Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B40: Focus Session: Transport Properties of Nanostructures I: Contacts |
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Sponsoring Units: DMP DCP Chair: A. Yu. Kasumov, Institute of Microelectronics Room: LACC 408A |
Monday, March 21, 2005 11:15AM - 11:51AM |
B40.00001: Quantum properties of atomic-sized conductors: Single atoms, chains of atoms, and molecules Invited Speaker: Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form a conducting nanowire. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics (N. Agra\"{\i}t, A. Levy Yeyati and J.M. van Ruitenbeek, Phys. Rep. \textbf{377} (2003) 81). More recently, similar techniques are being used to contact and study individual organic molecules. Junctions of single molecules such as H$_{2}$ and CO bonded between Pt electrodes can be characterized in great detail by vibration spectroscopy and the dependence of the vibration modes on the stretching of the junction (R.H.M. Smit, et al., Nature \textbf{419} (2002) 906; D. Djukic, J.M. van Ruitenbeek, K.S. Thygesen and K.W. Jacobsen, cond-mat/0409640). [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B40.00002: Electronic elastic and inelastic effects in atomic-sized structures and molecules Carlos Untiedt, Roel H. M. Smit, Gabino Rubio-Bollinger, Nicolas Agrait, Sebastian Vieira, Jan van Ruitenbeek Different techniques make possible the fabrication and measurement of the properties of atomic-sized structures. In such structures new effects can be found. The control over these is crucial for the development of new methods and techniques for molecular electronics. In atomic-sized structures we have studied in depth the variations of the conductance when different bias voltages are applied. We show that there are at least, two effects: One coming from the elastic scattering of electrons at the structure and with the impurities, and a second coming from their inelastic scattering with the excitations of the lattice such as the emission of phonons. The first one gives an oscillatory dependence of the conductance with the applied voltage or size of the structure and the second gives a decrease of the conductance. This second effect allows us to extend the technique of the Point Contact Spectroscopy (PCS) to use it in atomic-sized structures. The application of the PCS to the study of atomic-sized structures acting as molecular bridges in a circuit, gives useful information. As examples we show our latest measurements for the cases in which such a bridge consists of an atom, a chain of atoms or a hydrogen molecule. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B40.00003: Theoretical study of the conductance through hydrogen decorated Pd nanojunctions K.H. Khoo, J.B. Neaton, Steven G. Louie In a recent experiment, the conductance through Pt and Pd nanocontacts have been measured in an H$_{2}$ atmosphere.$^{1}$ This work suggested that electron transport is mediated by individual H$_{2}$ molecules, which are reported to have nearly perfect conductance, i.e., with a value of a single quantum unit conductance. However, subsequent measurements on Pd nanocontacts in H$_{2}$ yielded data with a half of a quantum unit of conductance, and this new result was attributed to hydrogen being dissolved in the Pd contacts.$^{2}$ In this study, we have computed the conductance of atomic and molecular hydrogen between Pd and PdH$_{x}$ nanocontacts using an {\it ab-initio} scattering-state approach$^{3}$ based on density functional theory and with a local-orbital basis set. Our results show that the conductance is highly sensitive to both the junction geometry and the hydrogen content of the lead. We find that significant hybridization reduces the conductance significantly below that of one quantum unit. The implications of our results for the interpretation of experiments are discussed.This work was supported by NSF Grant No. DMR04-39768 and by DOE under Contract No. DE-AC03-76SF00098. Computational resources have been provided by NERSC and NPACI. [1] R.H.M. Smit {\it et al.}, Nature {\bf 419}, 906 (2002). [2] Sz. Csonka {\it et al.}, Phys. Rev. Lett. {\bf 93}, 016802 (2004). [3] H.J. Choi, M.L. Cohen, Steven G. Louie, to be published. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B40.00004: Fermi surface studies of Ni nanowires through the proposed semi-metal-to-semiconductor transition T.E. Huber, A.A. Nikolaeva, M.J. Graf Theoretically, for Bi wire diameters near 60 nm (roughly the Fermi wavelength), quantum confinement effects cause Bi nanowires to undergo a semimetal-semiconductor transition involving modifications in the Fermi surface and band structure. Our recent measurements of the Fermi surface for 80-nm Bi nanowires show a dramatic decrease of the carrier density relative to that for the bulk, consistent with the model of quantum confinement, whereas for 30-nm nanowires we find significant discrepancies are found. Surface states, which have been observed via ARPES for surfaces of Bi crystals and via transport measurements on thin Bi films, may explain these discrepancies. We use a high-pressure, high-temperature injection technique with nanochannel dielectrics as a template structure to fabricate dense composites consisting of networks of trigonal-axis oriented bismuth nanowires. Electronic transport is studied over a wide range of temperatures down to low temperatures (0.3 K) and for magnetic fields up to 47 T. We focus on Shubnikov-de Haas oscillations of the magnetoresistance, which provide direct measurement of some cross-sectional areas of the carrier's Fermi surfaces and estimates for the carrier density, effective masses and relaxation times [Appl.Phys.Lett. 84, 1326 (2004)]. Supported by the Army Research Office, National Science Foundation, and CRDF. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B40.00005: A new expression for the conductance of quantum junctions within interacting {\it ab-initio} approaches Peter Bokes, Rex Godby We present a new expression for the conductance of a quantum junction~[1] that is both physically appealing and numerically adventageous. First, it shows that conductance represents a strength of the Drude singularity of the conductivity $\sigma(k,k';i\omega)$ and properly accounts for the experimental bias that results from the total field. The numerical advantage comes from the fact that instead of the current-current correlation function it is expressed in terms of density-density response which is far easier to evaluate. If formulated within time-dependent density-functional theory, it explicitly uses the exchange-correlation kernel $f_{xc}$ or, when used within many-body perturbation theory, it makes use of the irreducible polarisation function $P(k,k';i\omega)$. We will present its numerical implementation for a model metal-vacuum-metal interface. \\ \noindent [1] P. Bokes and R. W. Godby, Phys. Rev. B 69, 245420 (2004). [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B40.00006: Transport in closed nanoscale systems Neil Bushong, Na Sai, Massimiliano Di Ventra An alternative way to describe electrical transport in nanoscale systems has been recently proposed where two large but finite charged electrodes discharge across a nanoscale junction (M. Di Ventra and T. Todorov, J. Phys. Cond. Matt. 16, 8025 (2004)). We have applied this concept to describe the dynamics of a finite quasi-one dimensional gold wire using both a simple tight-binding model and time-dependent density-functional theory. After an initial transient, a quasi-steady state sets in whose lifetime increases with system size. This quasi-steady state is due to the wave properties of the electron wavefunctions and the resultant uncertainty principle and is established without inelastic effects. The corresponding current-voltage characteristics at steady state are in very good agreement with those calculated from the static scattering approach. We discuss local electron distributions, electrostatic potentials, and local resistivity dipoles formed at the quasi-steady state and compare these findings with the static open-boundary problem. A relation between information entropy and electron dynamics is discussed. Work supported by NSF. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B40.00007: Nonlocal Corrections to the DFT-LDA Electron Conductance in Nanoscale Systems Na Sai, Michael Zwolak, Giovanni Vignale, Massimiliano Di Ventra Using time-dependent current-density functional theory, we derive analytically the nonlocal exchange-correlation correction to the conductance of nanoscale junctions. The correction pertains to the conductance calculated in the zero-frequency limit of time-dependent density-functional theory within the adiabatic local-density approximation. In particular, we show that in linear response the correction depends nonlinearly on the gradient of the electron density; thus, it is more pronounced for molecular junctions than for quantum point contacts. We provide specific numerical examples to illustrate these findings. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:39PM |
B40.00008: New density functional for molecular conductance Invited Speaker: The ``ab initio'' descriptions of molecular conductance are routinely performed using density functionals that have spurious self-interaction. This may cause qualitative and severe quantitative errors in the conductance estimation. In an attempt to rectify the situation, we develop a new exact representation of the exchange-correlation energy, from which a new density functional theory is derived. The new functional has correct long-range behavior combined with a good description of the chemical bond. We show that this new functional excellently describes the polarizability of elongated molecules and yields quantitative electron affinity energies. We further show it has a more physical approach to describe the charge distribution in biased systems. The new method is encapsulated the required derivative discontinuities associated with charge transfer. The implication for a realistic description of molecular conductance is discussed. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B40.00009: Quantum transport calculations using periodic boundary conditions and plane wave basis Lin-Wang Wang Elastic quantum transport is a fast growing research area in nanoscience. The computational method for such elastic quantum transport is still under development. Although there are many methods for transport calculations, none of them is as fast and as reliable as the conventional ground state ab initio calculations (e.g., the plane wave pseudopotential local density approximation calculation). As a result, most transport calculations are restricted to small systems. We present an efficient new method [1,2] to solve the scattering states in the quantum transport problem using periodic boundary conditions and plane wave pseudopotential method. This method allows the use of conventional ground state ab initio programs without much change, and its computaional effort is similar to that of a ground state calculation. As a result, large systems containing hundreds of atoms can be calculated. Numerical results using this method for benzene molecules and doped silicon bars connected by Cu quantum wires will be presented. [1] L.W. Wang, //arxiv.org/abs/cond-mat/0408222 [2] L.W. Wang, //arxiv.org/abs/cond-mat/0408224 [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B40.00010: Persistent current in continuous one-dimensional quantum ring: Hartree-Fock and quantum Monte Carlo study. P. Vagner, R. N\'{e}meth, M. Mo\v{s}ko, L. Mitas We study numerically the effects of the electron-electron interaction on the persistent current of a one-dimensional quantum ring containing a single $\delta$-barrier. Using the self-consistent Hartree-Fock approximation for spinless electrons, we calculate the zero-temperature persistent current as a function of the ring circumference, magnetic flux threading the ring, barrier strength, electron-electron interaction strength, etc. Next, we explore the many-body quantum Monte Carlo method to obtain fully correlated solutions for the steady state wave functions and to evaluate the transport properties for the same problem. [Preview Abstract] |
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B40.00011: New Devices for Integrating Controlled Assembly, Imaging and Transport Measurements of Nanoscale Structures Michael Fischbein, Marija Drndic Experiments on charge transport in nanostructures require knowledge and control of the structural details in order to achieve an improved understanding of the transport mechanisms. Typical devices used to probe the electrical properties of nanometer scale objects do not allow for fluorescence imaging or the Angstrom resolution imaging that is available with Transmission Electron Microscopy. A substantial limitation on the knowledge of the sample's content and configuration is thus imposed. Furthermore, uncontrolled sample arrangement in these devices can introduce extraneous channels for transport. We have fabricated a new kind of device which resolves these issues. Silicon Nitride wafers are modified in a multi-step lithography process to yield a device which allows transmission based imaging of nanostructures positioned inside a submicron electrode gap. Additional lithographic steps make it possible to direct the assembly of the nanostructures. We discuss specific examples involving nanocrystals. This fabrication process allows for a range of device geometries which offer solutions to critical issues that are encountered when performing transport measurements on nanostructures. * This work is supported at Penn by the ONR Young Investigator Award {\#} N000140410489, the American Chemical Society and the startup funds at Penn. MF acknowledges funding from the NSF IGERT Program. [Preview Abstract] |
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