Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L24: Focus Session: Electronic Properties of Nanotubes |
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Sponsoring Units: DMP Chair: Sergei Tretiak, Los Alamos National Laboratory Room: 326 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L24.00001: Strongly Correlated Electron Phenomena in Carbon Nanotubes Invited Speaker: In this talk I will discuss our recent results demonstrating strongly correlated electron behavior in ultra-clean carbon nanotube quantum dots. Specifically, we have observed one-dimensional (1D) Wigner crystal behavior of dilute holes in semiconducting nanotubes, finding three distinct regimes of spin and valley quantum number ordering as the charge density and axial magnetic field are varied. The boundaries between the regimes in density and magnetic field are well-described by the theory of Levitov and Tsvelik for a narrow-gap Luttinger liquid. In the second part of the talk I will present results showing that the electrons in nominally metallic nanotubes comprise a 1D Mott insulator. This indicates that carbon nanotubes are never truly metallic, in agreement with theoretical predictions that account for Umklapp scattering at half-filling due to electron-electron interactions. Using inelastic cotunneling spectroscopy, we also observe neutral electronic excitations within the gap, yielding an additional signature of the Mott insulating state. Our results demonstrate nanotubes' promise for studying a variety of tunable correlated electron phenomena in 1D. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L24.00002: Relaxation and dephasing in a two-electron $^{13}$C nanotube double quantum dot Hugh Churchill, Ferdinand Kuemmeth, Jennifer Harlow, Andrew Bestwick, Emmanuel Rashba, Karsten Flensberg, Carolyn Stwertka, Thiti Taychatanapat, Susan Watson, Charles Marcus We use charge sensing of Pauli blockade (including spin and isospin) in a two-electron $^{13}$C nanotube double quantum dot to measure relaxation and dephasing times. The relaxation time, $T_1$, first decreases with parallel magnetic field then goes through a minimum in a field of 1.4 T. We attribute both results to the spin-orbit-modified electronic spectrum of carbon nanotubes, which suppresses hyperfine mediated relaxation and enhances relaxation due to soft phonons. The inhomogeneous dephasing time, $T_2^*$, is consistent with previous data on hyperfine coupling strength in $^{13}$C nanotubes. This work was supported by the National Science Foundation under grant no.~NIRT 0210736 and the GRFP, ARO/iARPA, the Department of Defense, and Harvard's Center for Nanoscale Systems. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L24.00003: Electrical evidence for the encapsulation of C$_{60}$ inside a carbon nanotube: Random telegraph signal and hysteric current-voltage characteristics Yung Woo Park, Han Young Yu, Dong Su Lee, Ursula Dettlaff-Weglikowska, Siegmar Roth We present electrical evidence for the encapsulation of C$_{60}$'s inside a carbon nanotube: random telegraph signals (RTSs), and hysteric current-voltage characteristics. The RTSs is ascribed to the instability of the quantum harmonic oscillations of C$_{60}$'s. RTSs are smeared out at a temperature which is consistent with the energy level of the vibrational quantum mediated by the van der Waals binding between the carbon nanotube and C$_{60}$'s. In addition, hysteric behavior in cyclic current-voltage characteristics is explained by the longitudinal motion and resettlement of the C$_{60}$'s with the modulation of the size of the quantum dot mediated by the C$_{60}$ insertion. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L24.00004: Breakdown of the Wigner-Mattis theorem in semiconductor carbon-nanotube quantum dots Massimo Rontani, Andrea Secchi, Franca Manghi The Wigner-Mattis theorem states the ground state of two bound electrons, in the absence of the magnetic field, is always a spin-singlet. We predict the opposite result --a triplet- for two electrons in a quantum dot defined in a semiconductor carbon nanotube. The claim is supported by extensive many-body calculations based on the accurate configuration interaction code DONRODRIGO (www.s3.infm.t/donrodrigo). The crux of the matter is the peculiar two-valley structure of low-energy states, which encodes a pseudo-spin degree of freedom. The spin polarization of the ground state corresponds to a pseudo-spin singlet, which is selected by the inter-valley short-range Coulomb interaction. Single-electron excitation spectra and STM wave function images may validate this scenario, as shown by our numerical simulations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L24.00005: Casimir Interactions Between Scatterers on Carbon Nanotubes Dina Zhabinskaya, Jesse Kinder, E. J. Mele We study the interactions between two short-range scatterers in metallic carbon nanotubes as a Casimir problem. In the massless Dirac Hamiltonian for the electrons, a defect can be represented by a scattering potential with a pseudospin polarization. Sublattice-asymmetric and bond-centered potentials may lead to small momentum backscattering, depending on the chiral angle of the nanotube. Quasibound states formed between two defects determine the forces at the boundaries. We develop a force operator approach within the Dirac model to calculate the forces on two square well potentials of finite width, and take the limit of sharp and strong scatterers to study the Casimir forces mediated by the fermions. For the special case of two identical scatterers we recover the conventional one-dimensional attractive Casimir force. For the general problem with inequivalent scatterers we find that the magnitude and sign of this force depends on the relative pseudospin polarizations of the two defect potentials. We will also discuss the effects intervalley scattering on the Casimir interactions between defects. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L24.00006: Water confined in carbon nanotubes: Magnetic response and proton chemical shieldings Patrick Huang, Eric Schwegler, Giulia Galli Carbon nanotubes (CNT) provide a well-defined environment for the study of confined water, whose behavior can differ markedly from bulk water. The application of nuclear magnetic resonance (NMR) to probe the local water structure and dynamics in these cases is hindered by ambiguities in the interpretation of the NMR spectra. We employ linear response theory to evaluate the $^1$H chemical shieldings of liquid water in semiconducting CNTs, where the electronic structure is derived from density functional theory with periodic boundary conditions. The shieldings are sampled from trajectories generated via first-principles molecular dynamics simulations at ambient conditions, for water in CNTs with diameters $d=11$~\AA\ and $14.9$~\AA\@. We find a large ($\sim -23$~ppm) {\em upfield} shift relative to bulk liquid water, which is a consequence of strongly anisotropic magnetic fields induced in the CNT by the applied magnetic field. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L24.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L24.00008: Effect of spin-orbit interactions on the static polarizability of single-wall carbon nanotubes* G. S. Diniz, S. E. Ulloa The electronic structure of carbon nanotubes (CNTs) is known to exhibit different metallic or insulating behavior as different chiral vectors are considered. Application of external electric fields and the presence of spin-orbit interaction (SOI) result in the Rashba effect modifying the level structure of CNTs, strongly coupling spin and orbital degrees of freedom, as demonstrated in recent experiments [1]. In this work we calculate the static long-wavelength limit of the dielectric response of different single-wall CNTs when subjected to uniform external electric fields both along and across the longitudinal direction (parallel to the nanotube axis). Our calculation uses a $\pi$-band tight-binding formalism, considers Rashba and intrinsic SOI, and utilizes the random phase approximation to evaluate $\epsilon(q\rightarrow 0, \omega=0)$ [2]. We use parameters from the literature and find that the metallic-to-semiconductor transition induced by the intrinsic SOI is suppressed as the Rashba field is taken into account. We further show that this behavior has a clear signature on measurable quantities, such as the static polarizability. We discuss its dependence on nanotube size and chirality and propose possible nanoprobe experiments to study this phenomenon. [1] F. Kuemmeth \emph{et al.}, Nature 452, 448-452 (2008). [2] M. L. Cohen \emph{et al.}, Phys. Rev. B 52, 8541 (1995). *Supported by Fulbright, CAPES and NSF-DMR MWN. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L24.00009: Low-Temperature Studies of Electrostatic Doping and Phonon Renormalization in Individual Single-Walled Carbon Nanotubes Gugang Chen, Tereza Paronyan, Gamini Sumanasekera, Elena Pigos, Avetik Harutyunyan Electrostatic doping of carbon nanotubes (CNTs) induced by applied external gate voltage (V$_{g})$ in the field-effect transistor (FET) configuration allows controllable variation of charge density and the Fermi level in nanotubes. Raman scattering from a CNT is very sensitive to doping and its interaction with the surrounding environment. Recently, the peculiarities of G-mode as a function of gate induced carrier at room temperature have been reported in [1] [Nature Nanotechnology 2, 725 (2007)]. In the present work, we used Raman scattering to study the p- and n- electrostatic doping of CNT as a function of V$_{g}$ in a wide range of temperature from 6.8 K to 300 K. Virtually symmetric blue-shifting of the G-mode for both p- and n- doping of CNTs has been observed. We found that the prehistory of the CNT measurement as well as environment interaction such as gas adsorption on CNT plays an important role on the observed phenomena. Explanation of our findings will be discussed based on phonon energy renormalization of carbon nanotubes [1] due to the carrier density variation during electrostatic doping. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L24.00010: Field Enhancement inside Carbon Nanotubes Hong Zhang, Yoshiyuki Miyamoto There are many investigations on photo-excitations with polarization vector parallel to tube axis (parallel-polarization), compared to it, few studys on those with polarization vector perpendicular to tube axis (cross-polarization) are reported because of early theoretical consideration on electric-field-depolarization effect of nanotubes with cross-polarization [1]. Using dynamical consideration and TDDFT analysis, we herein present the influence of external electric field perpendicular to the axis of semiconductor carbon nanotube (CNT). By adjusting frequency of applied E-field in corresponding wavelength of light from 800nm to 591 nm, the total E-field inside carbon nanotubes has been found to show great change depending on the frequency; incomplete screening and strong enhancement even without including the excitonic effect [2]. The enhancement comes from increase of oscillating amplitude of electron cloud with resonant frequency given by the applied E-field. Also the numerical stability and the satisfaction of energy conservation rule with application of dynamical E-field were numerically checked [3]. This finding should be taken into account in interpreting a measurement of optical response of molecules being encapsulated in CNTs. [1]. H. Ajiki and T. Ando, Physica B \textbf{201}, 349 (1994) [2]. S. Uryu and T. Ando, Phys. Rev. B \textbf{76}, 115420 (2007) [3]. Y. Miyamoto and H. Zhang, Phys. Rev. B \textbf{77}, 165123 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L24.00011: Quasi-particle Energy Gap of Metal-coated Carbon Nanotubes Yu Zhou, Li Chen, Yiming Zhang, Swastik Kar, Pulickel Ajayan, Saroj Nayak We have studied the electronic structures of metal-coated carbon nanotubes using density functional theory and many body corrections based on GW approximation (GWA). In particular, we will present energy band gap variation as a function of number of metal atoms on the nanotube surface. Our results are compared with recent experiments. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L24.00012: Microwave Rectification by Carbon Nanotube Schottky Diodes Enrique Cobas, Steven Anlage, Michael Fuhrer We report the fabrication and electrical characterization of carbon nanotube Schottky diodes (CNT-SDs) via photolithography on high-frequency-compatible substrates using dissimilar contacts of chromium and platinum. The diodes are well-described by the ideal diode equation (n = 1.0). DC and low-frequency behavior is compared to a model of a diode in series with a resistor. The diodes rectify microwave signals beyond 18GHz and produce dc currents of hundreds of nanoamperes. The frequency and voltage dependence is used to estimate the junction capacitance of 1aF and an intrinsic device cut-off frequency of 400GHz. \\[3pt] [1] Cobas, E. and Fuhrer, M., Applied Physics Letters 93, 043120 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L24.00013: Microwave Irradiation Induced Effects to Single-walled Carbon Nanotube Thin Films Lu Wang, Yao Xiong, Ziran Wu, Liwei Chen, Hao Xin Carbon nanotubes have been considered as potential building blocks for nano-scale circuits in virtue of their unique mechanical and electrical properties. However, one of the biggest obstacles for massive production of nanotube circuits is the difficulty of separating semiconducting tubes from metallic tubes or vice versa. In this work, a convenient method which may be potentially employed to selectively remove metallic tubes using microwave induced breakdown is proposed and investigated. Carbon nanotube thin films deposited on glass and quartz substrates are placed in a commercial microwave oven and heated for up to several minutes. The radial breathing mode in Raman spectra on the nanotube samples before and after the microwave irradiation suggests that the metallic-to-semiconducting ratios are reduced by around 20{\%}. Meanwhile, because in the thin film samples most of the nanotubes are entangled, smaller diameter nanotubes (both metallic and semiconducting) tend to be affected more. THz transmission measurements of these thin films are also performed before and after microwave irradiation. The significant increase of transmission after the microwave irradiation process confirms the loss of metallic tubes. [Preview Abstract] |
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