Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J38: Focus Session: Theory of Electron Transport Through Molecules II |
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Sponsoring Units: DCP Chair: Weitao Yang, Duke University Room: 410 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J38.00001: Transport in Molecular Junctions: Thoughts Coherent and Incoherent Invited Speaker: Current experimental efforts are clarifying quite beautifully the nature of charge transport in so-called molecular junctions, in which a single molecule provides the channel for current flow between two electrodes. The theoretical modeling of such structures is challenging, because of the uncertainty of geometry, the nonequilibrium nature of the process, and the variety of available mechanisms. The talk will center on the formulation of the problem in terms of non-equilibrium theory, and then on the generalizations needed to make that simple picture relevant to the real experimental situation. These include antiresonances, vibronic coupling and its control, structural disorder and representations for the electronic structure. Comments will be made on the measurements of inelastic spectra, and the information to be gained from them. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J38.00002: Theoretical aspects of modeling the conductance of molecular junctions Invited Speaker: In this talk I will discuss different semi-empirical and ab initio approaches for modeling the coherent electron transport of molecular junctions using the non-equilibrium Greens function formalism [1]. The most important effects for determining the conductance are the energies and coupling of the frontier molecular orbitals to the electrodes. I will discuss the accuracy of different levels of theory for calculating the HOMO-LUMO gap of various molecules, and present a simple correction that improves the accuracy of Density Functional based mean field theories [2]. The physical origin of the correction is illustrated using the Moshinsky atom as test system, and the accuracy is illustrated for a number of small molecules [3]. The coupling of the molecule to the electrodes is controlled by the terminal group on the molecule. We illustrate how a molecule with C60 terminal groups can have a very strong coupling with the electrodes [4]. \\[4pt] [1] Mads Brandbyge, Jose-Luis Mozos, Pablo Ordejon, Jeremy Taylor, and Kurt Stokbro, \textit{Density functional method for nonequilibrium electron transport}, Phys. Rev. B. 65, 165401 (2002).\\[0pt] [2] A. Cehovin, H. Mera, J. H. Jensen, K. Stokbro, and T. B. Pedersen, \textit{Role of the virtual orbitals and HOMO-LUMO gap in mean-field approximations to the conductance of molecular junctions}, Phys. Rev. B 77, 195432 (2008)\\[0pt] [3] H. Mera and K. Stokbro, \textit{Using Kohn-Sham-DFT to describe charged excitations in finite systems, }submitted\\[0pt] [4] C. A. Martin et. Al., Fullerene\textit{-Based Anchoring Groups for Molecular Electronics}, J. Am. Chem. Soc. $130$, 13198 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J38.00003: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J38.00004: Many-body theory of electron transport in single-molecule junctions Charles Stafford, Justin Bergfield Currently, there is no general theory to treat the many-body problem of a single molecule coupled to metallic electrodes. Mean-field approaches such as density-functional theory---the dominant paradigm in quantum chemistry---have serious shortcomings because they do not account for important interaction effects like Coulomb blockade. We develop a systematic theoretical framework for this nonequilibrium many-body problem, starting from an exact diagonalization of the few-body problem of an isolated molecule, and including lead-molecule coupling perturbatively in a novel application of nonequilibrium Green's functions. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J38.00005: Quantum theory of image potential and resonant tunneling in molecular junctions Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner It has recently been realized that the image potential plays an important role in charge transport through single organic molecules. In most cases, the classical image potential -1/4z is used to calculate the modified energy spectrum of the charge carriers in the molecule. In this talk, we will present the theory of resonant tunneling transitions that include the quantum mechanical effects of dynamic image potential due to the polarization interaction of the tunneling charge carrier (electron or hole) with surface plasmons. The application of this theory to organic molecular junctions of experimental interest will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J38.00006: Charge and Spin Memory Effects in Molecular Junctions P. D'Amico, D.A. Ryndyk, G. Cuniberti, K. Richter In the field of molecular electronics, effects like charge-memory, bistability and switching between charged and neutral states have been observed in STM [1] and single-molecule junctions [2] experiments. In this work we use model hamiltonians to describe molecular junctions, including electron-electron and electron-vibron interactions as wel as tunneling coupling to the leads. For a molecular level coupled to a vibron and in the presence of leads, we show that upon applying gate or bias voltage, it is possible to observe charge-bistability and hysteretic behavior. Physical quantities like lifetimes, charge-voltage and current-voltage curves are calculated by the master equation method for weak coupling to the leads [3] and at stronger coupling by the equation-of-motion method for noneq. Green functions, performing a systematic analysis of the bistable behaviour of the system for different internal parameters such as the electron-vibron and the lead-molecule coupling [4]. In the case of a spin-degenerate molecular level in a single and double dot molecule with vibrational coupling and in presence of ferromagnetic leads, we consider the possibility to obtain a spin-memory effect. [1] J.Repp et al, Science 305, 493 (04); [2] E.Lortscher et al, Small 2, 973 (06); [3] D.A.Ryndyk et al, PRB 78, 085409 (08); [4] P.D'Amico et al, NJP 10, 085002(08). [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J38.00007: A simple model for the description of correlation effects in molecular conductors Matthias Ernzerhof, Francois Goyer To model transport through molecular electronic devices (MEDs), we use a non-Hermitian Hamiltonian [1] for the description of open systems that exchange current density with their environment. The infinite contacts are replaced by complex source-sink potentials (SSPs) [1]. Employing a Hubbard interaction term, we include electron-correlation effects in our approach [2]. Electron interaction is considered in the molecule and neglected in the contacts. Among other strongly correlated problems, we discuss the change in conductance upon bond breaking. In the limit where the electron repulsion is strong compared to the binding energy (as it is the case in a stretched bond) a strong suppression of conductance is observed due to the localization of electrons. Other interesting phenomena, which cannot be accounted for with conventional (independent electron) approaches, are discussed as well. [1] F. Goyer, M. Ernzerhof, and M. Zhuang, J. Chem. Phys. 126, 144104 (2007); M. Ernzerhof, J. Chem. Phys. 127, 204709 (2007). [2] A. Goker, F. Goyer, and M. Ernzerhof, J. Chem. Phys. 129, 194901 (2008); M. Ernzerhof, J. Chem. Phys. 125, 124104 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J38.00008: Effects of dephasing on molecular conduction Jesse Maassen, Ferdows Zahid, Hong Guo In this work, we theoretically investigate effects of dephasing on electron transport in molecular wires. The quantum transport analysis is carried out using the density functional theory (DFT) combined with the non-equilibrium Green's function framework (NEGF). The dephasing effect is included at a phenomenological level by introducing fictitious voltage probes to the NEGF-DFT formalism that mimics the randomisation of quantum phase information of the charge carriers. For three systems: (i) a 1,4-benzenedithiol (BDT) molecule connected to Al(001) leads; (ii) an atomic gold chain in contact with Au(001) leads; and (iii) a very narrow Al(001) nanowire, our results indicate that there are two behaviours. When the wires are not conductive as (i,ii), the dephasing effects can increase conduction for a range of system parameters; while for conducting systems (iii), the effect is opposite. These effects can be understood from a quantum interference point of view. We also compare results for two different models on how the phenomenological dephasing effects are introduced into the NEGF-DFT formalism. [Preview Abstract] |
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