Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H34: Focus Session: Atomic, Molecular, and Memristive Junctions |
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Sponsoring Units: DMP Chair: Douglas Natelson, Rice University Room: C141 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H34.00001: Memory effects in nanoscale systems Invited Speaker: Memory emerges quite naturally in systems of nanoscale dimensions: the change of state of electrons and ions is not instantaneous if probed at specific time scales, and it generally depends on the past dynamics. This means that the resistive, capacitive and/or inductive properties of these systems generally show interesting time-dependent (memory) features when subject to time-dependent perturbations. In other words, nanoscale systems behave as a combination of (or simply as) memristors, memcapacitors or meminductors, namely circuit elements whose resistance, capacitance and inductance, respectively, depend on the past states through which the system has evolved. After an introduction to the theory and properties of memristors, memcapacitors and meminductors, I will discuss several memory phenomena in nanostructures associated to charge, ion and spin dynamics and their far-reaching applications ranging from information storage to computation to biologically-inspired systems. Work supported in part by NSF, NIH, and DOE. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H34.00002: Multiple switching modes and multiple level states in memristive devices Feng Miao, J. Joshua Yang, Julien Borghetti, John Paul Strachan, M.-X. Zhang, Ilan Goldfarb, Gilberto Medeiros-Ribeiro, R. Stanley Williams As one of the most promising technologies for next generation non-volatile memory, metal oxide based memristive devices have demonstrated great advantages on scalability, operating speed and power consumption. Here we report the observation of multiple switching modes and multiple level states in different memristive systems. The multiple switching modes can be obtained by limiting the current during electroforming, and related transport behaviors, including ionic and electronic motions, are characterized. Such observation can be rationalized by a model of two effective switching layers adjacent to the bottom and top electrodes. Multiple level states, corresponding to different composition of the conducting channel, will also be discussed in the context of multiple-level storage for high density, non-volatile memory applications. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H34.00003: Stochastic memory: getting memory out of noise Alexander Stotland, Massimiliano Di Ventra Memory circuit elements, namely memristors, memcapacitors and meminductors [1], can store information without the need of a power source. These systems are generally defined in terms of deterministic equations of motion for the state variables that are responsible for memory. However, in real systems noise sources can never be eliminated completely. One would then expect noise to be detrimental for memory. Here, we show that under specific conditions on the noise intensity memory can actually be enhanced. We illustrate this phenomenon using a physical model of a memristor in which the addition of white noise into the state variable equation improves the memory and helps the operation of the system. We discuss under which conditions this effect can be realized experimentally, discuss its implications on existing memory systems discussed in the literature, and also analyze the effects of colored noise. \\[4pt] [1] M. Di Ventra, Y.V. Pershin, L.O. Chua, Circuit elements with memory: memristors, memcapacitors and meminductors, Proc. IEEE 97, 1717 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H34.00004: Nano-Ionic in Molecular Nanojunctions Lam Yu, Matthew Roberson Metal filament growth can be induced on the surfaces of electrochemically active metals such as silver and copper by an external electric potential. We study the voltage-driven formation and dissolution of nanoscale silver filaments in silver/molecular-layer/gold junctions. In this system, an applied electric voltage causes oxidation at the silver surface and the resulting silver cations from the reaction are transported away from the silver surface toward the gold surface. The silver cations are reduced at the gold surface and from the accumulation of the transported silver particles, filaments are formed from the gold surface. We found that the energy required for silver nanofilament formation depends critically on the thickness and electrostatic property of the molecular layer, while the energy required for the dissolution of silver nanofilaments is virtually molecule-independent. I will discuss what our results tell us about chemical reactions in the nanoscale, and the practical application of our results in the area of electronic memory and chemical sensors. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H34.00005: Bias dependent shot noise measurement in STM style Au junction at room temperature Ruoyu Chen, Patrick Wheeler, Amanda Whaley, Douglas Natelson Shot noise in nanoscale junctions is suppressed strongly near certain conductance values because of nearly fully transmitted modes. Using a gold tip in a STM style motion as the source, combining with high-frequency techniques, we simultaneously measured the conductance and the mean square current noise in nanoscale junctions at a series of voltage biases at room temperature. We have observed peaks in the conductance histogram and related shot noise suppression at different biases near integer multiples of the conductance quantum G0, especially the first three. It demonstrates that quantized electronic transport through quantum channels takes place. We will discuss the relevant noise processes and their evolution with bias across the junctions. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H34.00006: Large bandwidth measurements of break junctions for molecular electronics at microwave frequencies Gabriel Puebla-Hellmann, Andreas Wallraff The controlled breaking of a thin gold wire, by mechanical stress or by electromigration, has not only been successfully employed to produce the nm-spaced electrodes necessary for creating single molecule devices, but has also attracted attention as the means to produce optical field enhancement for surface enhanced Raman scattering on the few to single molecule level. Although frequently employed, such break junctions usually have a low bandwidth when performing electrical transport measurements of single molecule devices. We investigate such junctions and single molecule devices by using microwave reflectrometry, where a break junction is integrated into a superconducting impedance matching circuit. This allows the impedance and thus the state of the junction to be deduced from the measured reflection coefficient with a bandwidth of 10-100 MHz. We electrically characterize such impedance matching circuits at microwave frequencies as well as gold break junctions at DC. We also show measurement results of the combined system, where the break junction is formed either by electromigration or by mechanical means. This setup will be used to study the transport properties of single molecule devices with a bandwidth larger than that of standard low frequency techniques. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H34.00007: Shot Noise Measurements in Individual Electromigrated Nanoscale Jsunctions Patrick Wheeler, Douglas Natelson Shot noise provides insight into the correlated motion of electrons in nanostructures. Previous measurements have examined shot noise in mechanically controlled break junctions (MCBJs), looking at a large ensemble of junction configurations. Electromigrated, lithographically created Au structures at liquid nitrogen and helium temperatures allow for the shot noise measurement of individual junction configurations. High frequency excess noise is amplified by a rf amplifier chain and measured in combination with lock-in techniques simultaneously with the dc conductance. Preliminary noise measurements across bias and temperature are compared to previous experiments preformed with MCBJs at room temperature, with an emphasis on the dependence of the noise on bias conditions. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H34.00008: Entanglement dynamics within the micro-canonical approach to transport Massimiliano Di Ventra, Chih-Chun Chien, Michael Zwolak When a central barrier is located between two biased electrodes, the tunneling of electrons may build a quasi steady-state current and the entanglement entropy between the two sides increases. We study these quantities using the micro-canonical picture of transport [1]. The quasi steady-state current from our simulations agree with that obtained from single-particle scattering theory. In addition, we find that the entanglement entropy increases linearly in time and with bias, so long as the barrier is only partially transmitting, which agrees qualitatively with previous results derived under restrictive assumptions. The micro-canonical approach also allows us to investigate this system highly out-of-equilibrium and under a range of conditions. We present further results on barriers with different tunneling probabilities, biases, and time-dependent fields. \\[4pt] [1] M. Di Ventra and T. N. Todorov, J. Phys. Cond. Matt. 16, 8025 (2004). [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H34.00009: The number of transmission channels through a single-molecule junction Stafford Charles, Justin Bergfield, Joshua Barr We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting pi-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level. Thus for Pt-benzene-Pt junctions we predict two dominant transmission channels, and for Pt-butadiene-Pt junctions only one. Pt-buckyball-Pt junctions are predicted to exhibit up to five dominant transmission channels. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H34.00010: Vibrational heating in molecular junctions Daniel Ward, David Corley, James Tour, Douglas Natelson Energy injection, distribution and dissipation are of great important in understanding molecular electronics. One method of characterizing the distribution of energy in a system is to measure the effective temperature. Using surface-enhanced Raman spectroscopy (SERS) of molecular nanojunctions, we measure the effective vibrational temperatures of a molecular nanojunction as a function of bias. We observe significant mode-specific vibrational pumping by both optical excitation and DC current, with effective temperature changes exceeding several hundred Kelvin. These measurements provide direct information about heat generation and dissipation in molecular-scale junctions and allow direct comparisons with theories of nanoscale heating. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H34.00011: Effective field theory of interacting pi electrons in molecular junctions Joshua Barr, Justin Bergfield, Charles Stafford We present an effective field theory that allows the two-body Hamiltonian for a $\pi$ electron system to be expressed in terms of three effective parameters: the $\pi$ orbital quadrupole moment, the on-site repulsion, and a dielectric constant. As an application of this theory, we present a model of screening in single-molecule junctions based on the image charge method, and use this technique to calculate the van der Waals interaction between a neutral molecule and a metallic conductor. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H34.00012: Temperature dependence of charge transport in conjugated single molecule junctions Eek Huisman, Masha Kamenetska, Latha Venkataraman Over the last decade, the break junction technique using a scanning tunneling microscope geometry has proven to be an important tool to understand electron transport through single molecule junctions. Here, we use this technique to probe transport through junctions at temperatures ranging from 5K to 300K. We study three amine-terminated (-NH$_{2})$ conjugated molecules: a benzene, a biphenyl and a terphenyl derivative. We find that amine groups bind selectively to undercoordinate gold atoms gold all the way down to 5K, yielding single molecule junctions with well-defined conductances. Furthermore, we find that the conductance of a single molecule junction increases with temperature and we present a mechanism for this temperature dependent transport result. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H34.00013: Charge transport in mechanically controlled single-molecule break-junctions Youngsang Kim, Florian Strigl, Hans-Fridtjof Pernau, Elke Scheer, Hyunwook Song, Takhee Lee, Thomas Hellmuth, Fabian Pauly, Linda A. Zotti, Juan Carlos Cuevas We present inelastic electron tunneling spectroscopy (IETS) measurements carried out on single molecules incorporated into a mechanically controllable break-junction (MCBJ) at low temperature. The single molecules contacted with a MCBJ or with a STM show various conductance values under stretching depending on the contact geometry and the molecular conformation ($e.g.$, \textit{trans} or \textit{gauche}). In such single-molecule devices, the metal of electrodes ($e.g.$, gold or platinum) and anchoring groups ($e.g.$, thiol (-SH) or amine (-NH$_{2}))$ can also significantly influence the charge transport through the single molecules. Here we demonstrate how these individual aspects influence the conductance properties of single-molecule junctions. [Preview Abstract] |
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