Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U40: Focus Session: Transport Properties of Nanostructures V: Molecules |
Hide Abstracts |
Sponsoring Units: DMP DCP Chair: J. v. Ruitenbeek, Universiteit Leiden Room: LACC 408A |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U40.00001: Theory of electron hopping through atomic-scale contacts Kenji Hirose, Nobuhiko Kobayashi With the rapid progress of constructing atomic-scale nanostructure devices, understanding of electron transport between electrodes becomes an important problem. In such systems, the transfer of an electron is achieved {\it via} tunneling or ballistically through atomic-scale contacts. Here, we consider the junction system with atomic-scale contacts and evaluate the current-voltage characteristics as a function of the distance between electrodes. We use the recursion transfer matrix (RTM) method with plane-wave basis sets to obtain accurate scattering states between electrodes under finite bias voltages. Then we construct the nonequilibrium Green's function (NEGF) to evaluate charge densities and current-voltage characteristics from the obtained scattering states. Based on the density-functional method, we perform self-consistent calculations of the charge density and effective potential for the junctions systems. This method enables us to treat accurate scattering states from tunneling to ballistic transport regimes. As the distance between electrodes becomes large, we find a strong nonlinear behavior in the current-voltage (I-V) characteristics and correspondingly a gap structure appears in conductance. We consider the mechanism to appear such nonlinear behavior in I-V characteristics and compare the experimental observations. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U40.00002: Encapsulated Organic Molecules in Carbon Nanotubes: Novel p-n junctions Yi Liu, Nicholas Kioussis Recently Lu et al [1] have shown that both p- and n-type doping can be realized on single-walled carbon nanotubes (SWCNTs) by encapsulating various organic or organometallic molecules, such as TCNQ, F$_{4}$TCNQ, TTF, and TDAE, with different electron affinities or ionization potentials. Using a method that combines density functional theory and Keldysh nonequilibrium Green's functions, we have investigated the charge transport properties of such encapsulated SWCNTs. We have found that the current-voltage characteristics of p-type and n-type doped SWCNTs are very different, which can be explained in terms of their individual band structures. More importantly, if these p-type and n-type doped SWCNTs are joined together, p-n junctions can be achieved. Current-voltage curves for such p-n junctions will be presented. \begin{enumerate} \item J. Lu, S. Nagase, D. Yu, H. Ye, R. Han, Z. Gao, S. Zhang, and L. Peng, Phys. Rev. Lett. \textbf{93,} 116804 (2004). \end{enumerate} *Supported through the NSF grant No. DMR-0097187 [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U40.00003: Nonequilibrium Kondo Prashant Sharma We consider a quantum dot connected to Fermi liquid leads in the Kondo regime, when ac voltages are applied to the leads and also to the dot. For temperatures well-below the Kondo temperature ($T_K$) we identify an effective time-dependent Hamiltonian that contains all the relevant (in the renormalization group sense) operators. With this as the starting point, we solve the ac transport problem by including the role of leading irrelevant operators. The theoretical results are compared to the recent experimental observation by Kogan {\it et al.}~[Science~\bf{304}, 1293 (2004)] of satellite peaks in the differential conductance of a single electron transistor in the Kondo regime. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 9:12AM |
U40.00004: Superconductivity in Long and Short Molecules Invited Speaker: We present the results of experimental study of superconductivity in individual molecules of carbon nanotubes [1,2], DNAs [3] and metallofullerenes [4]. Critical currents of supeconductor-molecule-superconductor junctions were extensively studied as a function of temperature and magnetic field. The mechanism of current induced superconductor-normal state transition for a long molecule (carbon nanotubes and DNAs) is the creation of phase slip centers and for a short molecule (metallofullerens) -- multiple Andreev reflections. We observe an influence of spin state of encapsulated atom on the induced superconductvity in a metallofullerene molecule. \begin{enumerate} \item A.Yu.Kasumov, et.al, Science 284, 1508 (1999). \item A.Yu.Kasumov et al., Phys. Rev. B 68, 214521 (2003). \item A.Yu.Kasumov et al., Science 291, 280 (2001). \item A.Yu.Kasumov et al., cond-mat/0402312, submitted to Phys.Rev.Lett. \end{enumerate} [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:48AM |
U40.00005: Using molecular transistors to study the Kondo effect in the presence of ferromagnetism Invited Speaker: I will describe a technique to study spin-polarized electron transport through nm-sized molecular transistors. We create nm-sized gaps in nanofabricated ferromagnetic wires by electromigration, into which organic molecules can be incorporated. We can tailor the shapes of the ferromagnetic electrodes using electron-beam lithography so that the relative magnetization orientation can be switched from parallel to antiparallel. We use this technique to make contact to C$_{60}$ molecules using nickel electrodes. We are able to observe signatures of the Kondo effect in low- resistance devices. The ferromagnetism in the electrodes splits the Kondo resonance, resulting in two symmetric peaks in the differential conductance as a function of bias voltage. This splitting is decreased (even to zero) when the electrode magnetizations switch from parallel to antiparallel. Our measurements are in good agreement with theories that predict an exchange splitting of the Kondo resonance. The Kondo effect leads to values of magnetoresistance that are several times larger than the Julliere value. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U40.00006: Inelastic effects on the transport properties of alkanethiols Yu-Chang Chen, Michael Zwolak, Massimiliano Di Ventra We discuss inelastic scattering effects in alkanethiols of different lengths sandwiched between metal electrodes. In particular, we examine local heating and the inelastic contribution to the current. We observe that the intensities of certain peaks in the inelastic scattering profile alternate depending on odd and even number of alkyl carbons. The cause of the odd-even effect is the alternating strength of the coupling between electrons and the longitudinal component of CH3-group motion. We also find that in the absence of heat dissipation into the bulk electrodes the local temperature of alkanethiols is relatively insensitive to the length of the wires. This is due to the fact that the rates of heating and cooling processes scale similarly with length. On the other hand, when considering heat dissipation into the bulk electrodes, the local temperature of alkanethiols decreases as their length increases. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U40.00007: Electron Transport Through Molecules: Gate Induced Polarization and Potential Shift San-Huang Ke, Harold U. Baranger, Weitao Yang We analyze the effect of a gate on the conductance of molecules by separately evaluating the gate-induced polarization and the potential shift of the molecule relative to the leads. The calculations use {\it ab initio} density functional theory combined with a Green function method for electron transport. For a general view, we study several systems: (1) atomic chains of C or Al sandwiched between Al electrodes, (2) a benzene molecule between Au leads, and (3) (9,0) and (5,5) carbon nanotubes. We find that the polarization effect is small because of screening. The effect of the potential shift is significant, providing a mechanism for single-molecule transistors. This work was supported in part by the NSF (DMR-0103003). [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U40.00008: Franck-Condon Blockade and Giant Fano Factors in Transport Through Single Molecules Felix von Oppen, Jens Koch We show$^1$ that Franck-Condon physics leads to a significant current suppression at low bias voltages (termed {\it Franck-Condon blockade}) in transport through single molecules with strong coupling between electronic and vibrational degrees of freedom. We find that transport in this regime is characterized by remarkably large Fano factors ($10^2$ - $10^3$ for realistic coupling parameters), which arise due to avalanche-like transport of electrons. Avalanches occur in a self-similar manner over a wide range of time scales, as reflected in power-law dependences of the current noise on frequency and vibrational relaxation rate. \\ \\ $^1$ Jens Koch and Felix von Oppen, cond-mat/0409667 [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U40.00009: Controlled Fabrication of Nanogaps for Molecular Electronics D. R. Strachan, D. E. Smith, D. E. Johnston, A. T. Johnson, D. A. Bonnell, T.-H. Park, S. P. Wu, M. J. Therien, F. V. Cochran, W. F. DeGrado We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance has several advantages over the typical method at liquid-helium temperatures. One advantage is that it will make feasible electrical measurements of molecules that do not survive a sub-freezing environment. A second advantage is that it yields nanogaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. We discuss how the nanogap evolves through three regimes -- a bulk-neck regime where electromigration is triggered at constant temperature, then a few-atom regime characterized by quantized plateaus in the conductance, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum ($G_o=2e^2/h$). We end with a discussion on the electronic properties of molecules measured using the new electrodes. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U40.00010: Orbital resolved electron transport in single molecule contacts Kristian Thygesen, Robert Stadler, Mikkel Strange, Karsten Jacobsen We have performed a theoretical study of the phase-coherent electron transport in three different molecular junctions consisting of hydrogen, CO and bi-pyridine connected to Pt and Au leads. In all three cases we find good agreement with experimental results. We use a Green's function transport scheme in combination with density functional theory and a Wannier function basis set. The transport characteristics of the three molecules differ substantially ranging from complete transparency in the case of hydrogen to resonant tunneling through the LUMO state of the bi-pyridine molecule. By reconstructing the molecular orbitals of the contacted molecule and subsequently calculating the transmission with different orbitals removed from the basis set, we can directly test the individual contributions to the transmission from each molecular orbital and thereby obtain a detailed picture of the conduction mechanism. [Preview Abstract] |
|
U40.00011: Tunneling mechanisms of transport in single organic molecules Ivan Oleynik, Mortko Kozhushner, Vladimir Posvyanskii Electron transport in one-dimensional organic molecular structures exhibits unique and intriguing properties that cannot be explained using traditional concepts of solid state physics and/or quantum chemistry of organic molecules. We present a new theoretical approach to study tunneling phenomena in single organic molecular systems that provides an explanation of the experimental observations within a conceptually simple and unified framework. The tunneling in metal/organic-molecule/metal systems is considered as a sub-barrier scattering of tunneling electrons off the electrons and nuclei of the molecular wire and is described using the powerful technique of scattering operators. We will discuss the unique features of the tunneling electron spectrum, the combined mechanisms of ordinary and resonant tunneling, and other phenomena that are important for understanding and interpreting experiments. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700