Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L44: Focus Session: Nanoscale Transport - Molecules II |
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Sponsoring Units: DMP Chair: Harold Baranger, Duke University Room: Colorado Convention Center 507 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L44.00001: Image potential and molecular conductance Ivan Oleynik, Mortko Kozhushner The image potential plays an important role in condensed matter physics including tunneling phenomena at surfaces, but its role in molecular conductance has not been thoroughly investigated. We discuss the influence of the image potential on molecular conductance. It is known that the predominant mechanism of conductance in relatively long organic molecules is resonant tunneling,, i.e. the current between electrodes is due to resonant transfer of electrons via the resonant levels of the negative molecular ion (electron states) or/and the levels of positive molecular ion (hole states). Both the energies and wave functions of these resonant states are influenced by the dynamic image potential of the tunneling electron due to the presence of both electrodes. The physics of the image potential is governed by the relative balance between the plasmon energy of electrodes and the band width of the molecular resonant states. In particular, the image potential may lower the resonant electronic levels by as much as 1 eV or raise the resonant hole levels by the same amount. We will discuss interesting phenomena associated with image potential including possible diode effects in symmetric molecules. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L44.00002: Real-space pseudopotential method for charge and spin transport properties of nanoscale junctions Lingzhu Kong, James R. Chelikowsky, Jeffrey B. Neaton, Steven G. Louie We present an {\sl ab initio} method for the electronic transport of nano-scale junctions under finite bias. Our method is based on density functional theory using real space pseudopotentials. The scattering wave function is obtained by solving a set of linear equations with a sparse coefficient matrix. Our method does not require a matrix inversion. We apply the method to Na or Mg atomic point contacts coupled to two planar electrodes, and good agreement with previous work is obtained. We also extend this study and examine spin-dependent transport in select magnetic atomic point contacts, where trends in magnetoresistence are examined as a function of junction bias, magnetic moment, and electronic coupling. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L44.00003: Ab initio investigation of rotaxane-based molecular switches Wenchang Lu, J. Bernholc A parallel crossbar architecture based on a bistable [2]rotaxane has been achieved experimentally [1]. The rotaxane molecule contains two recognition sites and a macrocyclic ring. Depending on the applied bias, the ring is expected to move from one site to the other and the molecule switches from a high to a low conductance state. We investigate the energetics, forces and quantum transport properties of rotaxane structures using a massively parallel real-space multigrid method. Several stable and metastable configurations are identified and investigated as a function of the applied bias and the current flowing through the molecule. To account for the effects of the current and the open boundary conditions, we use a linear-scaling non-equilibrium Green's function method in a basis of optimized localized orbitals. We discuss current-induced charge redistribution, forces as a function of applied bias, interactions between the ring and the recognition sites, and the I/V characteristics. 1. Luo et al, Chem. Phys. Chem. 3, 519 (2002). [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L44.00004: Is Density Functional Theory adequate for quantum transport? Invited Speaker: Kieron Burke Density functional calculations for the electronic conductance of single molecules attached to leads are now common. I'll examine the methodology from a rigorous point of view, discussing where it can be expected to work, and where it should fail. When molecules are weakly coupled to leads, local and gradient-corrected approximations fail, as the Kohn-Sham levels are misaligned. In the weak bias regime, XC corrections to the current are missed by the standard methodology. Finally, I will compare and contrast several new methodologies that go beyond the present standard approach of applying the Landauer formula to ground-state DFT. \newline \newline {\em Self-interaction errors in density functional calculations of electronictransport}, C. Toher, A. Filippetti, S. Sanvito, and K. Burke, Phys. Rev. Lett. {\bf 95}, 146402 (2005) \newline {\em The Dramatic Role of the Exchange-Correlation Potential in ab initio Electron Transport Calculations}, S-H. Ke, H.U. Baranger, and W. Yang, cond-mat/0609367. \newline {\em Zero-bias molecular electronics: Exchange-correlation corrections to Landauer's formula}, M. Koentopp, K. Burke, and F. Evers, Phys. Rev. B Rapid Comm., {\bf 73}, 121403 (2006). \newline {\em Density Functional Theory of the Electrical Conductivity of Molecular Devices}, K. Burke, Roberto Car, and Ralph Gebauer, Phys. Rev. Lett. {\bf 94}, 146803 (2005). \newline {\em Density functional calculations of nanoscale conductance}, Connie Chang, Max Koentopp, Kieron Burke, and Roberto Car, in prep. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L44.00005: Ab initio calculation of transmission and I-V curve for $\pi $-stacked polythiophene layers sandwiched between gold electrodes. Sergey Faleev, Francois Leonard, Mark van Schilfgaarde We have applied an implementation of recently developed [Faleev et. al. PRB 71, 195422 (2005)] non-equilibrium Green's Function method in framework of the tight-binding LMTO approach in its atomic sphere approximation to calculate the transmission function and I-V curves of $\pi $-stacked polythiophene layers sandwiched between Au(111) electrodes. Our approach is a fully \textit{ab initio} all-electron approach that treats the central region and electrodes on equal footing. To the best of our knowledge, this is first application of an \textit{ab initio} approach to calculation of transport properties of multiple polymer layers arranged \textit{parallel} to the metal surface, as opposed to previously studied systems of a small molecule or oligomer attached at both ends to the electrodes. We found that for a number of layers L $>$ 1, an increasingly pronounced dip in the transmission function is formed at energies from E$_{F}$ to E$_{F}$ + 0.5 eV, reflecting the semiconductor nature of a polythiophene multilayer film. The zero-bias conductance of the film exhibits large-L asymptotic behavior $\sigma \quad \approx $ G$_{0 }$exp(-1.2(L-5)), starting with L $\approx $ 6 that can be seen as a thickness of the thin polythiophene film at which a metal-semiconductor transition occurs. For L = 1, the current depends linearly on applied voltage, while at L $>$ 1, current is non-linear, reflecting strong bias and energy dependence of the transmission function. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L44.00006: First Principles Simulation of STM Image and Spectroscopy Yu Zhu In this talk, we shall present a framework for simulating STM images and transport spectroscopy based on density functional theory (DFT) carried out within nonequilibrium Green's function approach (NEGF). In our model, the STM tip and the sample are treated together on equal footing within the self-consistent NEGF-DFT formalism, in contrast to the usual practice where electronic structure of the tip and sample are calculated separately. The NEGF-DFT formalism allows one to do STM simulation whether the STM tip is far (weak coupling) or close (strong coupling) to the sample. The main implementation issues of this STM simulation tool will be discussed and several examples will be given. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L44.00007: Electron correlations in transport through molecular junctions: Coulomb blockade and hysteresis in the I-V characteristics of a model system. Catalin D. Spataru, Mark S. Hybertsen, Andrew Millis, Steven G. Louie Electron-electron interaction effects can play a very important role in explaining the mechanism of charge transport in molecular junctions. We use a simple tight-binding model to describe the leads and the electron-ion interaction inside the molecule. The electron-electron interaction inside the molecule is treated at the Hartree-Fock level. We study the model as a function of the number of sites in the molecule and the alignment of molecular energy levels relative to the average chemical potential in the leads. This model captures important phenomena such as the Coulomb blockade. We find that depending on the gate voltage and applied bias, there can be more than one Hartree-Fock steady-state solution for the system, which may give rise to a hysteresis in the I-V characteristics. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L44.00008: Quantum quasi-steady states in current transport Roberto D'Agosta, Michael Zwolak, Massimiliano Di Ventra We investigate quasi-steady state solutions to transport in quantum systems by finding states which at some time minimize the change in density throughout all space and have a given current density flowing from one part of the system to another [1]. Contrary to classical dynamics, in a quantum mechanical system there are many states with a given energy and particle number which satisfy this minimization criterion. Taking as an example spinless fermions on a one-dimensional lattice, we explicitly show the phase space of a class of quasi-steady states. We also discuss the possibility of coherent and incoherent mixing of these steady state solutions leading to a new type of noise in quantum transport. [1] M. Di Ventra and T.N. Todorov J. Phys. Cond. Matt. {\bf 16}, 8025 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L44.00009: DC conductance of long molecular chains Roberto Car, Emil Prodan Inspired by the work of Kamenev and Kohn, we obtained a general and formally exact expression for the 2-terminal dc conductance of linear molecular structures, within the framework of Time Dependent Current-Density Functional Theory. In this talk we focus on the adiabatic conductance. For linear molecular chains, both insulating and metallic, we derive exact and asymptotic analytic expressions for the conductance. For insulating chains, for example, not only do we get the exponentially decaying factors, but also the coefficients in front of these factors, which are highly dependent on the contacts. The results are based on the analytic structure of the bands and a compact expression for the Green's functions. Applications will also be presented. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L44.00010: Nonequilibrium Electron Transport in Mott Insulators K. A. Al-Hassanieh, F. Heidrich-Meisner, I. Gonzalez, E. Dagotto, A. E. Feiguin, M. J. Rozenberg We study the nonequilibrium transport properties of a Mott insulator using a recently developed time-dependent DMRG procedure to study conductances [1]. As a setup, we use a Hubbard chain connected to two ideal leads. We find a simple functional form of the I-V characteristics, and a universal functional dependence of the current on the electric field and the Mott gap. A mechanism of transport is described based on these results. The properties of the conducting phase induced by a strong electric field are also studied. We compare these properties to those of the doped phase. We also compare the Mott insulator to the band insulator case and discuss the similarities and differences [2]. \newline \newline [1] K. A. Al-Hassanieh, A. E. Feiguin, J. A. Riera, C. A. Busser, and E. Dagotto, Phys. Rev. B 73, 195304 (2006). K. A. Al-Hassanieh, A. E. Feiguin, F. Heidrich-Meisner, I. Gonzalez, M. J. Rozenberg, and E. Dagotto, preprint. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L44.00011: Conductance, surface traps and passivation in doped Silicon Nanowires Marivi Fernandez-Serra, Christophe Adessi, Xavier Blase By means of {\it ab initio} total energy and conductance calculations within the Landauer Formalism we investigate the structural, electronic and transport properties of doped silicon nanowires (SiNWs). We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Impurities located in the core of the wire induce a strong resonant backscattering at the impurity bound state energies. Surface dangling bond defects have hardly any direct effect on conductance. Upon surface trapping, impurities become transparent to transport, as they are both electrically inactive and do not induce any resonant backscattering. \begin{itemize} \item{}M. V. Fern\'andez-Serra, Ch. Adessi and Xavier Blase, Phys. Rev. Lett. {\bf 96}, 166805 (2006). \item{}M. V. Fern\'andez-Serra, Ch. Adessi and Xavier Blase, NanoLetters. (In press) {\bf 12}, (2006) \end{itemize} [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L44.00012: Low-Temperature Nonequilibrium Transport through Multi-pathway Single-Molecule-Transistor Systems as Which-way Effect Detector Haizhou Lu, Zuo-zi Chen, Rong Lu, Bang-fen Zhu We theoretically investigate the electronic transport through an AB-ring with a quantum dot (QD) embeded and a double-quantum- dot (DQD) interferometer. The QDs in both structures are coupled to a vibrational mode by electron- phonon interaction. With the help of Non-equilibrium Green function formalism and an improved independent boson model approximation, low-temperature bias-induced phonon-assisted sidebands are resolved. Electrons can tunnel via the phonon sidebands, pure electronic levels, or directly from one lead to the other. For DQD interferometer, electrons tunneling via two dots must emit the same number of the identical phonons to interfere with each other. Emitting different numbers of phonons would make the two pathways distinguishable, thus destroys the coherence between two pathways. Therefore this system could act as an intrinsic which-way effect detector in energy space. For the AB-ring model, the which-way effect does not apply since the direct lead-lead tunneling is assumed to be independent of energy. As a result, all phonon side peaks show flux-dependent Fano lineshapes. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L44.00013: Observation of tribo-induced melting of an asperity contact with STM-QCM Jacqueline Krim, Sang-Min Lee, Doug L. Irving, Cliff W. Padgett, Don W. Brenner Heating, energy dissipation and melting at the interface of a nanoscale sliding contact can trigger a broad range of interfacial physical and chemical processes. A fundamental understanding of these processes is largely lacking, however, because they occur at buried interfaces that are extremely difficult to characterize during sliding. In the arena of nanotechnology, heating, energy dissipation and conversion issues are integral to successful device design, construction and operation. Particularly compelling is the fact that any frictional heat produced is available to destroy device function: If any portion melts, the nanoscale device itself is destroyed. We report here on our use of the unique capabilities resulting from combining a scanning tunneling microscope (STM) and a Quartz Crystal Microbalance (QCM) to characterize the melting of a buried sliding interface between a tungsten tip and an indium overlayer. An analytic heat transport model supports the conclusion of melting arising from tip-surface sliding. These combined results constitute the first compelling evidence for the observation of tribo-induced melting of an asperity contact. [Preview Abstract] |
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