Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V43: Electronic Properties in Nanostructures |
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Sponsoring Units: DCMP Chair: Ilya Ponomarev, Naval Research Laboratory Room: Colorado Convention Center 506 |
Thursday, March 8, 2007 11:15AM - 11:27AM |
V43.00001: $g$-factors and discrete energy level velocities in nanoparticles Eduardo R. Mucciolo, Caio H. Lewenkopf, Leonid I. Glazman We establish relations between the statistics of $g$ factors and the fluctuations of energy in metallic nanoparticles where spin-orbit coupling is present. These relations assume that the electron dynamics in the grain is chaotic. The expressions we provide connect the second moment of the $g$ factor to the root-mean square ``level velocity'' (the derivative of the energy with respect to magnetic field) calculated at magnetic fields larger than a characteristic correlation field. Our predictions relate readily observable quantities and allow for a parameter-free comparison with experiments. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V43.00002: Full counting statistics for a quantum nanoelectromechanical system Steven Bennett, Aashish Clerk Experiments on nanoelectromechanical systems often involve the effects of a mechanical oscillator on the current noise of a mesoscopic conductor. Coupling to the oscillator induces correlations between tunneling electrons in the conductor, leading to signatures in the shot noise. To better characterize such correlations it is useful to consider full counting statistics (FCS), which describe the complete probability distribution of tunneled charge. We study theoretically the FCS in a tunnel junction coupled to a nanomechanical oscillator. This system has been realized in experiment using an atomic point contact where one electrode is free to vibrate \footnote{N. E. Flowers-Jacobs, D. R. Schmidt, and K. W. Lehnert ({submitted}).}, and it has been predicted that the oscillator dynamics leads to large signatures in the shot noise that cannot be explained classically \footnote{A. A. Clerk and S. M. Girvin, {\it Phys. Rev. B} {\bf 70}, 121303(R) (2004).}. Thus motivated, we investigate the FCS using a reduced density matrix tracking the oscillator and the number of tunneled electrons, for which we obtain an equation of Caldeira-Leggett form with additional terms due to tunneling. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V43.00003: {\em Ab Initio} Calculations for the Surface Energy of Silver Nanoclusters Bharat Medasani, Igor Vasiliev, Young Ho Park We apply first principles computational methods to study the surface energy and the surface stress of silver nanoparticles. The structures, energies and lattice contractions of spherical Ag nanoclusters are calculated in the framework of density functional theory combined with the generalized gradient approximation. Our calculations predict the surface energies of Ag nanoclusters to be in the range of 1$-$2 J/m$^2$. These values are close to the bulk surface energy of silver, but are significantly lower than the recently reported value of 7.2 J/m$^2$ derived from the Kelvin equation for free Ag nanoparticles\footnote{ K. K. Nanda {\it et al.}, Phys. Rev. Lett. {\bf 91}, 106102 (2003)}. From the lattice contraction and the nearest neighbor interatomic distance, we estimate the surface stress of the silver nanoclusters to be in the the range of 1$-$1.45 N/m. This result suggests that a liquid droplet model can be employed to evaluate the surface energy and the surface stress of Ag nanoparticles. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V43.00004: The structures and energetics of interacting ionic nanocrystals from atomistic simulations. Paul Tangney, Steven G. Louie Self-assembled ordered aggregates of nanocrystals (NCs) of many different sizes, shapes and compositions have been synthesized in recent years. These ``supercrystals'' form a new class of material with potentially new and useful properties in which nanoparticles take the place of atoms as the fundamental building blocks of matter. However, at present neither the detailed structures of NCs themselves nor the interactions between them are well understood and it is not yet clear which forces are responsible for binding and ordering them in supercrystals. In this work, NCs of highly ionic materials are simulated using first principles molecular dynamics (MD) and MD based on accurate polarisable force-fields. Individual NCs and multiple NCs in close proximity are simulated and their structures and the electrostatic contributions to their energetics are studied in detail. From our understanding of NCs in this ionic limit we provide insight into the importance of electrostatic contributions to NC bonding in more covalent materials. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V43.00005: Fermi-edge singularity in a spin-incoherent Luttinger liquid Gregory A. Fiete We theoretically investigate the Fermi edge singularity in a spin incoherent Luttinger liquid. Both cases of finite and infinite core hole mass are explored, as well as the effect of a static external magnetic field of arbitrary strength. For a finite mass core hole the absorption edge behaves as $(\omega-\omega_{\rm th})^\alpha/ \sqrt{|\ln(\omega-\omega_{\rm th})|}$ for frequencies $\omega$ just above the threshold frequency $\omega_{\rm th}$. The exponent $\alpha$ depends on the interaction parameter $g$ of the interacting one dimensional system, the electron-hole coupling, and is independent of the magnetic field strength, the momentum, and the mass of the excited core hole (in contrast to the spin coherent case). In the infinite mass limit, the spin incoherent problem can be mapped onto an equivalent problem in a spinless Luttinger liquid for which the logarithmic factor is absent, and backscattering from the core hole leads to a universal contribution to the exponent $\alpha$. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V43.00006: Free Energy of a 1D Metal-Molecule Interface: C$_{60}$-Decorated Ag Islands T. J. Stasevich, C. Tao, E. D. Williams, T. L. Einstein We study the structural and dynamical properties of one- dimensional metal-molecule interfaces by investigating Ag monolayer islands on Ag(111) decorated by C$_{60}$. At 300K bare Ag(111) islands have hexagonal equilibrium shapes. When C$_{60}$ is deposited on the surface, it preferentially nucleates along island step edges, near the island corners, making them round.\footnote{C. Tao et al., Phys. Rev. B \textbf{73}, 125436 (2006).} We tune coverage so that a single chain of C$_{60}$ fully decorates the island, forming a closed ring, circular in shape. From a simple model for the C$_{60}$ step decoration, we derive the decorated step free energy as a function of step angle, yielding the equilibrium shape of the decorated islands via the Wulff construction. By comparing the model to experiment, we estimate the Ag-C$_{60}$ attraction. Using fast STM scanning, we also study the fluctuations of the C$_{60}$ decorated islands. By fitting the time correlation function of the fluctuation component Fourier modes, we show the decorated step dynamics are consistent with attachment-detachment (AD) kinetics, in contrast to the step-edge diffusion of the bare island. Finally, from our analysis, we extract the decorated step free energy and estimate the C$_{60}$-C$_{60}$ attraction. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V43.00007: Permanent polarization of small metallic particles Andrey Shytov, Michael Pustilnik Electric charge density in a metal fluctuates on the spatial scale of the Fermi wavelength due to various types of disorder. These fluctuations are usually compensated in the bulk due to Coulomb interaction between electrons. However, a small metallic particle may have a non-vanishing static electric dipole moment, owing to uncompensated density fluctations near the surface on the scale set by the screening length. We analyze these fluctuations statistically and find that the typical value of the dipole moment increases linearly with the particle size, and fluctuates strongly from particle to particle. Our results are applicable to small metallic clusters and nanocrystals. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V43.00008: Tuning the Conductivity of Semiconductor Nanostructures by Dielectric Engineering Aniruddha Konar, Debdeep Jena Electron transport properties in semiconductor nanoscale quantum structures grown by bottom-up techniques can be fundamentally different from those grown by epitaxial methods. Transport properties in 1D nanotubes and nanowires and 2D nanoscale thin films are strongly affected by carrier-impurity interactions mediated by the dielectric environment of the nanostructure. We show that by suitable dielectric engineering of this environment, electron mobility in 1D and 2D semiconductor nanostructures can be enhanced by 1-2 orders of magnitude. The enhancement takes place only when the smallest length scale of the nanostructure is less than the effective Bohr radius of the bulk semiconductor. The enhancement in mobility occurs predominantly due to a large damping of Coulombic scattering, which results in a reduction of electron scattering rates. When the dielectric constant of the environment is changed from 1 (air) to 100 (high-k oxide dielectric), the Coulomb scattering rate decreases from 700/ps to 4/ps for a 1D nanowire, and from 66/ps to 3/ps for a 2D sheet. When other scattering mechanisms such as surface roughness and phonon scattering are considered, we find that the total conductivity of the nanostructures can be enhanced by 1-2 orders of magnitude by coating them with high-k dielectrics. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V43.00009: Dielectrophoretic alignment of VO$_{2}$ nanowires in device geometries Irving Herman, Sarbajit Banerjee, Vladimir Blagojevic, Kellen Petersen, Manav Malhotra, Michael Steigerwald, Louis Brus Bulk VO$_{2}$ is characterized by a Mott metal---insulator phase transition at $\sim $68 $^{o}$C and has been widely studied for optical and electrical switching applications. However, nanostructured vanadium oxides have been challenging to fabricate and thus not much is known about their properties. Here, we present the AC dielectrophoretic alignment of hydrothermally grown VO$_{2}$ nanoribbons, $\sim $40 nm in width and several micrometers in length, in device geometries. The alignment process has been studied as a function of the applied voltage and frequency, gap distance, and concentration of the VO$_{2}$ dispersion. VO$_{2}$ nanowires have also been precisely positioned in different device geometries, such as across deep trench structures. The electrodes have been designed based on electric-field simulations. The nanowire devices show gate dependence at room temperature. The temperature dependence of the transport properties has also been examined. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0213574 and the New York State Office of Science, Technology and Academic Research (NYSTAR), and partially by the NSEC Program of the National Science Foundation under Award Number CHE-0641523. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V43.00010: Universal behavior of surface-dangling bonds in hydrogen-terminated Si, Ge, and Si/Ge nanowires. Ricardo Nunes, Ricardo Kagimura, H\'elio Chacham We report an {\it ab initio} study of the electronic properties of surface dangling bond (SDB) states in hydrogen-terminated Si, Ge, and Si/Ge nanowires with diameters between 1 and 2 nm. We find that the charge transition levels $\varepsilon(+/-)$ of SDB states are deep in the bandgap for Si wires, and shallow (near the valence band edge) for Ge wires. In both Si and Ge wires, the SDB states are localized. We also find that the SDB $\varepsilon(+/-)$ levels behave as a ``universal" energy reference level among Si, Ge, and Si/Ge wires within a precision of 0.1 eV. By computing the average bewteen the electron affinity and ionization energy in the atomi limit of several atoms from the III, IV and V columns, we conjecture that the universality is a periodic-table atomic property. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V43.00011: Superlattice nanowires via double-sided heteroepitaxy on compliant ultra-thin Si ribbons Clark S. Ritz, Frank S. Flack, Michelle M. Roberts, Douglas M. Detert, Yu Zhang, Donald E. Savage, Paul G. Evans, Feng Liu, Max G. Lagally We fabricate and characterize a novel type of Si/SiGe superlattice nanowire. Such structures, traditionally created by VLS growth, have been of great interest for thermoelectric applications for some time. We have developed a technique for creating a superlattice-like system using strained SiGe epitaxial islands. We pattern free-standing Si nanowire ribbons made from ultrathin silicon-on-insulator (SOI) substrates and use them as a substrate for the Stranski-Krastanov growth of coherent 3D islands. Interaction between islands growing on the top and bottom surfaces causes them to order laterally. The periodic strain induced in the substrate by the ordered islands affects the electronic band structure in a periodic way. The discussion will cover the fabrication and electrical properties of these strain superlattice structures. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V43.00012: Ground-state properties of quantum rings with a few electrons Yasuhiro Saiga, Dai Hirashima, Junko Usukura Quantum dots occupy an important position not only in the field of basic science, but also in the field of nanotechnology. Among various shapes of dots, a ring structure is a particularly interesting nanostructure, because the diameter and the ring width can be separately changed. In this talk, we discuss the ground-state properties of one-dimensional quantum rings with a few electrons, which interact with each other in the form of 1/r-Coulomb repulsion. By using exact diagonalization, we find that for three electrons, the fully spin-polarized ground state is uniquely realized when the diameter of the ring is sufficiently large. In contrast, for four and five electrons, the fully polarized state never becomes the unique ground state, however large the diameter is. These results can be understood in terms of multiple-spin exchanges. We also show that a magnetic field applied perpendicularly to the ring induces not only the persistent current but also the spin chirality. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V43.00013: Quantum Impurities and Persistent Currents: Decoupling through Integrability Hans-Peter Eckle, Johan Nilsson, Henrik Johannesson We consider the problem of a persistent current in a one-dimensional mesoscopic ring with the electrons coupled by a spin exchange to a magnetic impurity. We show that this problem can be mapped onto an integrable model with a quadratic dispersion (with the latter property allowing for an unambiguous definition of the persistent current). We have solved the model exactly by a Bethe ansatz and found that the current is insensitive to the presence of the impurity. From the structure of the Bethe ansatz equations we conjecture that this result holds for any integrable quantum impurity model with an electronic dispersion $\epsilon(k)$ that is an even function of $k$. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V43.00014: Energy Spectra and Oscillatory Magnetization of Two-Electron Self-Assembled InGaAs/GaAs Ring-Like Nanostructures V.M. Fomin, V.N. Gladilin, J.T. Devreese, N.A.J.M. Kleemans, H.C.M. van Genuchten, P.M. Koenraad We have analyzed the effect of the Coulomb interaction on the energy spectrum and the magnetization of two electrons in a strained In$_x$Ga$_{1-x}$As/GaAs ring-like nanostructure with realistic parameters inferred from our X-STM data. With increasing magnetic field, the lowest spin-singlet and spin- triplet states sequentially replace each other as the ground state. This is reminiscent of the Aharonov-Bohm effect for the ring-like structures. The exchange interaction leads to a more complicated oscillatory structure of the magnetic moment of the two electrons as a function of the magnetic field as compared to the magnetization pattern for a single-electron ring-like nanostructure. We discuss the relevance of the two-electron systems for the interpretation of the Aharonov-Bohm oscillations in the persistent current observed in low temperature magnetization measurements on self-assembled In$_x$Ga$_{1-x}$As/GaAs ring-like nanostructures. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V43.00015: Electric-field lithography of LaAlO$_{3}$/SrTiO$_{3}$ quasi-two-dimensional electron gas Cheng Cen, Jeremy Levy, Stefan Thiel, Jochen Mannhart Recent reports$^{2,3}$ have indicated that the existence of polar discontinuities at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ is unstable to the formation of a quasi-two-dimensional electron gas. Below a critical thickness electrons can still accumulate at the interface under the influence of an applied electric field$^{3}$. We use a biased conducting atomic force microscope (AFM) probe to create conducting nanowires at the LaAlO$_{3}$/SrTiO$_{3}$ interface without physical alteration of the interface. The conducting regions written by AFM probe can be written and erased repeatedly. This form of quasi-two-dimensional lithography demonstrates the utility of the LaAlO$_{3}$/SrTiO$_{3}$ interface as a rewritable medium, with the potential for creating passive as well as active circuits such as field-effect transistors. ( $^{2}$ A. Ohtomo and H. Y. Hwang, Nature \textbf{427}, 423 (2004). $^{3}$ S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart, Science \textbf{313}, 1942 (2006).) [Preview Abstract] |
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