Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y8: Electron-electron Interactions and Unconventional Structures |
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Sponsoring Units: DCMP Chair: Sumit Mazumdar, University of Arizona Room: 307 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y8.00001: Modeling graphene interactions beyond pairwise additivity John Dobson, Tim Gould Dispersion (van der Waals) interactions between graphenic systems are commonly modeled by summing energy contributions between pairs of atoms or `''elements''. This pairwise assumption is now known to be inaccurate for such highly polarizable, highly anisotropic systems [1-5]. Many-electron correlation theories of RPA type [6] are more accurate, but are computationally intensive. Here we present a relatively simple type of model, based on long-wavelength RPA dielectric function data for stretched bulk graphite, that captures the non-additive physics. [1] J. F. Dobson, A. White, and A. Rubio, Phys. Rev. Lett. 96, 073201 (2006) [2] H. Y. Kim, J. O. Sofo, D. Velegol, M. W. Cole, and A. A. Lucas, J. Chem. Phys. 124, 074504 (2006) [3] A. White and J. F. Dobson, Phys. Rev. B 77, 075436 (2008) [4] A. J. Misquitta, J. Spencer, A. J. Stone, and A. Alavi, Phys. Rev. B 82, 075312 (2010) [5] R.-F. Liu, J. G. Angyan and J. F. Dobson, J. Chem. Phys. 134, 114106 (2011) [6] S. Lebegue, J. Harl, T. Gould, J. G. Angyan, G. Kresse, and J. F. Dobson, Phys. Rev Lett. 105, 196401 (2010) [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y8.00002: Charge-Carrier Screening in Single-Layer Graphene David Siegel, William Regan, Alexei Fedorov, Alex Zettl, Alessandra Lanzara Unlike normal metals that have a true Fermi surface, the pointlike Fermi surface of undoped graphene allows for long-ranged coulomb interactions to be unscreened by free charges, leading to singular behaviors. Therefore, the introduction of charge to a neutral graphene sheet can have a profound effect on transport properties and device performance. In this talk I will demonstrate the effects of charge-carrier screening of the electron-electron and electron-impurity interactions on the electronic properties of graphene, as we have observed through angle-resolved photoemission spectroscopy (ARPES). These observations help us to understand the basis for the transport properties of graphene, and shed light on the fundamental physics in the vicinity of the Dirac point crossing. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y8.00003: Evidence for strong electron correlations in graphene molecular fragments: Theory and experiments on two-photon absorptions Karan Aryanpour, Adam Roberts, Arvinder Sandhu, Alok Shukla, Sumit Mazumdar Historically, the occurrence of the lowest two-photon state below the optical one-photon state in linear polyenes, polyacetylenes and polydiacetylenes provided the strongest evidence for strong electron correlations in these linear $\pi$-conjugated systems. We demonstrate similar behavior in several molecular fragments of graphene with $D_{6h}$ symmetry, theoretically and experimentally. Theoretically, we have calculated one versus two-photon absorptions in coronene, two different hexabenzocoronenes and circumcoronene, within the Pariser-Parr-Pople $\pi$-electron Hamiltonian using high order configuration interaction. Experimentally, we have performed z-scan measurements using a white light super-continuum source on coronene and hexa-peri-hexabenzocoronene to determine frequency-dependent two-photon absorption coefficients, for comparison to the ground state absorptions. Excellent agreement between experiment and theory in our work gives strong evidence for significant electron correlations between the $\pi$-electrons in the graphene molecular fragments. We particularly benchmark high order electron-hole excitations in graphene fragments as a key element behind the agreement between theory and experiment in this work. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y8.00004: Manipulating molecule-substrate exchange interactions via graphene Sumanta Bhandary, Olle Eriksson, Biplab Sanyal Organometallic molecules with a 3d metal center carrying a spin offers many interesting properties, e.g., existence of multiple spin states [1]. A recent interest has been in understanding the magnetic exchange interaction between these organometallic molecules and magnetic substrates both from experiments and theory [2]. In this work, we will show by calculations based on density functional theory how the exchange interaction is mediated via graphene in a geometry containing iron porphyrin(FeP)/graphene/Ni(111). The exchange interaction varies from a ferromagnetic to an antiferromagnetic one depending on the lattice site and type of defect in the graphene lattice along with the switching of spin state of Fe in FeP between S=1 and S=2, which should be detectable by x-ray magnetic circular dichroism experiments. This scenario of complex magnetic couplings with large magnetic moments may offer a unique spintronic logic device.\\[4pt] [1] S. Bhandary, S. Ghosh, H. Herper, H. Wende, O. Eriksson and B. Sanyal, Phys. Rev. Lett. {\bf 107}, 257202 (2011).\\[0pt] [2] H. Wende {\it et al.}, Nat. Mater. {\bf 6}, 516 (2007). [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y8.00005: Giant capacitance of a plane capacitor with a two-dimensional electron gas in a magnetic field Brian Skinner, Boris Shklovskii If a clean two-dimensional electron gas (2DEG) with small concentration comprises one (or both) electrodes of a plane capacitor, the resulting capacitance can be larger than the ``geometric capacitance" defined by the physical separation between electrodes. Such capacitance enhancement is a hallmark of the positional correlations that arise between electrons within the 2DEG at low electron density. Here we show that in the presence of a strong perpendicular magnetic field, such correlations are enhanced, leading to unusually large capacitance even for systems where the effective Bohr radius is large. The effect is perhaps most dramatic for ultrathin graphene-based capacitors, where strongly-correlated electron states appear at small filling factors, even though in the absence of magnetic field such correlated states are normally precluded by graphene's Dirac-like kinetic energy spectrum. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y8.00006: Electron-electron Interaction and Thermoelectricity in Graphene Fereshte Ghahari, Yuri Zuev, Carlos Forsythe, Kenji Watanabe, Takashi Taniguchi, Philip Kim In this presentation, we report thermoelectric power (TEP) measurements on graphene samples deposited on hexagonal boron nitride substrates where drastic suppression of disorder is achieved. Our results show that at high temperatures where the inelastic scattering rate due to electron-electron (e-e) interactions is higher than the disorder induced elastic scattering rate,~ the measured TEP deviates from the Mott relation, and can be explained by a non-relativistic hydrodynamic flow of electrons. We also investigated TEP in the quantum Hall regime at a high magnetic fields, where we observed symmetry broken integer quantum Hall due to the strong e-e interactions. The field dependence of TEP at these states reveals the important role that exchange interactions play. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y8.00007: Interface Inducing Interesting Effects on Thermal Transport in Graphene Based Systems Haiyuan Cao, Hongjun Xiang, Xingao Gong Using nonequilibrium molecular dynamics method (NEMD), we have studied how the interface affecting the thermal conductivity in multilayer graphene nanoribbons and the graphene grain boundaries. In multilayer graphene nanoribbons, the monotonous decrease of the thermal conductivity with the increase of the number of layers can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons. The thermal transport across the asymmetric tilt grain boundary between armchair and zigzag graphene has also been investigated by simulations. We have observed significant temperature drop and ultra-low temperature-dependent thermal boundary resistance. More importantly, we find an unexpected thermal rectification phenomenon. The thermal conductivity and Kapitza conductance is direction-dependent. The effect of thermal rectification could be amplified by increasing the difference of temperature imposed on two sides. Our results show the interface phonon coupling could greatly change the thermal conductivity. Besides that, we have proposed a new promising kind of thermal rectifier and phonon diode based on the asymmetric interface in graphene. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y8.00008: Direct visualization of reversible dynamics in a Si$_6$ magic cluster in a graphene pore Jaekwang Lee, Wu Zhou, Stephen Pennycook, Juan-Carlos Idrobo, Sokrates Pantelides Clusters containing only a handful of atoms have been the subject of extensive theoretical and experimental studies, but direct imaging of their structure and dynamics has not been possible so far, with information provided mainly by theory. We report a direct atomically-resolved observation of a single Si$_6$ magic cluster trapped in a graphene nanopore. We report a sequence of images that show a reversible, oscillatory, conformational change: one of the Si atoms jumps back and forth between two different positions. Density functional theory shows that the cluster is exploring metastable configurations under the influence of the beam providing direct information on the atomic-scale energy landscape. The capture of a magic cluster in a graphene nanopore suggests the possibility of patterning nanopores and either capturing or assembling atomic clusters with a potential for applications. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y8.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y8.00010: Brownian Dynamics Simulations of Dispersed Graphene Sheets Yueyi Xu, Micah Green Past simulations of the dynamics of dispersed graphene sheets are limited to static fluids on small timescales, with little attention devoted to flow dynamics. To address this need, we investigated how flow fields affect graphene morphology dynamics using a coarse-grained model; this relatively untouched area is critical given the importance of graphene solution-processing of multifunctional devices and materials. In particular, we developed a Brownian Dynamics (BD) algorithm to study the morphology of sheetlike macromolecules in dilute, flowing solutions. We used a bead-rod lattice to represent the mesoscopic conformation of individual two dimensional sheets. We then analyzed the morphology dynamic modes (stretching, tumbling, crumpling) of these molecules as a function of sheet size, Weissenberg number, and bending stiffness. Our results indicate the model can successfully simulate a range of dynamic modes in a given flow field and yield fundamental insight into the flow processing of graphene sheets. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y8.00011: Temperature-dependent levitation of a graphene flake due to Casimir forces Anh Phan, David Drosdoff, Lilia Woods, Igor Bondarev, Nguyen Viet We present theoretical investigations of temperature-dependent Casimir interactions of a graphene flake between substrates in a fluid. By properly choosing the materials, we propose that the graphene can be suspended in the fluid due to the balance between the Casimir, buoyancy and gravitational forces. The graphene properties, such as the Dirac-like nature of the carriers and universal optical conductivity, have a profound effect on the Casimir force making it completely thermal at room temperature. Since thermal contributions to the Casimir interaction in most materials are usually small, the graphene system offers a unique opportunity to demonstrate such effects without going to extreme temperatures. We show that the equilibrium position of the suspended flake is temperature dependent. We suggest that this maybe a promising system for observing thermal Casimir effects via levitation. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y8.00012: Properties of field-effect transistors of CVD grown MoS$_2$ single atomic layers on CVD grown hexagonal Boron Nitride Nihar Pradhan, Daniel Rhodes, Qiu Zhang, Ana Elias, N. Lopez, Zheng Liu, Sina Najmei, Jun Lou, Saikat Talapatra, Mauricio Terrones, Pulickel Ajayan, Luis Balicas Two dimensional crystalline layered materials such as MoS$_2$, WS$_2$, have recently become an intense focus of research activities due to their exceptional electronic and optical properties. A single- or a few atomic layers of these materials show quite promising charge conduction characteristics, such as large mobility or fast on/off switch ratios, which lead to a few recent examples of integrated circuits based on these materials. Here, we will present a comparison among the electronic transport properties of, either mechanically exfoliated or CVD grown MoS$_2$ under different substrates, i.e. on SiO$_2$, on exfoliated or on CVD grown h-BN, and suspended. We will also discuss results obtained from back and top gated configurations with different dielectrics. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y8.00013: Nanochannel Device with Embedded Nanopore: a New Approach for Single-Molecule DNA Analysis and Manipulation Yuning Zhang, Walter Reisner Nanopore and nanochannel based devices are robust methods for biomolecular sensing and single DNA manipulation. Nanopore-based DNA sensing has attractive features that make it a leading candidate as a single-molecule DNA sequencing technology. Nanochannel based extension of DNA, combined with enzymatic or denaturation-based barcoding schemes, is already a powerful approach for genome analysis. We believe that there is revolutionary potential in devices that combine nanochannels with embedded pore detectors. In particular, due to the fast translocation of a DNA molecule through a standard nanopore configuration, there is an unfavorable trade-off between signal and sequence resolution. With a combined nanochannel-nanopore device, based on embedding a pore inside a nanochannel, we can in principle gain independent control over both DNA translocation speed and sensing signal, solving the key draw-back of the standard nanopore configuration. We demonstrate that we can optically detect successful translocation of DNA from the nanochannel out through the nanopore, a possible method to 'select' a given barcode for further analysis. In particular, we show that in equilibrium DNA will not escape through an embedded sub-persistence length nanopore, suggesting that the pore could be used as a nanoscale window through which to interrogate a nanochannel extended DNA molecule. Furthermore, electrical measurements through the nanopore are performed, indicating that DNA sensing is feasible using the nanochannel-nanopore device. [Preview Abstract] |
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