Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H26: Focus Session: Nanotubes and nanowires: Theoretical Studies |
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Sponsoring Units: DMP Chair: M.P. Anantram, NASA Ames Room: LACC 501B |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H26.00001: Mesoscopic Band Gap Engineering in SWNTs: How the Symmetry Affects Electronic Properties Invited Speaker: We discuss mechanisms to control the band gap in carbon single-wall nanotubes (SWNTs) of certain symmetry. One can apply external perturbation, such as electric field, mechanical deformation or a combination of two above, to a pristine semiconductor/metal SWNT. Band structure, e.g., band gap and density of states (DoS) at the Fermi level, will reflect the perturbation. The SWNT symmetry and its doping level determine whether the external perturbation will open the gap (in a metallic NT), increase or decrease the gap (in semiconductor NT). When the perturbation is not uniform along the SNWT a possibility for creating molecular heterojunctions and heterostructures opens. The perturbations considered in the talk range from local electrostatic potentials (that change the electronic structure in a small region) to periodic potentials (that superimpose a periodic perturbation on the lattice potential of the SWNT) to uniform mechanical deformation and polarization. It is important to take into account long-range Coulomb interactions between charge carriers in SWNT because the screening changes the picture for the band gap/DoS modulation. \newline\newline References: \newline 1. Y.Li, et.al. ``Metal-Semiconductor Transition in Armchair Carbon Nanotubes by Symmetry Breaking,'' APL, 4178 (2004) \newline 2. SV. Rotkin, in {\it Applied Physics of Nanotubes: Fundamentals of Theory, Optics and Transport Devices}, Nanosc. and Nanotechn. Ser., Ph. Avouris, Ser.Ed. Springer Verlag (2005) \newline 3. SV. Rotkin, K. Hess, ``Possibility of Metallic Nanotube Field-Effect Transistors,'' APL 84, 3139 (2004) \newline 4. AG. Petrov, SV. Rotkin, ``Breaking of Nanotube Symmetry by Substrate Polarization,'' Nano Lett. 3, 701 (2003) \newline 5. Y. Li, et.al. ``Electronic response and bandstructure modulation of carbon nanotubes in a transverse electrical field,'' Nano Lett. 3, 183 (2003) [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H26.00002: Effect of diameter and contacts on electron transport in carbon nanotubes Alexei Svizhenko, M.P. Anantram We study the effect of nanotube diameter and contact quality on screening properties and current carrying capacity of single- and multi-wall carbon nanotubes. Depending on the diameter, the differential conductance shows qualitatively different behavior versus bias: decreasing for small diameter and increasing for large diameter nanotubes. We find that while the screening is better for smaller diameter nanotubes at low bias, this trend is reversed at high bias. The quality of the contacts can also make a significant difference between increasing or decreasing trends of differential conductance versus bias. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H26.00003: Near Ideal Operation of Nanometer Gate Length Carbon Nanotube FETs Khairul Alam, Roger Lake Performance of coaxially gated carbon nanotube FETs is investigated in their scaling limit with gate lengths down to 2 nm. Both single-walled carbon nanotube (SWCNT) and double-walled carbon nanotube (DWCNT) are used as the conducting channels. With SWCNT as the conducting channel, the device has nearly ideal subthreshold slope $\sim $ 63 mV/dec, high on/off ratio $\sim $ 10$^{6}$, and low intrinsic switching time $\sim $ 18 fs. While with DWCNT as the channel, the tube with larger diameter contributes more than 99{\%} to the total current, the subthreshold slope is more than 120 mV/dec, and the on/off current ratio is very low ($\sim $ 100) although the drive current is a few times larger. The leakage current is a combination of inter-band and intra-band tunneling. This current can be significantly reduced by changing the tube length as well as the tube diameter. The simulation model is based on the $\pi $-bond of CNT and self-consistent solution of non-equilibrium Green function (NEGF) equations and Poisson's equation. The NEGF equations are solved using recursive Green function algorithm. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H26.00004: Electron Orbital Filtering in Multiply Connected Carbon Nanotubes Gunn Kim, Sang Bong Lee, Tae-Suk Kim, Jisoon Ihm An electronic state in atoms, molecules, or solids has a particular spatial character usually represented by an orbital or a linear combination of such orbitals well- established in quantum chemistry. Since an enormous number of different kinds of orbitals coexist and overlap in a real material, it is difficult to select or probe a particular orbital in real solids. Recently, as fabrication techniques of nanometer-sized material units such as carbon nanotubes are developed, control over individual electronic states in a material is improving quite remarkably. Here we report that, in a multiply connected carbon nanotube system with a mirror-reflection symmetry, we can produce an electron current of one particular orbital character ($\pi$ orbital) and suppress the current flow of other electrons ($\pi^{*}$ orbital) over a significantly wide range of energy. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H26.00005: Shot Noise and Full Counting Statistics from Non-equilibrium Plasmons in a Double Junction Quantum Wire Jaeuk Kim, Jari Kinaret, Mahn-Soo Choi We consider a quantum wire double junction system with each wire segment described by a spinless Luttinger model, and study theoretically shot noise and full counting statistics. We find that the non-equilibrium plasmonic excitations in the central wire segment give rise to qualitatively different behavior compared to the case with equilibrium plasmons. In particular, shot noise is greatly enhanced by non-equilibrium plasmons, and exceeds the Poisson limit. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H26.00006: High frequency properties of a CNT-based nanorelay Jari Kinaret, Magnus Jonsson, Tomas Nord, Sven Axelsson, Susanne Viefers We investigate theoretically the high frequency properties of a carbon-nanotube-based three-terminal nanoelectromechanical relay. The intrinsic mechanical frequency of the relay is in the GHz-regime, and the electromechanical coupling shows a non-linear resonant behavior in this frequency range. We show that the electromechanical resonant frequencies can be tuned by a bias voltage, and discuss how the device may be used as a tunable HF filter or a voltage-controlled oscillator. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H26.00007: Dynamic friction force in a novel Ultra-High Frequency Oscillator Haibin Su, William Goddard III Very recently, it has been reported by Zettle group that frictional forces is very small, c.a. in the magnitude of 10-14 N per A$^{2}$, during the controlled and reversible telescopic extension of multiwalled carbon nanotubes. Moreover, a new type of nano-oscillator operating completely different from conventional quartz oscillator has been proposed based upon this interesting observation. Since then, designing this type of nano-oscillator has been carried out actively. Legoas and collaborators first simulated a 38 GHz nano-oscillator consisting of a (9,0) carbon nanotube (CN) inside of (18,0) CN. However, no successful experimental tests have been reported so far. This is probably due to the difficulty of preparing the bi-tube type oscillator unit from multiwalled carbon nanotubes with high quality, and the considerable amount of energy dissipation. We propose a new generation of fullerene nano-oscillator: a (10,10) single wall carbon nanotube with one buckyball inside. The molecular dynamics studies predict the operating frequency is ultra-high, c.a. 50 GHz. The energy dissipation from simulation shows significant effects of temperature, and impulse velocity on dynamic friction force. In particular, it has been shown that edge effects are the main cause of dynamic friction force. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H26.00008: Theoretical Studies of Charge Transport through Al atomic wires Khorgolkhuu Odbadrakh, Pawel Pomorski, Christopher Roland We have revisited the question of quantum transport through single-chain atomic Al wires, using a recently combined nonequilibrium/density functional-based formalism. Investigations of transport under a bias voltage brings additional insight into the current-voltage characteristics of this simple molecular electronic system. In agreement with previous work, we find that the Al-wires with (100) and (111) Al leads display prominant conductance oscillations with a period of four, that are qualitatively independent of lead size and configurations. A band structure analysis of the infinite Al-wire shows that this period may be explained in terms of induced resonace states localized at the atomic wire. In addition, we have investigated the induced charges in the wires as a function of the bias voltange, and the capacitance coefficients of the leads. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H26.00009: Variational Wavefunctions for a Quantum Wire of Cylindrical J.E. Hasbun In previous work, the quantum level structure in a quantum wire was investigated.$^{1}$ As in that work, an electron is considered to be free to move along the $z$ coordinate of a cylindrical wire but confined along the $\rho$ direction and include the Hartree term and exchange-correlation a la DFT, as done previously.$^{2}$ However, whereas in ref.1 a numerically obtained ground state was compared to a ground state variational model based on the wave function $f(\rho)=2\sqrt{2b_0}exp(-2b_0\rho^2)$, the present treatment makes use of a variational model ground state of the form $f(\rho)=J_0[a(1+b_0R^2)\rho/R_p]exp(-b_0\rho^2)$, where $J_0$ is the zeroth order Bessel function, $b_0$ is the variational parameter, $R$ is the radius of the wire, $a$ is a zero of the Bessel function, and $R_p$ is an adjustable effective radius. As will be shown, the motivation for this wavefunction is that it can be extended to at least two excited states and still be able to go beyond the Hartree approximation to include exchange-correlation using the LDA, and get reasonable convergence as well as reasonable agreement with the full numerical results. \newline $^{1}$ J. Hasbun, APS Bull. \underline {49}, 1135 (2004). \newline $^{2}$ J. Hasbun, APS Bull. \underline {48}, 888 (2003). [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H26.00010: Scanning probe microscopy of spin-charge separation Iddo Ussishkin, Leonid I. Glazman In one-dimensional systems, interaction between electrons modify the nature of the state, as described by Luttinger liquid theory. Spin and charge propagate independently with different velocities. We consider the possibility of probing this spin-charge separation with a scanning probe microscopy technique. Specifically, we consider an experimental setup in which the scanning tip is used to create a local potential that scatters electrons, without tunneling from tip to sample. We analyze the effect of this auxiliary scatterer on the conductance of a semi-infinite one-dimensional system. We find that the differential conductance for tunneling into the end of the quantum wire reflects the separation of the elementary excitations into spin and charge modes for realistic interaction strength. The separation is revealed as a specific moire pattern in the dependence of the differential conductance on bias and on the position of the scatterer (induced by the scanning tip). This pattern is associated with the difference between the velocities of the spin and charge modes. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H26.00011: STM Response of a Luttinger Liquid with a Kondo Impurity Paata Kakashvili, Henrik Johannesson We study the STM (Scanning Tunneling Microscopy) response of a Luttinger liquid with a Kondo impurity. The system can be realized by adding a magnetic impurity to a quantum wire or carbon nanotube. At low temperatures we map the system to a boundary problem, and use Bosonization and Boundary Conformal Field Theory techniques to calculate the finite-temperature Green's functions. The local density of states is calculated for different temperatures and distances from the impurity. We discuss properties of the uniform and Friedel terms in the STM current, and show how it takes fingerprints of the electron correlations. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H26.00012: Exchange Coupling in a One-Dimensional Wigner Crystal Alexios Klironomos, Revaz Ramazashvili, Konstantin Matveev We consider a long quantum wire at low electron densities. In this strong interaction regime, a Wigner crystal ground state can form, in which the electrons comprise an antiferromagnetically ordered Heisenberg spin chain. The coupling constant $J$ is exponentially small as it originates from tunneling of two neighboring electrons through the potential barrier that separates them. Because of the long range nature of the Coulomb interactions, tunneling is in fact a many body process. We calculate the exchange constant $J$ taking into account all electrons in the chain, for both an idealized one-dimensional quantum wire and a more realistic device of finite width. We show that in current experiments, $J$ can be smaller than the temperature, which should have an effect on the electron transport in the wire. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H26.00013: Spin-splitting induced by spin-orbit interaction in chiral nanotubes M.P. L\'opez-Sancho, L. Chico, M.C. Mu\~noz We show that chiral tubes present spin splitting at the Fermi level in the absence of magnetic field, whereas achiral tubes preserve spin degeneracy, as evidenced by electronic structure calculations with the inclusion of spin-orbit interaction. The nanotubes are modeled by the Slater-Koster empirical tight-binding Hamiltonian including $sp^3$ orbitals. These remarkably different behaviors of chiral and non-chiral nanotubes have a symmetry origin. Even though the spin-orbit interaction partially lifts degeneracy in achiral nanotubes, it does not affect the spin degenerate states, while in chiral tubes the lack of inversion symmmtry does not allow spin degeneracy. This result may provide a global explanation to recently reported spin-dependent transport experiments which were in apparent contradiction. [Preview Abstract] |
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H26.00014: Diffusion Of Lithium And Hydrogen In Nanotubes In Terms Of Fokker-Plank Equation John Adams, Russell Selva, Yuriy Malozovsky We present the theory of diffusion of atoms like Li and H and their diatomic molecules like Li$_{2}$ and H$_{2}$ in metallic nanotubes. We derived the Arrhenius type diffusion coefficient in terms of the kinetic Fokker-Plank equation. The diffusion coefficient was derived with the consideration both the motion of the particle in the cylindrical periodic potential of the nanotube and interaction of the diffuse particle with lattice vibrations of the tubule. We evaluated the activation energies of diffusion in terms of the pair interaction potential. We argue that there is an optimum diameter of the tubule below which the diffusion is significantly reduced. [Preview Abstract] |
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