Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z37: Focus Session: Nanoscale Conductance Theory III |
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Sponsoring Units: DMP Chair: Jeffrey Neaton, Lawrence Berkeley National Laboratory Room: Baltimore Convention Center 340 |
Friday, March 17, 2006 11:15AM - 11:27AM |
Z37.00001: First-principles studies of the electronic structure of cyclopentene on Si(100) Su Ying Quek, Jeffrey Neaton, Mark Hybertsen, Efthimios Kaxiras, Steven Louie Small organic molecules on silicon surfaces are promising candidates for active elements in nanoelectronic devices. The interplay between electronic states of the molecule and the silicon surface can change the molecular HOMO-LUMO gap, as well as result in interesting transport properties that depend sensitively on the alignment of molecular frontier orbitals with the silicon band structure. In this work, we determine this alignment quantitatively using the GW method for cyclopentene on Si(100), a prototypical organic-molecule/silicon junction of interest in molecular electronics. We will discuss our results in the context of recent STM experiments that observed negative differential resistance in this system. [Preview Abstract] |
Friday, March 17, 2006 11:27AM - 11:39AM |
Z37.00002: Hydrodynamical approach to transport in nanostructures Roberto D'Agosta, Massimiliano Di Ventra The electrical resistance induced by the viscous properties of the electron liquid has been recently derived.$^{1}$ In addition, it is known that the geometric constriction experienced by electrons flowing in a nanostructure gives rise to a fast ``collisional'' process.$^{2}$ These facts allow us to derive Navier-Stokes-type of equations, and therefore describe the electron flow on a par with a viscous and compressible liquid. By using this hydrodynamical approach we study electron transport in nanoscale systems and derive the conditions for the transition from laminar to turbulent flow in quantum point contacts. We also discuss possible experimental tests of these predictions. \\ $^{1}$ N. Sai, M. Zwolak, G. Vignale, and M. Di Ventra, Phys. Rev. Lett. {\bf 94}, 186810 (2005).\\ $^{2}$ M. Di Ventra and T.N. Todorov, J. Phys. Cond. Matt. {\bf 16}, 8025 (2004); N. Bushong, N. Sai and, M. Di Ventra, Nano Lett. (in press).\\ Work supported by the Department of Energy (DE-FG02-05ER46204) [Preview Abstract] |
Friday, March 17, 2006 11:39AM - 11:51AM |
Z37.00003: Source and sink approach to calculate transport properties of nanostructures Kalman Varga, Sokrates Pantelides We have calculated the transport properties of single molecule and carbon nanotube devices using the the Source and Sink method. We have carefully tested the convergence of the current by enlarging the basis. The fully converged current-voltage characteristics presented in this paper can serve as benchmark for transport calculations. We also compare our results to those obtained by other approaches and analyze the source of disagreements. [Preview Abstract] |
Friday, March 17, 2006 11:51AM - 12:03PM |
Z37.00004: Theory of the Conductance of a Single Molecule Connected to Metallic Single Wall Carbon Nanotube Electrodes Lei Zhang, Yiing-Rei Chen, Mark S. Hybertsen Metallic single wall carbon nanotubes intuitively are expected to be ideal electrodes for making contact to small, conjugated organic molecules. In this work, the conductance of ideal junctions consisting of armchair nanotube electrodes connected by conjugated carbon chains is studied. A pi-electron tight binding model is used with a Greenâ€™s function formulation of the electronic states in the junction and the conductance. Key factors that influence the conductance of these junctions include molecule length, electrode attachment topology, and even versus odd length. This study delimits the maximum conductance that can be ideally expected using metallic nanotube electrodes. [Preview Abstract] |
Friday, March 17, 2006 12:03PM - 12:15PM |
Z37.00005: Role of the evanescent states in the elastic quantum transport Arantzazu Garcia-Lekue, Lin-Wang Wang The role of evanescent states is always an issue in electronic transport, especially when the energy of the propagating electron is close to a band structure minimum. In this work, we investigate the effects of exact evanescent states on the calculation of the elastic quantum transport through a molecule connected by two quantum wires, based on the method presented in Ref.\,[1]. An exact treatment of evanescent (decaying) states involves the calculation of the complex band structure of the electrodes with imaginary parts of the k vectors. We have calculated the complex band structure under the plane-wave pseudopotential approach. The exact evanescent states are used in the transport calculations with auxiliary boundary condition as described in Ref.\,[1]. The results are exact numerical solutions of the scattering states under a plane-wave pseudopotential formalism. \newline \newline [1] L.W. Wang, Phys. Rev. B. {\bf 72}, 045417 (2005). [Preview Abstract] |
Friday, March 17, 2006 12:15PM - 12:27PM |
Z37.00006: First-principles investigation of electronic coupling effects between organic molecules and transition metal electrodes on conduction Gunn Kim, Marco Buongiorno Nardelli, J. Bernholc We investigate electronic transport in long chain molecules sandwiched between metallic electrodes. The calculations are carried out using the non-equilibrium Green function method and a basis set of localized orbitals, which are optimized to minimize the DFT total energy of the system. The optimization is performed on a grid, using multigrid techniques~~to accelerate convergence. Our model systems consist of saturated hydrocarbon (alkane) chains having thiol (-SH) and amino (-NO$_2$) end groups attached to transition metal electrodes. We show that the current-voltage characteristics strongly depend on the coupling between the molecule and the metal electrodes. [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 12:39PM |
Z37.00007: Effect of doping and molecular coverage on the I-V characteristics of organic molecules on silicon surfaces Wenchang Lu, V. Meunier, S. Wang, Q. Zhao, J. Bernholc Quantum transport properties of organic molecules on the silicon (001) surface have been studied by ab initio non- equilibrium Green function calculations in a basis of optimal localized orbitals. Our calculated results provide a qualitative picture and quantitative understanding of the importance of self-consistent screening and broadening of quasi- molecular orbitals under a large bias. Negative Differential Resistance (NDR) is found to be a general feature of organic molecules on Si surfaces [1]. By comparing the I-V characteristics of a monolayer of cyclopentene molecules with that of a single molecule, we show that interactions between the molecules attenuate the NDR, as seen in experiments. We have also investigated the effects of proximal dopant atoms on the NDR in a large unit cell. \newline \newline [1]. W. Lu, V. Meunier, J. Bernholc, Phys. Rev. Lett. 95, 206805 (2005). [Preview Abstract] |
Friday, March 17, 2006 12:39PM - 12:51PM |
Z37.00008: Calculation of Single-Electron Transport Through Molecules Jingbin Li, Nikita Simonian, Konstantin Likharev We have carried out numerical calculations of electron transport through OPE terminated with isocyanide groups. The electron spectra and orbitals are calculated using the NRLMOL DFT package (http://cst-www.nrl.navy.mil/$\sim $nrlmol/). The wave functions are then used to calculate the transmission of the interface energy barriers, within the Bardeen approximation. If the transmission is high, we calculate the current using the standard methods of the theory of ballistic field effect transistors, while if it is sufficiently low, we use the general theory of single-electron tunneling in systems with discrete energy spectrum [1]. (In order to understand the best way of resolving the problems due to the intrinsic limitations of the DFT approach, initial calculations have been carried out for a simple model: a Na atom sandwiched between two Au electrodes.) Our results yield I-V curves with substantial negative differential resistance (NDR) of conducting branches, due to a new mechanism: the enhancement of one of the tunnel barriers of the system by the applied electric field. The work is supported in part by AFOSR and NSF. [1]. D. V. Averin, A. N. Korotkov, and K. K. Likharev, Phys. Rev. B, 44, 6199 (1991). [Preview Abstract] |
Friday, March 17, 2006 12:51PM - 1:03PM |
Z37.00009: Contact Effects on Transport Properties through Single Molecules --- ab initio RTM/NEGF method study Kenji Hirose, Nobuhiko Kobayashi Recently much attention has been focused on the transport properties of single molecules sandwiched between electrodes, aiming at construction of ultimate functional devices with molecular electronics. Since the transfer of an electron through single molecules attached to electrodes is sensitivily affected by the atomic-scale contacts and also it is difficult at present to construct well-characterized nanostructures and to directly observe their atomic structures, theoretical approaches based on the ab inito calculations is indispensable to study the transport properties of molecular-scale devices. Using the ab initio RTM/NEGF method developed recently, we study the the transport properties through single molecules attached to electrodes. This method is based on the plane-wave basis sets and thus not dependent on the atomic positions, which enables us to treat accurate tails of wavefunctions in the tunneling regimes as well as those in the ballistic regimes on the same footing. We investigate especially atomic-scale contact effects on the I-V characteristics through single molecules, changing the distance to electrodes. We find strong non-linear behaviors appear in the I-V characteristics at some distances with and even without single molecules connected to electrodes. We clarify the relationship between electronic states of single molecules and the contact effect at the electrodes for the transport properties. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:15PM |
Z37.00010: All Electron Calculations of IV Characteristics For Molecular Junctions John Lawson, Charles Bauschlicher We present current-voltage (I-V) characteristics computed using all-electron basis sets on the conducting molecule. We consider benzene dithiol with gold contacts as our model system. The all-electron results are very similar to previous results obtained using effective core potentials (ECP). A hybrid integration scheme is used that keeps the all-electron calculations cost competitive with the ECP calculations. By neglecting the coupling of states to the contacts below a fixed energy cutoff, the density matrix for the core electrons can be evaluated analytically. The full density matrix is formed by adding this core contribution to the valence part that is evaluated numerically. Expanding the definition of the core in the all-electron calculations significantly reduces the computational effort and, up to biases of about 2~V, the results are very similar to those obtained using more rigorous approaches. The convergence of the I-V curve and the transmission function with respect to basis set is discussed. [Preview Abstract] |
Friday, March 17, 2006 1:15PM - 1:27PM |
Z37.00011: Spin-polarized electron transport in a molecular wire: tuning the conductivity through conformational changes L. Senapati, S.C. Erwin, R. Pati We investigate theoretically the electrical conductivity of a molecular wire consisting of three linked benzene molecules. By using magnetic electrodes the conductivity becomes spin-dependent, the degree of this dependence varying with the relative spin-alignment of the two electrodes. Of particular interest is how the the conductivity depends on the geometrical conformation of the benzene wire. Here we use density-functional theory and the Landauer-B\"{u}ttiker method to calculate this dependence from first principles. We find that the current depends sensitively on the conformation, with a near-planar configuration of the three benzene rings giving significantly higher current (by $\sim$30\%) than the ground-state non-planar structure. [Preview Abstract] |
Friday, March 17, 2006 1:27PM - 1:39PM |
Z37.00012: Ab initio modeling of molecular spintronics Derek Waldron, Vladimir Timochevski, Brian Larade, Hong Guo We report on theoretical studies of spin polarized quantum transport through molecular scale magnetic tunnel junctions. Our theoretical formalism is based on carrying out density functional theory (DFT) analysis within the Keldysh nonequilibrium Green's function (NEGF) formalism. The NEGF-DFT technique allows one to calculate nonlinear quantum transport features from atomic point of view. We will report these features for systems such as Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction, conventional Fe-MgO-Fe device, and spin injection through Fe-GaAs interfaces, including all the microscopic material properties. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 1:51PM |
Z37.00013: Theory of electron-vibration coupling in the electron transport of molecular bridges Masaru Tsukada, Kunihiro Mitsutake Electron transport through molecules connecting nano-electrodes is the key issue for molecular devices. The competition and coexistence of the coherent and dissipative transport are unresolved issue, in spite of its importance. In this work, this problem is investigated by a novel theoretical approach of an \textit{ab initio} molecular orbital model with combining polaron effect. When carriers are injected into molecules from electrodes, the structure of the molecule changes, which leads the coupling term of the electron/hole and the molecular vibration. The model Hamiltonian for the thiophene oligomer is solved by a variational approach, and a mixed states of dressed polaron with molecular orbital states mediated by the phonon cloud is found. The former and latter are predominant for small or large transfer integral, respectively. The excited states can be calculated in the same framework as the ground state. The overall carrier transport properties can be analyzed by solving the master equation with the transition rate estimated by the golden rule including the phonon degrees of freedom. In this theoretical approach, the coherent and dissipative electron transport through molecular bridges can be described in a uniform systematic way. [Preview Abstract] |
Friday, March 17, 2006 1:51PM - 2:03PM |
Z37.00014: Inter-strand coupling and base pairing sequences in DNA charge transport. Efta Yudiarsah, Sergio Ulloa The electronic transport properties of double-stranded DNA are studied using a tight-binding Hamiltonian. Transfer and scattering matrix methods for double strands are employed simultaneously in the calculation, guaranteeing numerical stability. Realistic on-site energies [1] and hopping constants are used in the model [2]. The role of inter-strand coupling is shown to be extremely important for random sequences typical of genetic DNA. In contrast, inter-strand coupling only changes slightly the charge transport properties for more periodic sequences. The effect of base-pairing across strands and details of the sequences were investigated. Our model shows that the resistance of DNA depends on the sequences and the ratio of the bases. This agrees with previous results by Roche [3]. The resistance is also shown to increase with the concentration of different bases in a homogenous strand, and we find that for certain sequences only short-range electronic transport is possible.\\ $[1]$ H. Sugiyama and I. Saito, J. Am. Chem. Soc. 118, 7063 (1996).\\ $[2]$ A. A. Voityuk, J. Jortner, M. Bixon, and N. Rosch, J. Chem. Phys. 114, 5614 (2001).\\ $[3]$ S. Roche, Phys. Rev. Lett. 91, 108101 (2003).\\ [Preview Abstract] |
Friday, March 17, 2006 2:03PM - 2:15PM |
Z37.00015: Rabi-assisted transport in diatomic molecules Edson Vernek, Enrique V. Anda, Sergio E. Ulloa, Nancy Sandler Electronic transport through few atoms, molecules and quantum dots has captured much attention recently among physicists. The confinement of electrons going alongin these systems gives rise to strong Coulomb repulsion that produces a very rich phenomenology. Electrons also interact with phonons, which affect transport characteristics under resonant conditions. We have modeled electronic transport in a diatomic molecule by a system composed of two orbital sites coupled in parallel between two leads [1]. We studied the regime where both electron- electron and electron-phonon interactions are important and have shown dramatic effects on the conductance.. Using Green's functions and equation of motion techniques we found new conducting channels in the presence of Coulomb interactions when the energy of phonons matches the energy difference between quasei-particle levels. We call this phenomenon Rabi-assisted tunneling. We now present a detailed study that includes the super-exchange interaction between the atoms in the molecules (indirectly coupled through the leads), which was not included in the previous work. We describe how the indirect coupling affects resonance conditions and the conductance of the system. \newline $[1]$ E. Vernek, et al., Phys.\ Rev.\ B (R) {\bf 72}, 121405 (2005). (Rapid comm.) [Preview Abstract] |
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