Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W24: Focus Session: Computational Nanoscience V: Transport |
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Sponsoring Units: DMP DCOMP Chair: Hyoung Joon Choi, Yonsei University, Korea Room: 326 |
Thursday, March 19, 2009 11:15AM - 11:51AM |
W24.00001: Quantum Monte Carlo Studies of Interaction-Induced Localization in Quantum Dots and Wires Invited Speaker: We investigate interaction-induced localization of electrons in both quantum dots and inhomogeneous quantum wires using variational and diffusion quantum Monte Carlo methods. Quantum dots and wires are highly tunable systems that enable the study of the physics of strongly correlated electrons. With decreasing electronic density, interactions become stronger and electrons are expected to localize at their classical positions, as in Wigner crystallization in an infinite 2D system. (1) Dots: We show that the addition energy shows a clear progression from features associated with shell structure to those caused by commensurability of a Wigner crystal. This cross-over is, then, a signature of localization; it occurs near $r_s \sim 20$. For higher values of $r_s$, the configuration symmetry of the quantum dot becomes fully consistent with the classical ground state. (2) Wires: We study an inhomogeneous quasi-one-dimensional system -- a wire with two regions, one at low density and the other high. We find that strong localization occurs in the low density quantum point contact region as the gate potential is increased. The nature of the transition from high to low density depends on the density gradient -- if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. We find no evidence for ferromagnetic spin polarization for the range of parameters studied. The picture emerging here is in good agreement with the experimental measurements of tunneling between two wires. Collaborators: C. J. Umrigar (Cornell), Hong Jiang (Fritz Haber Institut), Amit Ghosal (IISER Calcutta), and H. U. Baranger (Duke). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W24.00002: Magnetic impurities in Cu nanocontacts: Kondo effect and conductance from first principles David Jacob, Kristjan Haule, Gabriel Kotliar We present {\it ab initio} calculations of the electronic structure and coherent transport properties of Cu nanocontacts hosting a single magnetic impurity (Fe,Co or Ni) in the contact region. The strong electron correlations of the impurity $3d$-electrons are fully taken into account by combining density functional calculations with a dynamical treatment of the impurity $3d$-shell in the so called one-crossing approximation. We find that for all three impurities the strong electron correlations give rise to Kondo resonances at the Fermi level which in turn lead to Fano lineshapes in the coherent transport characteristics of the nanocontact. The exact shape of the Kondo and Fano lineshapes, however, depends strongly on the impurity type and the geometry of the contact. This is in agreement with recent experiments measuring the conductance of magnetic impurities on noble metal surfaces [1-4]. [1] P. Wahl {\it et al.}, Phys. Rev. Lett. 93, 176603 (2004). [2] N. N\'eel {\it et al.}, Phys. Rev. Lett. 98, 016801 (2007). [3] L. Vitali {\it et al.}, Phys. Rev. Lett. 101, 216802 (2008). [4] N. N\'eel {\it et al.}, arXiv:0810.0236 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W24.00003: Iterative real-time path integral approach to nonequilibrium quantum transport Michael Thorwart, Stephan Weiss, Jens Eckel, Reinhold Egger We have developed a numerical approach to compute real-time path integral expressions for quantum transport problems out of equilibrium. The scheme is based on a deterministic iterative summation of the path integral (ISPI) for the generating function of the nonequilibrium current. Self-energies due to the leads, being non-local in time, are fully taken into account within a finite memory time, thereby including non-Markovian effects, and numerical results are extrapolated both to vanishing (Trotter) time discretization and to infinite memory time. This extrapolation scheme converges except at very low temperatures, and the results are then numerically exact. The method is applied to nonequilibrium transport through an Anderson dot. [1] S. Weiss, J. Eckel, M. Thorwart, and R. Egger, Phys. Rev. B {\bf 77}, 195316 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W24.00004: Time dependent transport in nanostructures Kalman Varga Using the Lagrange-function representation [1] we present time-dependent density functional calculations of the transport properties of nanostructures. To avoid the complications related to the semiinfinite leads a complex absorbing potential (CAP) is added to the Hamiltonian [2,3]. This transformation leads to an effectively closed system which is computationally manageable. We will compare the results of the time dependent approach to those of time independent approaches for prototypical molecular devices such as benzene ring between gold electrodes and nanotubes.\\[4pt] [1] K. Varga, Z. Zhang, S.T. Pantelides, Phys. Rev. Lett. \textbf{93}, 176403 (2004).\\[0pt] [2] K. Varga, S.T. Pantelides, Phys. Rev. Lett. \textbf{98}, 076804 (2007).\\[0pt] [3] J. A. Driscoll, K. Varga, Phys. Rev. B. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W24.00005: Effective capacitance of small molecules and nanoscale devices in an electric circuit Xiaoguang Zhang, Jun-Qiang Lu, Sokrates Pantelides A quantum-mechanical definition of the capacitance of a molecule or nanodevice between two electodes is complicated by the fact that one cannot unambiguously partition the electron density between the metal electrodes and the molecule or device. We introduce a procedure that leads to an unambiguous partitioning and to practical calculations using a linear response formalism for alternating current (AC) transport. The linear response theory is derived for a closed quantum system including the molecule and two electrodes with a finite length. The mutual capacitance between the two electrodes in the absence of a molecule or device is subtracted to obtain an effective capacitance for the molecule in the presence of the electrodes. Numerical calculations show that the effective capacitance converges with the increasing length of the electrodes. The converged results for single molecules of CO$_2$, CO, CH$_4$, NH$_3$, H$_2$, H$_2$O, and benzene range from 0.18 to 2.832 ($10^{-22}$ F). [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W24.00006: Theoretical Study of Electron Transport across Carbon Nanotube Junctions Decorated with Au Nanoparticles Khoonghong Khoo, James Chelikowsky In recent years, there has been extensive research on carbon nanotube networks owing to their potential for applications in transparent electronics, and several experimental studies have found that electrical conductivity across these networks can be increased by metal nanoparticle doping. To aid in understanding the mechanism of this conductance increase, we have performed first-principles calculations on nanotube junctions decorated with small Au nanoparticles. Our calculations show that the conductance of nanotube junctions is significantly increased by the introduction of odd-numbered Au nanoparticles, and electron transport is mediated by resonant tunneling through Au nanoparticle states. In addition, we find that interesting interference effects modulate conduction across doped nanotube junctions that connect near nanotube tips. This work was supported in part by NSF under DMR-0551195 and the U.S. Department of Energy under DE-FG02-06ER46286 and DE-FG02-06ER15760. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W24.00007: Mechanism of Current-Induced Switching in Naphthalocyanine Molecular Device Tesfaye Abtew, Jerry Bernholc, Wenchang Lu Current-induced switching of inner cavity hydrogen atoms in a naphthalocyanine molecule has been reported experimentally [1]. The experiment shows a rotation of the lowest unoccupied molecular orbital (LUMO) image by 90$^{o }$when the switching occurs. We study transport properties and energetics of a naphthalocyanine molecule sandwiched between gold leads using a parallel real space multigrid method. A non-equilibrium Green's function formalism in a basis of optimized localized orbitals is employed to evaluate the current-voltage characteristics. Current-induced forces are evaluated and used to obtain bias-induced relaxations. The current-voltage characteristics indeed reveal contrasting high and low conductances depending on the orientation of the hydrogen atoms. However, a high energy barrier restrains the hydrogens from switching. We propose an alternative atomic configuration, which yields a much lower energy barrier for switching but still results in LUMO images that agree with the experimental results. [1]. P. Liljeroth, J. Repp, and G. Meyer, Science 317, 1203 (2007). [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W24.00008: Negative differential resistance in molecular junctions: The effect of the electrodes electronic structure Natalya Zimbovskaya, Mark Pederson We have carried out calculations of electron transport through a metal-molecule-metal junction with metal nanoclusters taking the part of electrodes. We show that negative differential resistance peaks could appear in the current-voltage curves. The peaks arise due to narrow features in the electron density of states of the metal clusters. The proposed analysis is based on the ab initio computations of the relevant wave functions and energies within the framework of the density functional theory using NRLMOL software package. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W24.00009: Transport Properties of DNA Bases Placed in Graphene Nano-gap Christian Wolowiec, Nick Kioussis, Dmitri Novikov There has been significant demand and research activity for the development of new DNA sequencing technologies employing transverse transport techniques. We present systematic first principles studies based on Density Functional Theory of the transport properties and current-voltage characteristics of nucleotide molecules of the DNA bases, placed in 1.2 nm gap formed between the zigzag edges of graphene nano-electrodes. The linear dispersion of the graphene electrons and the local spin-polarization associated with the zigzag edges allow the exploration of both the charge- and spin-current signatures of the DNA bases to sequence DNA. We will present results in the tunneling regime of the charge- and spin-transport properties as the geometrical conformation of the bases is varied. Such signatures may be used experimentally for developing an efficient means of sequencing larger strands of DNA. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W24.00010: Quantum mechanical pseudopotential atomistic simulations of nanosized CMOS devices Lin-Wang Wang, Xiang-Wei Jiang, Hui-Xiong Deng We have used empirical pseudopotential to calculate the electronic structures of million atom CMOS systems. This is done by using the linear combination of bulk band (LCBB) method. For a nonequilibirum CMOS system with an applied source-drain bias, we have devised three different ways to calculate the inverse carrier charge densities and the corresponding currents. The first is to use partition functions extended from source and drain using their respective Fermi energies. The second is to use a spatially dependent local quasi-Fermi energy, and the third is to calculate the current using Bardeen's tunneling current formula. In this talk, we will compare the results of these three different methods. We will also compare the quantum mechanical results with classical simulation results. This work was supported by U.S. Department of Energy under Contract No. DE-AC02-05CH11231. It has also been supported by Chinese National Natural Science Foundation. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W24.00011: Nagaoka instabilities and coherent pairing in various cluster topologies Armen Kocharian, Gayanath Fernando, Kalum Palandage, James Davenport Electron pairing and formation of various types of magnetic correlations in the ensemble of small clusters of different geometries are studied with emphasis on tetrahedron, square pyramid, etc under variation of interaction strength, electron doping and temperature. These exact calculations of charge and spin collective excitations and pseudogaps yield intriguing insights into level crossing degeneracies, phase separation, condensation and spatial inhomogeneities. Separate condensation of electron charge and spin degrees offers a new route to superconductivity in inhomogeneous HTSC systems, different from the BCS scenario. Phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen recently in high Tc cuprates, manganites and CMR nanomaterials. \\[4pt] [1] A.~N.~Kocharian, G.~W.~Fernando, K.~Palandage, and J.~W.~Davenport, Phys.~Rev. B{\bf 78} 075431 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W24.00012: Calculation of complex band structure for low symmetry lattices Manoj Srivastava, Xiaoguang Zhang, Hai-Ping Cheng Complex band structure calculation is an integral part of a first-principles plane-wave based quantum transport method. [1] The direction of decay for the complex wave vectors is also the transport direction. The existing algorithm [1] has the limitation that it only allows the transport direction along a lattice vector perpendicular to the basal plane formed by two other lattice vectors, e.g., the c-axis of a tetragonal lattice. We generalize this algorithm to nonorthogonal lattices with transport direction not aligned with any lattice vector. We show that this generalization leads to changes in the boundary conditions and the Schrodinger's equation projected to the transport direction. We present, as an example, the calculation of the complex band structure of fcc Cu along a direction perpendicular to the (111) basal plane. [1] Hyoung Joon Choi and Jisoon Ihm, Phys. Rev. B 59, 2267 (1999). [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W24.00013: Reduced Bloch mode expansion for fast band structure calculations Mahmoud Hussein In this paper, we present reduced Bloch mode expansion for fast band structure calculations in lattice dynamics. The expansion employs a natural basis composed of a selected reduced set of Bloch eigenfunctions. The reduced basis is selected within the irreducible Brillouin zone at high symmetry points determined by the medium's crystal structure and group theory. At each of the reciprocal lattice selection points, a number of Bloch eigenfunctions are selected up to the frequency/energy range of interest for the band structure calculations. Being in line with the well known concept of modal analysis, the proposed approach maintains accuracy while reducing the computation time by up to two orders of magnitudes or more depending on the size and extent of the calculations. Results are presented for Si-Ge quantum dot superlattice band structures. [Preview Abstract] |
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