Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J30: Nanowires & Nanotubes: Electronic Properties |
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Sponsoring Units: DCMP Chair: Mark Reed, Yale University Room: C147/154 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J30.00001: Interplay between structural and electronic properties of bundled Mo$_6$S$_{9-x}$I$_x$ nanowires: an {\em ab initio} study Seoung-Hun Kang, Young-Kyun Kwon, David Tomanek We use first principles density functional theory to investigate the structural, electronic and magnetic properties of isolated and bundled Mo$_6$S$_{9-x}$I$_x$ nanowires with $x=3,4.5,$ and $6$. The skeleton of these nanowires consists of linear arrays of Mo$_6$ octahedra decorated with S and I atoms that are connected by flexible S$_3$ linkages. Due to the bi-stability of each sulfur linkage, free-standing and bundled nanowires are capable of stretching or compressing axially at almost no energy cost, giving rise to many structural minima. We explore the structural stability, elastic behavior and electronic structure at all these minima for different compositions. We find that axial strain and inter-wire interaction in bundles modify significantly the electronic structure. Most intriguing changes occur in nanowires with $x=4.5$ and $6$, which change from metal to semiconductor or undergo a magnetic transition upon axially stretching or compressing the nanowires or upon changing the inter-wire separation. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J30.00002: A Hybrid Density Functional Study of Capped Silicon Carbide Nanotubes Kapil Adhikari, Asok Ray A systematic study of fullerene hemisphere capped finite SiC nanotubes of type 1 using cluster approximation is presented. Nanotubes (3,3) and (5,0) are capped by C$_{20}$-fullerene hemisphere(C$_{10})$ and (5,5) and (9,0) are capped by C$_{60}$-fullerene hemisphere (C$_{30})$. Geometries of the tubes have been spin optimized using the functional B3LYP, 3-21G* basis set and the GAUSSIAN 03 software. The study indicates that fullerene capping of a SiC nanotube changes the electronic and geometric structure properties of SiC nanotubes. For example, the binding energy per atom for infinite nanotube (5,5) is 4.993eV whereas the same nanotube with C- and Si-caps has the binding energy per atom of 5.989eV and 4.812eV, respectively. C-capped nanotubes are energetically more preferable compared to Si-capped. The HOMO-LUMO gaps of the capped nanotubes are significantly lower compared to those of infinite nanotubes. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J30.00003: Low-energy spectral weights of the 1D Hubbard chain Stefan Soeffing, Imke Schneider, Alexander Struck, Sebastian Eggert We investigate the low-energy spectral weights of the 1D Hubbard chain by means of Density Matrix Renormalization Group (DMRG) calculations in comparison with Bosonization results. We identify the bosonic excitations of the underlying Luttinger liquid and analyze their evolution upon increasing the interaction strength in terms of their density of states (DOS). Comparing analytical and numerical results we point out the competition of spin/charge degrees of freedom vs. non-interacting spin up and down particles, which here become important due to the lattice nature of the model and higher order operators. Furthermore, we discuss the spatially resolved (local) DOS that can be calculated analytically by a recursive formula vs. numerically using DMRG. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J30.00004: Implications of time-reversal symmetry for band structure of single-wall carbon nanotubes Serguei Goupalov When electron states in carbon nanotubes are characterized by two-dimensional wave vectors with the components $K_1$ and $K_2$ along the nanotube circumference and cylindrical axis, respectively, then two such vectors symmetric about a ${\bf M}$-point in the reciprocal space of graphene are shown to be related by the time-reversal operation. To each nanotube there correspond five relevant ${\bf M}$-points with the following co\"ordinates: $K_1^{(1)}={\cal N}/2R$, $K_2^{(1)}=0$; $K_1^{(2)}={\cal M}/2R$, $K_2^{(2)}=-\pi/T$; $K_1^{(3)}=(2 \, {\cal N} -{\cal M})/2R$, $K_2^{(3)}=\pi/T$; $K_1^{(4)}=({\cal M} +{\cal N})/2R$, $K_2^{(4)}=-\pi/T$, and $K_1^{(5)}=({\cal N} -{\cal M})/2R$, $K_2^{(5)}=\pi/T$, where ${\cal N}$ and ${\cal M}$ are the integers relating the chiral, ${\bf C}_h$, symmetry, ${\bf R}$, and translational, ${\bf T}$, vectors of the nanotube by ${\cal N} \, {\bf R}={\bf C}_h + {\cal M} \, {\bf T}$, $T=|{\bf T}|$, and $R$ is the nanotube radius. We show that the states at the edges of the one-dimensional Brillouin zone which are symmetric about the ${\bf M}$-points with $K_2=\pm \pi/T$ are degenerate due to the time-reversal symmetry. Explicit expressions are obtained for the co\"ordinates of the ${\bf K}$-points in the reciprocal space of graphene relevant to a given nanotube. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J30.00005: Ab initio simulations of the eletronic and transport properties of nanotube bundles used as gas sensors Alexandre Rocha, Rodrigo Amorim, Adalberto Fazzio, Ant\^onio J.R. da Silva Carbon nanotubes (CNT) have exceptional mechanical and - particularly - electronic properties that make this material of great potential interest for applications in different areas of materials science. One of the possibilities which raises the highest hopes is the area of nanotube-based gas sensors. From the fabrication point of view, one is probably going to use bundles of CNTs instead of a single tube. In this work we initially use density functional theory (DFT) calculations to determine the electronic structure properties of different molecules interstitially positioned between the nanotubes in a bundle. From the most stable structures we couple the DFT calculations to a recursive Green's function method to simulate. The electronic transport properties of a disordered nanotube bundle containing a large number of molecules randomly distributed along the different tubes forming the ropes. This way one is able to simulate a realistic sensor based on three-dimensional nanotube bundles taking into consideration the effects of disorder. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J30.00006: Theoretical studies of the electronic and transportation properties of Gd disilicide nanowires on Si(001) Wenjie Ouyang, Yanning Zhang, Shengyong Qin, Anping Li, Ruqian Wu The scanning tunneling microscopy data demonstrate the successful growth of isolated GdSi$_{2}$ nanowires and wire bundles on Si (100) surface and the nano transport measurement shows the isolated nanowires exhibit a metal-insulator transition (MIT) upon cooling while the wire bundles maintain a metallic state. We investigate the structural and electronic properties of isolated GdSi$_{2}$ nanowires and wire bundles surface through extensive density functional calculations. A 8aSi-wide supercell was used to mimic the environment of a single nanowire, and a 5aSi-wide supercell was used for wire bundles. Interestingly, we found that the bundle structures frustrate the Perils-type structural transition that occurs easily in single nanowires. This can be regarded as the reason for the observed MIT. We also explored the effect of Si adatoms on top of wires and wire bundles. The electrical transport behaviors of GdSi$_{2}$ nanowires are further explained using the calculated local electronic density of states and band structures. The special magnetic ordering and its effect on other properties of nanowires will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J30.00007: Structural and Electronic properties of a bismuth nanowire encapsulated inside a boron nitride nanotube Chi-Hsuan Lee, Chih-Kai Yang The structural and electronic properties of a bismuth nanowire (BiNW) encapulated inside the boron nitride nanotube (BNNT) are investigated by first principles calculation. The results show that they depend both on the configuration of BiNW and the diameter of the BNNT. The interaction between the two constituents induces hybridization of energy bands from each subsystem, causing unexpected variation of dispersion and splitting of energy bands near the Fermi level. The role of spin-orbit interaction is especially decisive in the later outcome. It enhances the stability of the hybrid structure and produces more band-edge states. These results should be observable with the tool of scanning tunneling spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J30.00008: Jastrow-Correlated Wavefunctions for Flat-Band Lattices Hao Wang, V.W. Scarola The electronic band structure of many compounds, e.g., carbon-based nanostructures, can exhibit essentially no dispersion. Models of electrons in such flat-band lattices define non-perturbative strongly correlated problems by default. We construct a set of Jastrow-correlated ansatz wavefunctions to capture the low energy physics of interacting particles in flat bands. We test the ansatz in an example honeycomb ribbon. The model Hamiltonian is projected on a flat band of the ribbon, thus containing only the Coulomb interaction term. The properties of the ground states are studied using numerical diagonalization. We find that the ansatz wavefunction accurately captures the ground state in a transition from a crystal to a uniform quantum liquid. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J30.00009: Electronic structures of potassium-doped C$_{60}$ encapsulated in BN nanotubes Takashi Koretsune, Susumu Saito, Jesse Noffsinger, Marvin L. Cohen Boron-nitride nanotubes have large band gap independent of chirality and are promising candidates for nanostructure control. Here, we investigate the electronic structure of potassium-doped C$_{60}$ encapsulated in boron-nitride nanotubes using first-principles methods based on the density functional theory. We demonstrate that the material is one-dimensional metal where conducting electrons are only in the C$_{60}$ chain. Interestingly, the material can have a large Fermi-level density of states, which indicates the possibility of various phase transitions including superconductivity as in the case of fcc K$_3$C$_{60}$. We therefore discuss the electron-phonon couplings as well as the pressure dependence of the electronic structures of this material. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J30.00010: Effect of atomic defects and interwire coupling on the electronic properties of one-dimensional Gd silicide nanowires Shengyong Qin, Tae-Hwan Kim, Arthur P. Baddorf, An-Ping Li, Hanno H. Weitering, Chih-Kang Shih, Wenjie Ouyang, Yanning Zhang, Ruqian Wu Metallic nanowires have attracted great interest for understanding the electronic interactions and conductivity in one dimension. Electron transport is often dictated by quantum instabilities and strong localization at low temperature. Well-ordered and uniformly oriented GdSi2 nanowires are self-assembled on Si(100) in the form of either isolated nanowires or wire bundles with atomic interwire spacing. The effects of interwire coupling and atomic defects in these quasi-one-dimensional systems are studied by correlating the 4-probe STM electrical transport with STM local density of states of individual nanowires. While the isolated nanowires exhibit a metal-insulator transition associated with atomic defects, the wire bundles remain metallic at low temperature which we believe the interwire coupling suppress the lattice disorder and stabilize a robust metallic conductance. This research at ORNL's CNMS was sponsored by the Scientific User Facilities Division, Office of BES, U.S. DOE. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J30.00011: Negative curvature energy in magnesium-boride nanotubes Hui Tang, Sohrab Ismail-Beigi Mg-boride nano-materials have attracted much attention due to constant quest for novel superconducting materials on nanoscale. A recent experiment on Mg borides nanostructures has hinted at a possible superconducting temperature as high as 80K. More generally, studying the physics of pure and metal-doped boron nanosystems enhances understanding of novel properties that emerge in reduced dimensions. Here, based on first principles calculations, we describe an unusual nanoscale curvature effect in Mg-boride nanotubes and discuss its origin. We show that a number of 2D Mg-boride sheets prefer to spontaneously curve themselves into small diameter nanotubes and thus have negative curvature energies. This is rather unique when compared to other nanotubular materials: usually, curving the parent 2D sheet to create a nanotube imposes an energy cost. We explain the reason for the negative curvature energy by analyzing the charge state of the Mg atoms, its relation to the type of boron sublattice present in the nanostructure, and its consequences for the Mg-Mg interactions and hence the energetics. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J30.00012: Quantum Size Effect and Electronic Stability of Freestanding Metal Atom Wires Haiping Lan, Ping Cui, Jun-Hyung Cho, Qian Niu, Jinlong Yang, Zhenyu Zhang Using DFT calculations, we present a thorough study of the quantum size effects on the stability of freestanding metal atom wires. Our systems include Na, Ag, Au, In, Ga and Pb atom wires, i.e. s, sd, and sp electron prototypes. We found that the total energy always oscillates with the wire length, which clearly indicates the existence of preferred lengths. Increasing the length, the s-system exhibits even-odd oscillations following a 1/x decay law in the stability, which can be attributed to electron band filling and quantum confinement along the wire. The sd-system exhibits a similar oscillation pattern, even in the presence of sd hybridization. In sp-system, the energy oscillations are beyond the simple even-odd nature, likely due to unpaired p orbitals and the corresponding nontrival band filling. Our findings clearly demonstrate that electronic contribution is quite critical to the stability of freestanding wires, and this stability may be important even when wires are deposited on substrates or strained. This study sheds light on the underlying formation mechanism of metal atom wires. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J30.00013: Gate controlled donor activation in silicon nanowires Adam Gali, Binghai Yan, Thomas Frauenheim Due to the proximity to an embedding medium with low dielectric constant (e.g., oxides), semiconductor nanowires have higher impurity ionization energy than their bulk counterparts, resulting lower free carrier density. Using ab initio calculations within density functional theory, we propose a way to reduce the ionization energy in nanowires by fabricating a special cross section with appropriate engineering of doping and an applied gate voltage. We demonstrate on a phosphorus-doped silicon nanowire that the ionization energy can be effectively tuned and the impurity backscattering can also be reduced. For instance, the free carrier density may increase by 40{\%} in a silicon nanowire with 15 nm diameter and special cross section without special engineering of doping. Our proposal has profound implications to fabricate nanowire devices with high carrier density. Our proposed Si NW device realizes a fine manipulation of the interaction between electron and nuclear spins by using an external electric field which is a fundamental step to a silicon-based nuclear spin quantum computer. Moreover, with a negative voltage the ionization energy of P-donors can be increased even in larger silicon nanowires which opens up the possibility to manipulate the donor electron spin at room temperature [1]. [1] B. Yan, Th. Frauenheim and A. Gali, \textit{Nano Lett.}, 2010, 10, 3791 [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J30.00014: Non-Collinear Ferromagnetic Luttinger Liquids Nicholas Sedlmayr, Sebastian Eggert, Jesko Sirker In the now classic Tomonaga-Luttinger model the presence of the electron-electron interaction in one dimension is shown to profoundly change the properties of the system. We consider here the magnetic and electronic properties of a \emph{ferromagnetic} Luttinger liquid when it has a region of non-collinearity present, i.e. a domain wall. Spin-charge separation does not survive in this system, and the absence of both spin-charge separation and coherent spin-charge excitations has consequences for the spin-transfer-torque effects which cause domain wall motion. Furthermore the presence of the domain wall introduces a spin dependent scatterer into the problem, which will alter both the transport, and the static electronic, properties of the system. Finally we show how the magnetization dynamics of the domain wall will be modified for a Luttinger liquid. [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J30.00015: How to Extract Luttinger Liquid Velocity from Carbon Nanotubes Darryl H. Ngai, Chang-Yu Hou, Eun-Ah Kim We propose direct detection of Luttinger Liquid velocity of the charge collective mode in carbon nanotubes using optical conductivity and Coulomb blockade effect. We note that detection of such fractionalized excitation needs to exploit the energy or frequency scale tied to the finite length of the nanotube. This is why previous experimental attempts have been unsuccessful.\footnote[1]{Z. Zhong {\it{et al.}}, Nature Nanotechnology {\bf{3}}, 201 (2008)} We will discuss features in the optical conductivity sensitive to the velocity of the collective mode which would be observable in the high temperature limit. In the low temperature limit, spacing between the Coulomb blockade peaks in the conductance as a function of gate voltage will be a sensitive probe. [Preview Abstract] |
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