Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V27: Focus Session: Computational Nanoscience VIII - Nanotransport, Contact and Conduction |
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Sponsoring Units: DMP DCOMP Room: Colorado Convention Center 301 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V27.00001: Understanding Molecular Conduction: Old Wine in a New Bottle? Invited Speaker: Molecules provide an opportunity to test our understanding of fundamental non-equilibrium transport processes, as well as explore new device possibilities. We have developed a unified approach to nanoscale conduction, coupling bandstructure and electrostatics of the channel and contacts with a quantum kinetic theory of current flow. This allows us to describe molecular conduction at various levels of detail, -- from quantum corrected compact models, to semi-empirical models for quick physical insights, and `first-principles' calculations of current-voltage (I-V) characteristics with no adjustable parameters. Using this suite of tools, we can quantitatively explain various experimental I-Vs, including complex reconstructed silicon substrates. We find that conduction in most molecules is contact dominated, and limited by fundamental electrostatic and thermodynamic restrictions quite analogous to those faced by the silicon industry, barring a few interesting exceptions. The distinction between molecular and silicon electronics must therefore be probed at a more fundamental level. Ultra-short molecules are unique in that they possess large Coulomb energies as well as anomalous vibronic couplings with current flow -- in other words, strong non-equilibrium electron-electron and electron-phonon correlations. These effects yield prominent experimental signatures, but require a completely different modeling approach -- in fact, popular approaches to include correlation typically do not work for non-equilibrium. Molecules exhibit rich physics, including the ability to function both as weakly interacting current conduits (quantum wires) as well as strongly correlated charge storage centers (quantum dots). Theoretical treatment of the intermediate coupling regime is particularly challenging, with a large `fine structure constant' for transport that negates orthodox theories of Coulomb Blockade and phonon-assisted tunneling. It is in this regime that the scientific and technological merits of molecular conductors may need to be explored. For instance, the tunable quantum coupling of current flow in silicon transistors with engineered molecular scatterers could lead to devices that operate on completely novel principles. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V27.00002: Ab initio Green's function method and Boltzmann averaging for electrical conductance of a single molecular junction Tomofumi Tada, Arihiro Tawara, Toshiya Matsuyama, Satoshi Watanabe, Satoru Tanibayashi, Hideo Sekino The electrical conductance through benzene-dithiolate (BDT) between gold electrodes is studied using ab initio Green's function method coupled with GAUSSIAN 03 [1]. To simulate break junction experiments of BDT [2], we consider the energetic stability of transient structures of the BDT junction and possibility of the fluctuation among several structures, because these points have not been examined yet in spite that they may affect measurements results considerably. For this purpose, the most probable conductance of BDT is estimated by taking an average using Boltzmann factor [3]. The averaged conductance shows good agreement with the observed conductance and also shows a flat plateau just before the break of the junction, which is also observed by Xiao et al. [2]. We further investigate solvent effects on conductance by including solvent molecules between electrodes. The results for solvent effects will be presented at the meeting. [1] T. Tada et al., J. Chem. Phys. 121, 8050 (2004). [2] Xiao et al., Nano Lett. 4, 267 (2003). [3] S. Tanibayashi et al., Chem. Phys. Lett. 428, 367 (2006). [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V27.00003: The current-voltage characteristic of a metal-molecule-metal junction studied by an integrated and piecewise thermal equilibrium approach Y.-H Tang, T.-H Lu, M.-H. Tsai The current-voltage characteristic of a metal-molecule-metal junction has been studied by a new approach. The Au electrodes are modeled by 3-layer (111) films and the self-assembled monolayer (SAM) of Au-benzene-1,4-dithiol-molecule-Au molecules is sandwiched between them. The non-equilibrium electron distribution function is approximated by the Fermi-Dirac distribution function with a position dependent chemical potential to reflect spatial variation of the local electrostatic potential. The electronic states of the whole Au-film-SAM-Au-film system are calculated and are regarded as standing waves, which can be decomposed into +z and --z moving waves, $\Psi_{+}$ and $\Psi_{-}$, respectively, where z is the coordinate normal to the films. The current per molecule is obtained from the standard quantum mechanical current densities of the $\Psi_{+}$ and $\Psi_{-}$ states. With this approach the calculated I-V characteristic is improved substantially with respect to those obtained by the conventional transmission-probability-Green-function type approaches.. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V27.00004: First-Principles Investigation on Atomic and Electronic Transport in Ag-Ag$_{2}$S-Ag Zhongchang Wang, Takuya Kadohira, Tomofumi Tada, Satoshi Watanabe A novel atomic switch using Ag-Ag$_{2}$S-Ag heterostructure has seized a wide range of attentions recently. Its switching mechanism, however, has not been understood sufficiently. As a first step to clarify the mechanism, we investigated migration pathways of Ag ions and activation energies for the migration in Ag$_{2}$S, and then examined the interface structures, electronic states and electric properties of the Ag-Ag$_{2}$S-Ag system, using the density functional theory. The calculated activation energies for the migration are between 0.31 to 0.50 eV, which are comparable to the experimental values of 0.43 to 0.48 eV. The calculated transmission coefficient of Ag-Ag$_{2}$S-Ag at the Fermi level increases from 0.04 before atomic relaxation to 0.455G$_{0}$ after relaxation, which shows the opening of a conduction channel in the relaxed structure. Further analysis of atomic configuration in the relaxed structure shows formation of a chain-like arrangement of Ag in Ag$_{2}$S. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V27.00005: GW electronic Correlations in Quantum Transport : Renormalization and finite lifetime effects on real systems Pierre Darancet, Andrea Ferretti, Didier Mayou, Valerio Olevano We present an {\it ab initio} approach to electronic transport in nanoscale systems which includes electronic correlations through the GW approximation. With respect to Landauer approaches based on density-functional theory (DFT), we introduce a physical quasiparticle electronic-structure into a non-equilibrium Green's function theory framework. We use an equilibrium non-selfconsistent $G^0W^0$ self-energy considering both full non-hermiticity and dynamical effects. The method is applied to a real system, a gold mono-atomic chain. With respect to DFT results, the conductance profile is modified and reduced by to the introduction of diffusion and loss-of-coherence effects. The linear response conductance characteristic appear to be in agreement with experimental results. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V27.00006: Non-equilibrium Transport in Carbon based Adsorbate Systems Joachim F\"urst, Mads Brandbyge, Kurt Stokbro, Antti-Pekka Jauho We have used the Atomistix Tool Kit(ATK) and TranSIESTA[1] packages to investigate adsorption of iron atoms on a graphene sheet. The technique of both codes is based on density functional theory using local basis sets[2], and non-equilibrium Green's functions (NEGF) to calculate the charge distribution under external bias. Spin dependent electronic structure calculations are performed for different iron coverages. These reveal adsorption site dependent charge transfer from iron to graphene leading to screening effects. Transport calculations show spin dependent scattering of the transmission which is analysed obtaining the transmission eigenchannels for each spin type. The phenomena of electromigration of iron in these systems at finite bias will be discussed, estimating the so-called wind force from the reflection[3]. [1] M. Brandbyge, J.-L. Mozos, P. Ordejon, J. Taylor, and K. Stokbro. Physical Review B (Condensed Matter and Materials Physics), 65(16):165401/11-7, 2002. [2] Jose M. Soler, Emilio Artacho, Julian D. Gale, Alberto Garcia, Javier Junquera, Pablo Ordejon, and Daniel Sanchez-Portal. Journal of Physics Condensed Matter, 14(11):2745-2779, 2002. [3] Sorbello. Theory of electromigration. Solid State Physics, 1997. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V27.00007: First-Principles Analyses of Capacitance and Conductance of Atomic Point Contacts Michiko Tanaka, Shinnosuke Furuya, Satoshi Watanabe Conductance of atomic point contacts has been actively examined. On the other hand, there are only few investigations on their capacitance in the atomic scale, and thus its behavior has not been understood sufficiently yet. Here, we examine the capacitance of atomic point contacts together with their conductance. To calculate electronic states under applied bias voltages self-consistently, we adopt the boundary-matching scattering-state density functional method developed by our group. We investigate three models: (a) Two Al (100) electrodes with attached Al pyramidal clusters, (b) Al electrode with an attached Al pyramidal cluster and flat Al electrode, and (c) two flat Al electrodes. We found that the behavior of capacitance depends on structure: In (a) and (c), the capacitance first increases and then decreases with the increase in inter-electrode distance, while it shows a monotonic decrease in (b). For the conductance, our calculation reproduces the observed behavior that during stretching processes the conductance increases just before breaking of the contact in (a). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V27.00008: Atomic dimer shuttling and two-level conductance fluctuations in Nb nanowires Robert N. Barnett, Chun Zhang, Alexei Marchenkov, Zhenting Dai, Uzi Landman We describe density-functional structural optimization and conductance calculations which were carried out to explain high-resolution conductance measurements of niobium nanowires. In particular, the observed bistability manifesting itself as telegraph noise in the measured conductance is associated with the formation of a niobium dimer between the opposing electrodes, with the dimer shuttling between symmetric, high-conductance, and asymmetric, low-conductance, configurations. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V27.00009: Electronic Structure of Metal-Semiconductor Nanocontacts Denis Demchenko, Lin-Wang Wang Future nanoelectronics will depend on the electron/hole transport in a nanostructure and across nanostructure/metal electrode interfaces. Measurements of nanoscale transport are often conducted by contacting a semiconductor nanostructure with large metallic electrodes. Theoretical interpretation of such experiments, however, is often based on electronic structure of an isolated nanostructure, ignoring the influence of the electrodes. Here we address this issue by calculating the classical electrostatic polarization potential $P(r)$, and incorporating it into the atomistic pseudopotential method, to calculate the electronic structure of experimentally-relevant sizes of nanorods. We calculate several electrode/nanorod geometries, with varying contact depths. We show that the presence of an electrode can produce localized electron and hole states near the electrode. The localization is caused by the spatial variation of the $P(r)$. We have calculated the effects of the applied bias necessary to overcome the electron/hole localization, as well as the change of the band gap and the binding energy of the localized state as functions of the nanorod-electrode separation. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V27.00010: Benchmark Quantum Monte Carlo Calculations of Optical gaps of carbon Nanotubes Fernando Reboredo, Paul Kent Optical properties of single wall carbon nanotubes SWCNT have attracted considerable experimental and theoretical attention because they are strongly dependent on the details of the atomic structure (chiral vector). In these systems electronic correlations have been shown to play a dominant role both theoretically [1] and experimentally [2] as electron-electron interactions are increased in low dimensions. In this talk we present ongoing calculations of the optical gaps and quasi-particle energies of SWCNT with an alternative ab-initio technique: Diffusion Quantum Monte Carlo (DMC). We take advantage of a novel algorithm based on non-orthogonal localized orbitals that allows almost linear scaling calculations for $\sim $1000 electrons. DMC is a complementary technique to methods based on the GW approximation and the Bethe-Salpeter equation avoiding strong approximations. While the full absorption spectra cannot be obtained with DMC, we provide accurate benchmark values for the quasiparticle energy gaps and exciton binding energies. Research sponsored by the Division of Materials Sciences and Engineering, U. S. DOE, under Contract DE-AC05-00OR22725 with UT-Battelle, LLC. and by~ the Division of Scientific User Facilities, U. S.~ DOE. This work used resources of the NCCS at ORNL. [1] C. S. Spataru, PRL \textbf{92}, 077402 (2004). [2] Z. Wang PRL 96, 047403 (2006) [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V27.00011: First-principles study of effects of metallic electrode contacts on transport properties of carbon nanotubes Nobuhiko Kobayashi, Taisuke Ozaki, Kenji Hirose Towards a development of constructing nanometer-scale devices, considerable effort has been made in experiments using carbon nanotubes for fabricating nanoscale field-effect transistors. To detect electric signals, electrodes must be connected to the conductors. Contact with the electrodes sensitively influences the transport properties. Therefore, we have studied the transport properties on the basis of the detailed electronic state calculation that includes the effect of contact with the electrodes. We have investigated quantum transport in carbon nanotubes bridged between metallic electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We have analyzed transport properties of the finite size of carbon nanotubes bridged between Al, Au, Pt, Pd metallic electrodes, and discuss the contact effect of the electrodes on the transport properties. We reveal their dependency on the electrode materials. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V27.00012: Nano-helix based quantum transistor, charge pump and motor Xiaoliang Qi, Shoucheng Zhang We propose several novel device concepts based on nano-scale helical wires. Applying a static electric field transverse to the helical wire induces a metal to insulator transition, enabling the construction of a new type of transistor switch. The band gap is purely determined by the applied transverse voltage, and can be continuously tuned. The resulting light-emitting-diode can emit light with a tunable color spectrum. With a quadrupolar electrode configuration, the electric field could rotate in the transverse plane, leading to a quantized dc charge current proportional to the frequency of the rotation. Such a device could be used as a new standard for the high precession measurement of the electric current. The inverse effect implies that passing an electric current through the helical wire in the presence of a transverse static electric field leads to a mechanical rotation of the helix. This effect can be used to construct nano-scale electro-mechanical motors. Finally, our methodology also enables new ways of controlling and measuring the electronic properties of helical biological molecules such as the DNA. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V27.00013: Low temperature transport properties of semiconducting nanocrystal arrays Andreas Glatz, Igor Beloborodov, Valerii Vinokur We study the electron transport in semiconducting nanocrystal arrays at temperatures $T\ll E_c$, where $E_c$ is the charging energy for a single grain. In this temperature range the electron transport is dominated by co-tunneling processes. We discuss both elastic and inelastic co-tunneling and show that for semiconducting nanocrystal arrays the inelastic contribution is strongly suppressed at low temperatures. We also compare our results with available experimental data. [Preview Abstract] |
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