Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y30: Low Dimensional Systems |
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Sponsoring Units: DCMP Chair: Michael Mehl, Naval Research Laboratory Room: Morial Convention Center 222 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y30.00001: Inelastic neutron scattering from confined molecular oxygen. Paul Sokol, Duncan Kilburn We report results from experiments measuring the generalized density of states in confined solid molecular oxygen. It is known from previous experiments that fundamental properties of liquids and solids, such as phase transition temperatures and intermolecular structure can be altered by confining them in porous media (pores typically in the angstrom to nanometer range). It is reasonable therefore to ask the question: what is the effect of confinement on collective excitations in the material, and can these changes be exploited in a technological setting? Using inelastic neutron scattering we find that both the structure and generalized density of states of solid molecular oxygen are altered by confining it in a templated porous glass with a mean pore diameter of 100 Angstroms. The structure, in the Q-range which we were able to measure, resembles that of an amorphous material and the density of states are shifted to lower energy excitations. One possible application for such a material is as moderator material in a very cold neutron source. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y30.00002: Imaging One-Electron Quantum Dots in InAs/InP Nanowires Halvar Trodahl, Erin Boyd, R.M. Westervelt, Linus Froberg, Lars Samuelson Quantum dots formed in InAs/InP heterostructure nanowires are attractive candidates for nanoelectronics, spintronics, and quantum information processing. It has been shown that with the use of a liquid helium cooled scanning probe microscope (SPM) the electronic charge state of a single quantum dot can be chosen by merely moving the tip of the SPM with respect to the dot [1]. Simulations show that this technique can also be implemented for double quantum dots with interdot separation less than 30nm. By applying a magnetic field the electronic states of a quantum dot can be spin split. It is possible to image this splitting with the local, movable electrostatic gating of the SPM. A magnetic field applied to a double dot system can be used to create a spin filter; a simple movement of the SPM tip with respect to the dot structure can determine which spin passes through the filter. \newline [1] A. Bleszynski et al., 28th Int. Conf. Physics of Semiconductors, 2006. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y30.00003: Imaging the Wavefunction of a One-Electron Quantum Dot Erin E. Boyd, Halvar J. Trodahl, Parisa Fallahi, R.M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots grown in InAs/InP nanowires are promising contenders for nanoelectronics. A fundamental understanding of the quantum behavior of the electron is important for the design of quantum devices. We have developed an imaging technique to image the electron wavefunction of a quantum state inside a long InAs dot (length$>$diameter) formed by InP barriers, using a liquid He-4 cooled scanning probe microscope [1]. The electrostatic potential of the tip dents the wavefunction and changes the energy of the quantum state by an amount proportional to the electron probability density at the tip position. Using Coulomb blockade conductance images of the dot, the energy change vs. tip position can be found. By deconvolving the measured energy shift with the tip potential, one can extract the electron probability density, using first-order perturbation theory. [1] P. Fallahi, PhD Thesis, Harvard Univ (2006). [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y30.00004: Theoretical study of spin relaxation in a carbon nanotube quantum dot Brian Bezanson, Xuedong Hu Carbon nanotubes offer an attractive environment for coherent spin manipulation due to the small population of nuclear spins and weak spin-orbit interaction. While a couple of specific spin relaxation mechanisms have been investigated theoretically[1][2], there is still no comprehensive study of spin lifetimes in carbon nanotubes. In the present study we calculate the spin decay rate for electrons in gate-defined quantum dots on carbon nanotubes due to the spin-orbit and electron-phonon interactions. More specifically, we explore effects of magnetic field strength and orientation, tube diameter and chirality, and confinement. \newline [1] Y. G. Semenov, K. W. Kim, G. J. Iafrate, Phys. Rev. B 75, 045429 (2007) \newline [2] K. M. Borysenko, Y. G. Semenov, K. W. Kim, J. M. Zavada, arXiv 0710.3382 (2007) [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y30.00005: Analysis of unusual splitting of Kondo peak in the differential conductance of a carbon nanotube quantum dot Jeffrey Stephens, Jerome Licini, A.T. Charlie Johnson, Doug Strachan, Danvers Johnston, Sam Khamis Carbon nanotubes grown by chemical vapor deposition on an oxidized silicon substrate were contacted to form a gated sample of parallel tubes. Testing was done at low temperature and high magnetic field using a dilution refrigerator and superconducting magnet. The current versus voltage graph shows asymmetry with respect to zero volts. The differential conductance (dI/dV) is computed and yields some intriguing behavior. The previous asymmetry is more apparent as is a sharp increase in conductivity near zero voltage. Temperature data further suggests a conductance peak at near zero voltage consistent with the Kondo effect. High magnetic fields, 0 to 11 Tesla in 0.5 Tesla increments, are used to probe the conductance behavior. The magnetic field tests yield unusual shapes and splitting at two critical fields. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y30.00006: Probing edge-localized states of graphene quantum dots on Co(0001) Daejin Eom, Kwang Rim, Hui Zhou, Michael Lefenfeld, Li Liu, Shengxiong Xiao, Colin Nuckolls, George Flynn, Tony Heinz Two-dimensional graphene sheets of finite lateral extent are expected to show characteristic edge states at their boundaries. In particular, for zigzag edges, highly degenerate localized states have been predicted theoretically (Ref. 1) and probed by STM (Ref. 2). Such boundary effects are expected to be particularly prominent for nanometer-scale graphene quantum dots, structures for which the proportion of edge atoms is significant. In this paper we present investigations of graphene quantum dots that we have prepared by annealing carbon- bearing precursor molecules on a Co(0001) surface. Using scanning tunneling microscopy as a local probe of the physical and electronic structure, we report results on the nature of edge states for quantum dots of differing geometrical shape. We observed prominent edge-localized states for triangular quantum dots, whereas these features are suppressed for quantum dots of hexagonal shape. These observations are consistent with numerical simulations of the expected electronic structure. 1. M. Fujita et. al., J. Phys. Soc. Jpn. 65, 1920 (1996) 2. Y. Niimi et. al., Phys. Rev. B 73, 085421 (2006) [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y30.00007: Surface Functionalized Carbogenic Quantum Dots A.B. Bourlinos, A. Stassinopoulos, A. Anglos, S.H. Anastasiadis, R. Zboril, M. Karakassides, E.P. Giannelis Surface functionalized carbon-based quantum dots (C-QDs) are formed in-situ in a single-step process via thermal carbonization of suitable molecular precursors based on ammonium citrate salts. The as-synthesized nanoparticles have near spherical morphology and size around 7nm. Using different surface modifiers, we can form hydrophobic or hydrophilic capped C-QDs, which can be dispersed in organic or aqueous solvents, respectively. These C-QDs fluoresce strongly upon optical excitation. We believe that the fine size of the C-QDs combined with their disorder structure favor a high concentration of defect sites at the surface of the nanoparticles that, upon stabilization by the attached organic groups, give rise to the observed emissions. It is further noted that the emission band shifts to shorter wavelengths as the excitation is blue-shifted. In a series of studies, the emission quantum yield of C-QDs was found to be around 4\%. These types of materials are promising as fluorescent tags for biological application. Sponsored by the ULF-FORTH (Laserlab- Europe) and by the Greek GSRT. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y30.00008: Scanning Probe Microscopy Characterization of Electrical Properties of Bimetallic Core Shell Nanostructures Regina Ragan, Sangyeob Lee, Aniketa Shinde, Satoru Emori Metallic nanoparticles have shown enhanced catalytic activity compared to their bulk counterparts potentially due to changes in electronic properties at the nanoscale. Challenges in nanoscale catalysis studies include the fabrication of monodisperse nanostructures as well as a fundamental knowledge of the electronic properties at the nanometer length scale. Our group addresses these issues by fabricating dense ordered arrays of bimetallic core-shell nanostructures and characterizing structures with scanning tunneling spectroscopy and Kelvin probe force microscopy. Self assembled rare earth disilicide nanowires are used as templates for Pt and Au nanostructures on Si(001). We will present electronic characterization of these structures with nanometer scale resolution using STS and KPFM. STS measurements of RESi2 nanowires will be presented that show enhanced tunneling as compared to thin films as well as size dependant rectification ratios when comparing islands and wires of various width. KPFM is used to measure the work function of various sizes of RESi2 nanostructures providing a fundamental basis for understanding catalytic behavior in terms chemical activity of the nanostructures. KPFM data reveals a higher CPD for DySi2 nanowires than islands with $\Delta \Phi$ nanoisland-nanowire found to be 230meV. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y30.00009: Controlled fabrication and electrical properties of long quasi-one-dimensional superconducting nanowire arrays Ke Xu, James Heath Quasi-one-dimensional superconducting nanowires are an interesting and ideal system to examine fundamental superconductivity physics and size effects on superconductivity. We report a general method for reliably fabricating quasi-one-dimensional superconducting nanowire arrays, with good control over nanowire cross section and length, and with full compatibility with device processing methods. We investigate Nb nanowires with individual nanowire cross sectional areas that range from bulk-like to 10 $\times $ 11 nm, and with lengths from 1 to 100 micrometers. Nanowire size effects are systematically studied. In particular, a comprehensive investigation of influence of nanowire length on superconductivity is reported for the first time. All results are interpreted within the context of phase-slip models. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y30.00010: Optical Interferometry of Gas Pressure Damped Silicon Nanobridges and Nanocantilevers O. Svitelskiy, N. Liu, V. Sauer, J. Losby, M. Belov, E. Finley, K.M. Cheng, M. Freeman, W. Hiebert The growing interest in NEMS, in particular in nanobridges and nanocantilevers, is determined by the prospective of their usage as hypersensitive sensors of various physical factors: mass, tension, pressure, viscosity, etc. In order to investigate their properties under damping, a series of NEMS with different sizes was prepared from standard SOI wafers by the chemical etching after electron beam lithography. The surfaces were coated by layers of Al, Au and/or Cr in different combinations. The quality of the fabricated NEMS was evaluated by SEM imaging. The resonant frequencies of the NEMS varied in the range of 10-1000 MHz. The damping was introduced by means of pressurized gas in specially built optical pressure chamber capable to hold up to 5 atmospheres with glass window and not less than 160 atmospheres if equipped with sapphire window. We demonstrate that the NEMS Q- factor, the amplitude and the frequency of their resonances show considerable dependence on the value of the pressure in the chamber. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y30.00011: Periodic and Quasiperiodic Nanostructures: Accessing Complex Architectures Through Designer Phase Masks Cheong Yang Koh, Edwin Thomas In this work, we show how one may design phase mask architectures in order to achieve complex 3-dimensional periodic and quasiperiodic nanostructures through considerations of the symmetries of the phase masks. By making use of the fact that phase mask interference lithography is essentially a case of light propagation through the non-modulated direction of a finite photonic crystal slab, we show that the diffracted beams obey the symmetry restrictions of the corresponding phase mask, which allow us to determine and design the polarizations and directions of the exiting beams which interfere coherently in the substrate, subsequently leading to the formation of 3-dimensional nanostructures which are periodic or quasiperiodic. The extension of this approach towards quasiperiodic structures is straightforward when working in Fourier space, which identifies the propagating eigen-modes within the phase mask, or photonic crystal slab. This allows us to rationally design structures with targeted properties, utilizing this Fourier space approach. We show several examples of this approach in achieving this method of fabrication for both periodic and quasiperiodic nanostructures. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y30.00012: Field Emission Enhancement and the Field-Screening Effect Reduction using Carbon Nanopipettes as Cold Cathodes Abdelilah Safir, David Mudd, Mehdi Yazdanpanah, Vladimir Dobrokhotov, Gamini Sumanasekera, Robert Cohn In this work, we report a recent experimental study of high emission current densities exceeding 10mA/cm$^{2}$ and breakdown electric field lower than 5Volts/$\mu $m from novel cold cathodes such as conical shaped carbon nanopipettes (CNP). CNP were grown by CVD on Pt wire and have apex as sharp as 10nm with length between 3-6$\mu $m. The emission experiments were conducted under vacuum in a scanning electron microscope for individual CNP and in a dedicated chamber for bulk samples. CNP's conical bases and low density contribute significantly to the reduction of the screening effect and to the field emission enhancement. The experimental value for the field enhancement factor, $\gamma$, was about 867. Comparing emission results taken from CNP and aligned multiwall carbon nanotubes (MWNT) show that the ratio between $\gamma_{CNP}$ and $\gamma_{MWNT}$ is $\sim $1.6 which contributes to the reduction of screening effect. The emission from multilayers of graphene was also studied. High emission current (20$\mu $A) demonstrates promising emission properties of graphene. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y30.00013: Dicke effect in a multi-ripple electron waveguide. Hoshik Lee, Linda Reichl We compute the electron transmission through a bi-ripple electron waveguide. We numerically observe a resonance splitting, in this {\it open quantum system}, which is analogous to the Dicke effect in quantum optics. We also plot S- matrix poles in the complex energy plane, and find that two symmetry related poles contribute to the resonance splitting. We find that the symmetric resonant states are easily coupled to the leads, but the anti-symmetric states are not. We show the resonance splitting is due to a indirect interaction between wavefunctions in each cavity using a simple model. We also show that one of S-matrix poles withdraws from the real axis as a ripple is added. It turns out that the width of the resonance for $N$-ripple waveguide is $N$ times larger than the resonance width of a mono-ripple waveguide. It agrees with the result of the Dicke model. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y30.00014: What is Quantum in Quantum Pumping: The role of the Phase and Asymmetries Kunal Das, Tomas Opatrny By studying several examples, we show that quantum pumping does not always necessitate a strictly quantum description, neither is phase always a necessary concept. The same quantum mechanical picture of pumping encompass a variety of distinct mechanisms, some can be simulated by classical mechanisms while others can be explained only in a quantum picture; the role of the phase of the wavefunction is the crucial differentiator. We also show that most pumping processes have a previously unconsidered antisymmetric component which contributes significantly to the instantaneous current at each terminal but causes no net charge transfer . We have also computed the exact pumped current for arbitrary rates of time variation for certain potentials, not just in the adiabatic regime as has been previously studied. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y30.00015: Evanescent-wave current through nanometer-scale conductor with generalized channel decomposition in non-equillibrium Green's function theory Hiroshi Shinaoka, Takeo Hoshi, Takeo Fujiwara In optics, evanescent wave is known as a decay mode without dissipation, which appears at total reflecting surfaces and surfaces of nanoparticles. Even though such decay mode can be found, in principle, also in electronic current in nanometer-scale conductors, the evanescent wave effect has not yet been investigated systematically, in materials with electronic structure. We present a novel eigen-channel decomposition method in non-equilibrium Green's function formalism. By applying this method to nanometer-scale d-band metal wires, we found decaying behavior of electron density and backward current flows near electrodes, which is evidence of evanescent waves. We also found that the evanescent waves cause conductor-length dependence of the transmission, which is detectable in experiments. Dependence of the evanescent waves on materials and structures are also discussed from a point of view of band structures and their connectivity at electrodes. [Preview Abstract] |
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