Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B15: Graphene: Magnetotransport |
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Sponsoring Units: DCMP DMP Chair: Aubrey Hanbicki, Naval Research Laboratory Room: 314 |
Monday, March 14, 2016 11:15AM - 11:27AM |
B15.00001: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 11:27AM - 11:39AM |
B15.00002: Magnetoresistance and Anti-Ferromagnetic Coupling in FM-Graphene-FM Trilayers Enrique D. Cobas, Olaf M. J. van 't Erve, Shu-Fan Cheng, Berend T. Jonker Both high-magnetoresistance(MR) minority spin filtering[1] and anti-ferromagnetic (AFM) coupling[2-3] have been predicted for FM\textbar Graphene\textbar FM vertical heterostructures. Our previous experiments[4-5] demonstrated ordinary magnetoresistance in NiFe-Graphene-Co heterostructures and no evident AFM coupling. Here we present experimental results that confirm both MR minority spin filtering and AFM coupling in high-quality FM\textbar Graphene\textbar FM heterostructures. The heterostructures were fabricated by a combination of sputtering, chemical vapor deposition and electron beam evaporation. The stack was patterned into symmetric cross-bar structures using Ar ion milling. Measurements show negative magnetoresistance in excess of 10 percent, confirming spin-filtering, and weak anti-ferromagnetic coupling throughout the temperature range 15K to 300K. The temperature dependence of the MR was studied and found consistent with thermal excitation of spin waves in the ferromagnetic electrodes. Junction resistance-area products are in the range of 10 $\Omega $cm$^{2}$. These heterostructures provide a fast and low-power magnetic field sensor in the sub-100 Oe range and are a step towards high-MR low RA-product MRAM junctions. [1] Karpan, et al. Phys. Rev. Lett 99, 176602, 2007. [2] Li et al., App. Phys. Lett 98 (13), 133111, 2011. [3] Kim, D. et al., App. Phys. Lett 102 (11), 112403, 2013 [4] Cobas et al., Nano Lett. 12, 3000, 2012. [5] Cobas et al., IEEE Trans. Mag., 49 (7), 4343, 2013. [Preview Abstract] |
Monday, March 14, 2016 11:39AM - 11:51AM |
B15.00003: Magneto Transport in Three Dimensional Carbon Nanostructures Timir Datta, Lei Wang, Jan Jaroszynski, Ming Yin, Dheyaa Alameri Electrical properties of self-assembled three dimensional nanostructures are interesting topic. Here we report temperature dependence of magneto transport in such carbon nanostructures with periodic spherical voids. Specimens with different void diameters in the temperature range from 200 mK to 20 K were studied. Above 2 K, magnetoresistance, MR $=$ [R(B) -- R(0)] / R(0), crosses over from quadratic to a linear dependence with the increase of magnetic field [Wang et al, APL 2015; DOI:10.1063/1.4926606]. We observe MR to be non-saturating even up to 18 Tesla. Furthermore, MR demonstrates universality because all experimental data can be collapsed on to a single curve, as a universal function of B/T. Below 2 K, magnetoresistance saturates with increasing field. Quantum Hall like steps are also observed in this low temperature regime. Remarkably, MR of our sample displays orientation independence, an attractive feature for technological applications. [Preview Abstract] |
Monday, March 14, 2016 11:51AM - 12:03PM |
B15.00004: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 12:03PM - 12:15PM |
B15.00005: Study of magnetotransport across the neutrality point in CVD graphene Ramesh G. Mani Hall effect compensation and a residual resistivity $\rho_{xx} \approx h/4e^{2}$ are experimentally examined over the p$\leftrightarrow$n transition about the nominal Dirac point in CVD graphene. The observed characteristics are reproduced in a model with a parabolic distribution $f(V_{N})$ of neutrality potentials, $V_{N}$, and simultaneous electron- and hole- conduction. The results suggest that, broadly about the gate-induced n $\leftrightarrow$ p transition, charge transport is characterized by domain confined ambipolar currents, which leads to compensation in the global Hall effect and the observed residual resistivity. [Preview Abstract] |
Monday, March 14, 2016 12:15PM - 12:27PM |
B15.00006: Magnetotransport measurements in graphene/ferromagnetic insulator$_{\, }$heterostructures Aaron Sharpe, Wenmin Yang, Menyoung Lee, David Goldhaber-Gordon, Takashi Taniguchi, Kenji Watanabe, Robert Cava Through proximity effects, it is possible for two-dimensional graphene sheets to inherit order parameters from another two-dimensional substrate. Specifically, graphene has been seen to exhibit ferromagnetism when placed on a ferromagnetic insulator. Ferromagnetic graphene is a very promising platform for devices potentially useful for spintronics applications. We present here magnetotransport measurements of graphene/ferromagnetic insulator heterostructures. [Preview Abstract] |
Monday, March 14, 2016 12:27PM - 12:39PM |
B15.00007: Magnetotransport in Graphene on the Nano Scale measured by Scanning Tunneling Potentiometry Philip Willke, Thomas Druga, Thomas Kotzott, Rainer Ulbrich, Alexander Schneider, Martin Wenderoth The method of scanning tunneling potentiometry (STP) has been introduced by Muralt and Pohl [1] as a technique for mapping the electrochemical potential locally. Here we present a new home-built low-temperature STP setup with applicable magnetic field of up to 6T to study the spatial evolution of the voltage drop at extended defects in graphene with high-resolution.[2] We show that the voltage drop at a monolayer-bilayer boundary in graphene clearly extends spatially up to a few nanometers into the bilayer and hence is not located strictly at the structural defect. Moreover, different scattering mechanisms can be disentangled. Besides, we perform magnetotransport STP measurements mapping the local electrochemical potential as a function of the applied magnetic field. This allows us to identify localized and delocalized contributions to the magnetoresistance in epitaxial-grown graphene and to reveal the contribution of defects. [1] P. Muralt, D. W. Pohl, Scanning tunneling potentiometry, Appl. Phys. Lett.. 48, 514 (1986) [2] P. Willke, et al. Spatial Extent of a Landauer Residual-Resistivity Dipole in Graphene quantified by Scanning Tunneling Potentiometry, Nature Commun. 6, 6399 (2015) [Preview Abstract] |
Monday, March 14, 2016 12:39PM - 12:51PM |
B15.00008: Universality of Effective Medium and Random Resistor Network models for disorder-induced linear unsaturating magnetoresistance Silvia Lara, Ying Tong Lai, Cameron Love, Navneeth Ramakrishnan, Shaffique Adam In recent years, the Effective Medium Theory (EMT) [1] and the Random Resistor Network (RRN) [2] have been separately used to explain disorder induced magnetoresistance that is quadratic at low fields and linear at high fields. We demonstrate that the quadratic and linear coefficients of the magnetoresistance and the transition point from the quadratic to the linear regime depend only on the inhomogeneous carrier density profile. We use this to find a mapping between the two models using dimensionless parameters that determine the magnetoresistance and show numerically that they belong to the same universality class. [1] J. Ping, I. Yudhistira, N. Ramakrishnan, S. Cho, S. Adam, and M. S. Fuhrer, Phys. Rev. Lett. 113, 047206 (2014). [2] M. Parish and P. Littlewood, Nature 426, 162 (2003) [Preview Abstract] |
Monday, March 14, 2016 12:51PM - 1:03PM |
B15.00009: Linear unsaturating magnetoresistance in disordered systems Ying Tong Lai, Silvia Lara, Cameron Love, Navneeth Ramakrishnan, Shaffique Adam Theoretical works [1, 2] have shown that disordered systems exhibit classical magnetoresistance (MR). In this talk, we examine a variety of experimental systems that observe linear MR at high magnetic fields, including silver chalcogenides, graphene, graphite and Weyl semimetals. We show that a careful analysis of the magnitude of the MR, as well as the field strength at which the MR changes from quadratic to linear, reveal important properties of the system, such as the ratio of the root-mean-square fluctuations in the carrier density and the average carrier density. By looking at other properties such as the zero-field mobility, we show that this carrier density inhomogeneity is consistent with what is known about the microscopic impurities in these experiments. The application of this disorder-induced MR to a variety of different experimental scenarios underline the universality of these theoretical models. \\ \\ [1] J. Ping, I. Yudhistira, N. Ramakrishnan, S. Cho, S. Adam, and M. S. Fuhrer, Phys. Rev. Lett. 113, 047206 (2014). \\ [2] M. Parish and P. Littlewood, Nature 426, 162 (2003) [Preview Abstract] |
Monday, March 14, 2016 1:03PM - 1:15PM |
B15.00010: Valley-symmetry-preserved transport in ballistic graphene layers with gate-defined carrier guiding Minsoo Kim, Ji-Hae Choi, Sang-Hoon Lee, Kenji Watanabe, Takashi Taniguchi, Seung-Hoon Jhi, Hu-Jong Lee Zigzag graphene nanoribbons are predicted to exhibit interesting electronic properties stemming from its Dirac band structure. However, to date, investigation of them is highly limited because of the defects and the roughness at the edges, which mix different valley properties of graphene. Here, we report the signature of conservation of valley symmetry in two types of quasi-1D ballistic graphene transport devices; one is a quantum point contact (QPC) and another is an Aharonov-Bohm (AB) interferometer. In measurements, charge carriers were confined in a potential well formed by the dual gates operation and the four-terminal magnetoconductance (MC) was measured with varying the carrier density, dc bias, and temperature. It exhibits the conductance quantization in steps of $\Delta G \quad =$ 4$e^{\mathrm{2}}$/$h$ starting from $G \quad =$ (2, 6), 10$\times e^{\mathrm{2}}$/$h$ in a constricted conducting channel of QPC-type devices. This behavior is similar to the one observed in zigzag graphene nanoribbons having edge localized channels. Our tight-binding calculation shows that quasi-1D charge flow on a graphene plane acts a zigzag-type nanoribbon, unless it is perfectly aligned along the armchair direction. In the AB interferometry, we observed $h$/$e$ periodic modulation of MC and the zero-field conductance minimum with a negative MC background. [Preview Abstract] |
Monday, March 14, 2016 1:15PM - 1:27PM |
B15.00011: Visualization of phase-coherent electron interference in a ballistic graphene Josephson junction Monica Allen, Oles Shtanko, Ion Cosma Fulga, Joel Wang, Daniyar Nurgaliev, Kenji Watanabe, Takashi Taniguchi, Anton Akhmerov, Pablo Jarillo-Herrero, Leonid Levitov, Amir Yacoby Graphene provides an appealing platform to explore electronic analogs of optics-like effects due to the nonclassical nature of ballistic charge transport. By coupling superconductors to a ballistic graphene sheet, we explore a new regime of superconducting transport in which phase-coherent interference of electron waves is a dominant feature. We employ Fraunhofer interferometry to achieve spatial imaging of cavity modes in a graphene Fabry-Perot resonator, embedded between two superconductors to form a Josephson junction. By visualizing current flow using Fourier methods, our measurements provide evidence of separate interference conditions for bulk and edge currents and elucidate the microscopic nature of interference at the crystal boundaries. We also observe modulation of the multiple Andreev reflection amplitude on and off resonance, a direct measure of cavity transparency. These results constitute a strong departure from conventional Josephson behavior and motivate further exploration of new effects at the intersection of superconductivity and electron-optics. [Preview Abstract] |
Monday, March 14, 2016 1:27PM - 1:39PM |
B15.00012: Magnetostrain-driven quantum engine on a grapheme flake Enrique Munoz, Francisco Pena The concept of a quantum heat engine (QHE) has been discussed [1-3] as an alternative to efficiently recover, on a nanoscale device, thermal energy in the form of useful work. In a QHEN the working substance is in a mixed quantum state determined by a density matrix. Interesting examples of this concept are constituted by photosynthesis in plants as well as human-designed photocells [4]. In this work [1], we propose a graphene-based quantum engine, driven by a superposition of mechanical strain and an external magnetic field. Engineering of strain in a nanoscale graphene flake creates a gauge field with an associated uniform pseudo-magnetic field. The combination leads to the emergence of discrete relativistic Landau levels [1]. The inter-level distance and hence their statistical population can be modulated by quasi-statically tuning the imposed magnetic field along a sequence of reversible transformations that constitute a quantum mechanical analog of the classical Otto cycle. References [1] F. J. Pena and E. Munoz, Phys. Rev. E 91 (2015) 052152. [2] E. Munoz and F. J. Pena, Phys. Rev. E 89 (2014) 052107. [3] E. Munoz and F. J. Pena, Phys. Rev. E 86 (2012) 061108. [4] M. O. Scully, M. S. Zubairy, G. S. Agarwal, and H. Walther, Science 299 (2013) 862. [Preview Abstract] |
Monday, March 14, 2016 1:39PM - 1:51PM |
B15.00013: Magnetotransport of Epitaxial Graphene on Hexagonal SiC Surface Grown with Metal Plate Capping Kibog Park, Han Byul Jin, Sungchul Jung, Junhyoung Kim, Dong-Hun Chae, Wan-Seop Kim, Jaesung Park High quality epitaxial graphene (EG) was grown on a Si-face hexagonal SiC substrate by capping the surface with a metal plate (Molybdenum, Tungsten) during UHV annealing. The growth temperature was $\sim $ 950 degree C, significantly lower than the conventional UHV annealing. The crystallinity of EG film was examined with Raman spectrum measurements. Almost no D-peak and a large narrow 2D-peak ensure that a thin (mono- or bi-layer) EG film was grown with a negligible number of defects. The electrical properties of EG film were also characterized by performing magnetotransport measurements with Hall-bar structures. The carrier type was found to be n-type, the sheet carrier density be (3.6-9.2)x10\textasciicircum 12 /cm\textasciicircum 2, and the Hall mobility be \textasciitilde 2100 cm\textasciicircum 2/Vs. Due to the relatively high carrier density, the Quantum Hall Effect was observed only for high filling factors up to 14 T. However, clear Shubnikov-de-Hass oscillations were observed, indicating that the random carrier scattering due to impurities or defects is minimal in the EG film grown with metal plate capping. [Preview Abstract] |
Monday, March 14, 2016 1:51PM - 2:03PM |
B15.00014: Thermal Smearing of the Magneto-Kohn Anomaly for Dirac materials and comparison with the Two-dimensional electron Liquid Dipendra Dahal, Antonios Balassis, Godfrey Gumbs, M. L. Glasser We compute and compare the effects due to a uniform perpendicular magnetic field and the temperature on the static polarization functions for monolayer graphene (MLG) associated with the Dirac point with that for the two-dimensional electron liquid (2DEL). Previous results for the 2DEL are discussed and we point out a flaw in reported analytic derivation to exhibit the smearing of the Fermi surface for 2DEL. The relevance of our study to the Kohn anomaly in low-dimensional structures and the Friedel oscillations for the screening of the potential for a dilute distribution of impurities is reported. [Preview Abstract] |
Monday, March 14, 2016 2:03PM - 2:15PM |
B15.00015: Hofstadter butterfly and quantum transport in graphene on hexagonal boron nitride from multiscale lattice simulations Nicolas Leconte, Rafael Martinez-Gordillo, Allan MacDonald, Jeil Jung Clear signatures of the Hofstadter butterfly have been experimentally observed in graphene on hexagonal boron nitride (G/BN), thanks to an appropriate balance between the length scale and the quality of the moiré superlattices. During this talk, I will present a methodology to map the continuum moiré pattern of incommensurable G/BN crystals obtained from ab initio calculations onto supercell lattice tight-binding Hamiltonians. Using efficient Lanczos recursion techniques for simulating large scale systems containing millions of atoms, the density of states and the dc conductivity are obtained as a function of energy or carrier density and magnetic field. The calculated Hofstadter butterflies and Landau fan diagrams show that the site potential variations, the mass, and substantial virtual strain contributions that appear even in the absence of real strains in the band Hamiltonian sensitively affect the electron-hole asymmetry, the gaps at the secondary Dirac points, as well as the tertiary features that appear at high-carrier densities. [Preview Abstract] |
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