magnetic fields and linear magnetoresistance at larger fields.~ At lower temperatures, quantum phase-coherent effects can be observed in the magnetotransport, and this provides information about the dominant scattering mechanism in these materials. References: [1] J. Ping, I. Yudhistira, N. Ramakrishnan, S. Cho, S. Adam, M. S. Fuhrer, \textit{Phys. Rev. Lett.} \textbf{113,} 047206 (2014); [2] H. Schmidt, S. Wang, L. Chu, M. Toh, R. Kumar, W. Zhao, A. H. Castro Neto, J. Martin, S. Adam, B. \"{O}zyilmaz, and G. Eda, \textit{Nano Letters}, \textbf{14}, 1909 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J17.00003: I-V characteristics of graphene quantum dots Venkata Chaganti, Apalkov Vadym Quantum dots in graphene-like materials with honeycomb crystal structures are studied numerically within tight-binding model of graphene. The energy spectra and corresponding I-V characteristics are found for both isolated graphene islands, i.e., graphene ``atoms,'' and coupled graphene islands, i.e. graphene ``molecules.'' The results were obtained for difference sizes of graphene quantum dot and different values of the coupling constants between the dot and the contacts. The current is found to increase with increase of the size of the graphene quantum molecule and increase of the value of the coupling constant. We also study the dependence of the I-V characteristics of graphene quantum dot on the size and the placement of the contacts. For the same size of graphene quantum ``atom'' and graphene quantum ``molecule'' the I-V characteristics is almost the same. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J17.00004: Effects of fluorination on graphene spin transport Zhisheng Lin, Bowen Yang, Jing Shi Although ideal graphene has extremely weak intrinsic spin-orbit coupling (SOC), by introducing adatoms, proximity effect, hydrogenation, etc., SOC in graphene can be effectively enhanced. In this work, we study the effects of fluorination on graphene nonlocal transport which can probe the spin Hall effect and inverse spin Hall effect as SOC is introduced. The nonlocal resistance is compared between the pristine and fluorinated graphene devices. Upon fluorination, the nonlocal resistance rises clearly above the ohmic contribution and increases as the dosage increases, which is interpreted as a consequence of increased SOC in fluorinated graphene. Raman spectroscopy is used to monitor the D-peak intensity as a function of the fluorination. We find that by controlling fluorination, the magnitude of the nonlocal resistance enhancement is correlated with the D-peak in Raman spectra. We will discuss the effects of the enhanced SOC on spin diffusion length in fluorinated graphene devices. This work was supported by DOE/BES and NSF/NEB. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J17.00005: Spin injection and transport in graphene via spin Hall effect in Au Bowen Yang, Jing Shi Graphene is a promising material for spintronics due to its negligible intrinsic spin-orbit coupling (SOC, 1-50 $\mu $eV in perfect flat graphene) derived from the light weight of carbon atoms. Experimentally, graphene shows a spin diffusion length greater than 1 $\mu $m at room temperature even though it is coupled to a substrate, as demonstrated by nonlocal spin valves. In this work, we investigate the spin injection and transport in graphene using a nonlocal Hall bar geometry with Au strips along the Hall bar arms. When a charge current flows along one Au Hall bar arm, it induces a pure spin current in Au due to the spin-Hall effect (SHE). The spin current is injected into graphene and propagates along the etched graphene channel. As it arrives at the other Au strip, the spin current is detected by the inverse spin-Hall effect (ISHE) in Au. In a 2 $\mu $m long, 0.2 $\mu $m wide Hall bar, the nonlocal resistance we obtained is 10 $\mu \Omega $. Considering the small spin-Hall angle (0.01-0.02) of evaporated Au, the magnitude of the nonlocal resistance suggests efficient spin injection from metal directly into graphene. The simplicity and high injection efficiency of this method is suitable for further exploration of spin transport mechanism in graphene. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J17.00006: Transport phenomena in deformed graphene: Magnetic field versus curvature Thomas Stegmann, Nikodem Szpak The current flow in deformed graphene nanoribbons is studied theoretically. Using a tight-binding model, we apply the nonequilibrium Green's function (NEGF) method to investigate how a localized deformation and a perpendicular magnetic field affect the current flow. While a magnetic field acts differently on electrons and holes due to their oposite charges, the deformation treats them equivalently. Applying the eikonal approximation to the Dirac equation, which is effectively satisfied at long wavelengths, we show that the obtained geodesic lines are compatible with the current flow paths of our NEGF calculations. The solution of the Mathisson-Papapetrou equations also shows that the effect of the deformation can be subdivided in two parts. First, a pseudo-magnetic field with sixfold symmetry of attractive and repulsive regions, which acts differently on electrons and holes, but changes its sign when going from the K to the K' point. Second, an attractive force due to the curvature of the ribbon, which treats electrons and holes equivalently. We conclude with an outlook on how to use deformed graphene ribbons for geometrical focusing of the current flow. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J17.00007: Two-dimensional metal-insulator transition in functionalized graphene Michael Osofsky, Sandra Hern\'andez, Anindya Nath, Virginia Wheeler, Scott Walton, Clifford Krowne, Kurt Gaskill Since its discovery, graphene has held great promise as a metal with massless carriers and thus extremely high mobility. This feature is the result of the two-dimensional character of the band structure due to the so-called Dirac cone for the ideal, perfectly ordered crystal structure. One of the implications of this ideal case is that the transport properties of this material should be immune to lattice disorder. In reality graphene, which is subject to varying amounts of disorder that depends on preparation method, environment, impurities and other extrinsic variables, has been shown to exhibit an effective mass. Thus, metallic behavior with a wide range of mobilities has been reported. This situation contradicts the prediction that all two-dimensional systems must be insulating. Furthermore, there have also been reports of damaging graphene so severely that it becomes an insulator. Therefore, graphene, with its single layer structure, is a model system for studying the two-dimensional metal-insulator transition (MIT). In this work, we systematically increase the resistivity of epitaxial graphene via the introduction of chemical moieties using very low temperature plasmas. These results reveal the existence of a two dimensional MIT in epitaxial graphene. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J17.00008: Electronic transport in Chemically-doped Graphene Abdollah Dadgar Chemical doping is a well-known technique to introduce carriers into semiconductors. Previously, we have studied the atomic and electronic structure of graphene doped with nitrogen, and have shown that the dopants primarily incorporate in the graphitic form. Such graphitic dopants are sources of strong intervalley scattering in graphene. In this work, we will describe the effect of these scattering centers on electronic transport in N-doped graphene films produced on SiO2 and BN substrates. We will discuss the mobility and charge carrier concentration inferred from field effect measurements and will discuss the temperature dependent elastic and inelastic scattering rates obtained from weak localization measurements. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J17.00009: Near-field study in graphene/hBN heterostructures Guangxin Ni, Lei Wang, Michael Goldflam, Martin Wagner, Zhe Fei, Alex Swinton McLeod, Mengkun Liu, Fritz Keilmann, Barbaros \"Ozyilmaz, Antonio H. Castro-Neto, Philip Kim, James Hone, Michael Fogler, Dimitri N. Basov Graphene is proposed as one of the most promising candidates for novel plasmonic devices, owning to its versatile tunability and ultrafast operation speed. Although exciting results of graphene plasmonics have been obtained, there is keen interest in improving device quality to explore additional plasmon physics and functionalities. Here we present studies of surface plasmons in graphene/hBN devices using scanning near-field optical microscopy. We find that by using these high quality devices, plasmon dissipation rate can be greatly reduced. Moreover, we demonstrated ultrafast hot carriers induced plasmon dispersion in real space. Our study would be important for novel graphene plasmonic applications. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J17.00010: Vertical transport through a misoriented graphene / hexaonal boron-nitride / graphene heterostructure Supeng Ge, Darshana Wickramaratne, Roger Lake Hexagonal boron nitride has an atomically smooth self-passivated surface and minimal lattice mismatch with graphene, which makes it an ideal substrate material for achieving high-mobility graphene devices. There is also a growing interest in tunneling devices fabricated by vertically stacking h-BN between two layers of graphene. Mechanical stacking of these individual layers leads to interfaces that are naturally misoriented with respect to each other. Furthermore, the number of layers of h-BN between the graphene layers in such devices can also vary. The combined effect of the twist angle and the thickness of the h-BN layer on the vertical transport properties is still an open question. Using ab-initio calculations we calculate transmission across unrotated and rotated graphene/h-BN/graphene heterostructures. For a single layer of BN, misorientation increases the tunneling transmission. The transmission as a function of h-BN layer thickness and different commensurate rotation angles is discussed. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J17.00011: Nonlinear Hall effect in h-BN/graphene on ferri-magnetic substrates Chi Tang, Bin Cheng, Zhiyong Wang, Zilong Jiang, Mohammed Aldosary, Yafis Barlas, Marc Bockrath, Jing Shi, T. Taniguchi, K. Watanabe In this work, we present a magnetotransport study of fabricated graphene devices transferred on atomically flat ferri-magentic insulator yttrium iron garnet (YIG) thin films. The graphene sheet is sandwiched between a hexagonal boron nitride (h-BN) top gate dielectric and YIG. Due to the atomically smooth surfaces of both h-BN and YIG, graphene devices exhibit high mobility. Furthermore, unlike traditional back-gated devices, the h-BN top-gated devices show negligible gate hysteresis and can achieve high carrier densities with relatively small gate voltages. To investigate the magnetotransport properties of graphene arising from the proximity-induced exchange interaction, we explore the behavior of the nonlinear Hall component over a wide range of carrier densities. The shape of nonlinear Hall part tracks the magnetization of the underneath YIG film after removal of the linear background that originates from the ordinary Hall Effect. A sign reversal of nonlinear Hall contribution is observed when graphene is tuned from electron- to hole- dominated transport regimes. The magnitude of the nonlinear Hall decreases as the density increases for both carrier types. The h-BN top gate dielectric enables us to probe the intrinsic proximity interaction of multilayered graphene heterostructures more efficiently. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J17.00012: Controlling the electrostatic accumulation at the graphene edge, and effects on carrier transport Menyoung Lee, Patrick Gallagher, Grace Pan, Kenji Watanabe, Takashi Taniguchi, David Goldhaber-Gordon In graphene without doping, the local carrier density at any point is directly proportional to the net charge density induced by the electric field effect. For any finite sized sample, this induced charge distribution is inhomogeneous over macroscopic length scales and of a form determined by the actual geometry of the device. In contrast to gate-confined 2DEGs in modulation-doped semiconductor heterostructures, the carrier density accumulated at the boundary is greater than in the 2D bulk $[1]$, and many experiments and potential applications need to take this effect into account. We will discuss experiments on boron nitride-encapsulated graphene devices (with low disorder and high mobility) that are designed to explicitly tune the electrostatic accumulation at the boundary, from over-accumulation (the situation in typical experiments) to a nearly flat band condition, and further into a regime where the 2D bulk and the edge have opposite charge. In clean devices where the mean free path exceeds the sample width and boundary scattering dominates, the effect of the electrostatic accumulation on carrier transport is pronounced. \\[4pt] [1] P. G. Silvestrov and K. B. Efetov, PRB 77 155436 (2008) [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J17.00013: Gate-tunable Topological Pseudospin Transport in Bilayer Graphene Mengqiao Sui, Guorui Chen, Liguo Ma, Wenyu Shan, Kenji Watanabe, Takashi Taniguchi, Xiaofeng Jin, Wang Yao, Di Xiao, Yuanbo Zhang Extra quantum degree of freedom, generally referred to as pseudospin, arises in condensed matter systems when electrons from two sublattices of a crystal form degenerate bands at Fermi level. Here we describe a pseudospin system based on the ``which-layer'' quantum degree of freedom in bilayer graphene that is fully tuned by top and bottom gates. We detect topological pseudospin current - a result of the broken symmetry induced by the top and bottom gate electric fields - in a nonlocal geometry. The nonlocal pseudospin transport persists up to room temperature owing to the large, tunable band gap in our bilayer graphene devices. The gate-tunable pseudospin quantum degree of freedom in bilayer graphene may lead to future pseudospin-based electronic applications. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J17.00014: All-electrical control of RKKY interaction in graphene P-N junction Shuhui Zhang, Wen Yang, Kai Chang Graphene is a promising material for spintronic devices. In this development, one way is to dope graphene with magnetic impurity spins. Controllable long-range coupling between different spins is a key ingredient for these applications. The electron-mediated RKKY interaction provides a possible solution. However, there lacks efficient way to control this interaction. Here we demonstrate that by focusing the electron waves across a P-N junction, the long-range RKKY interaction can be controllably amplified by electrical gating. This provides a possible route towards scaling up graphene-based spintronic devices. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J17.00015: Length and density dependence of the critical current in long diffusive graphene-based Josephson junctions Chung-Ting Ke, Ivan Borzenets, Anne Watson, Yura Bomze, Gleb Finkelstein We study the critical current in graphene-based Josephson junctions as a function of the channel length and gate voltage. Previous works on normal metal SNS junctions have established that the product of the critical current, Ic, and normal resistance, Rn, is determined by the Thouless energy Eth. Several recent studies have addressed the IcRn $\sim$ Eth/e relationship in Josephson junctions made of graphene. These measurements are challenging, especially near the Dirac point, where the critical current of long junctions is small; many of these~junctions also tend to be underdamped, resulting in premature switching to the normal state before the critical current is reached. In this work, we study several junctions of different lengths fabricated in CVD graphene, which allows for wider junctions with larger critical currents. We present the dependence of the critical current on the channel length, both away from and close to the charge neutrality point. The latter regime is particularly interesting because the phase coherence should be established through the electron and hole puddles on the length scales of up to 800 nm. [Preview Abstract] |
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