Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D36: Carbon Nanotubes: Low Temperature Electronic Properties |
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Sponsoring Units: DCMP Chair: Xia Hong, Pennsylvania State University Room: 408 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D36.00001: Non-equilibrium tunneling spectroscopy in carbon nanotubes Nadya Mason, Yung-Fu Chen, Travis Dirks, Gassem Al-Zoubi, Norman Birge We report measurements of the non-equilibrium electron energy distributions in carbon nanotubes. Carbon nanotubes can be considered model one-dimensional systems whose transport is strongly affected by electron interactions. Using tunneling spectroscopy via a superconducting probe, we have studied electron energy distribution functions, and hence inelastic electron scattering, in nanotubes that have bias voltages applied between their ends. We find that at low temperatures, electrons interact weakly in nanotubes of a few microns channel length, independent of end-to-end conductance values. Surprisingly, the energy relaxation rate can increase substantially when the temperature is raised to only 1.5 K. In general, tunneling spectroscopy with a superconducting probe may be a powerful new tool for characterizing electron behavior in carbon nanotubes. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D36.00002: Method for determining the conductance tensor of quantum junctions from the ground state alone Armin Rahmani, Chang-Yu Hou, Claudio Chamon, Ian Affleck Conductance is related to dynamical correlation functions and is considered a non-equilibrium quantity. Here we propose a method to obtain the small-bias low-temperature conductance tensors of quantum junctions through equilibrium calculations such as time-independent DMRG. Using the dependence of a finite system ground state energy on the boundary conditions, we determine the junction conductance by finding the S-matrix. The method is applicable to interacting junctions connected to an arbitrary number of non-interacting leads. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D36.00003: Transition from the Sequential to the Resonant Tunneling in a Dissipative Environment Yuriy Bomze, Henok Mebrahtu, Ivan Borzenets, Alex Makarovski, Gleb Finkelstein We study the shape of the single-electron conductance peaks in a quantum dot coupled to a dissipative environment. In the regime of sequential tunneling through a single quantum level, the peak height increases as the temperature is lowered, although due to the dissipative environment it scales slower than the conventional $\sim $ 1/T. As the temperature is lowered further into the resonant tunneling regime, the peak width approaches saturation, while the peak height starts to decrease. To our knowledge, the non-monotonic peak height dependence on temperature is experimentally observed for the first time. We associate this behavior with coherent tunneling through a single quantum level in the presence of dissipative environment. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D36.00004: Possible electric-field induced one dimensional excitonic insulators in carbon nanotubes pairs Jay Sau, Marvin Cohen Recently there has been significant interest in the possibility of realizing excitonic insulator states in semiconducting systems in electric fields. Using a tight-binding formulation of the GW and Bethe-Salpeter methods parametrized from first-principles density functional theory calculations, we show that an electric field strength of 0.06 eV/\AA~ fails to close the quasiparticle gap of the system but closes the excitonic gap. This can cause a phase transition of the system into an excitonic phase where the ground state is populated with a quasi-one dimensional repulsive gas of excitons. Such a state provides a realization of a one-dimensional excitonic insulator phase with a spin degree of freedom which can lead to novel phases. We discuss some of the properties of the resulting excitonic phase and the transition and also discuss how similar properties may be observed in experiments on nanotube bundles. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D36.00005: A recursion formula for the local density of states in finite Luttinger liquids Sebastian Eggert, Imke Schneider The local density of states (LDOS) in quantum wires is one of the most central quantities for the experimental verification of the preditions from Luttinger Liquid theory. By now it has been well understood how boundaries lead to a crossover of powerlaws in the LDOS as a function of position and energy. It is also possible to calculate the LDOS for individual levels in finite wires analytically and numerically. However, the connection from finite wavefunctions to a semi-infinite powerlaw description remains unclear. We now present a simple recursion formula that ties together both limits and even allows to express the crossover of powerlaws in a closed analytic form in terms of hypergeometric functions. With the help of the formula it is now also possible to calculate the LDOS of long range interacting systems explicitly. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D36.00006: Superconducting Tunneling Spectroscopy of a Carbon Nanotube Quantum Dot Travis Dirks, Yung-Fu Chen , Nadya Mason, Norman Birge We report results on tunneling spectroscopy of a carbon nanotube quantum dot. Using a three-probe technique that includes a superconducting tunnel probe, we map out changes in conductance due to band structure, excited states, and applied bias. We also see features due to the unique nature of the superconducting probe, including enhancement of weak tunneling processes. In addition, we see~conduction inside the superconducting gap when an end to end bias is applied, which suggests some inelastic, possibly assisted, tunneling process inside the quantum dot. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D36.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 3:54PM - 4:06PM |
D36.00008: A DMRG approach to impurities and interactions in carbon nanotubes Alexander Struck, Sebastian Reyes, Sebastian Eggert Carbon nanotubes (CNTs) are well suited to study strong electronic correlations in quasi-one-dimensional systems experimentally and theoretically. Of particular interest is the interplay of interactions between the conducting electrons and impurities in the nanotube. Impurities include the boundaries of short tubes as well as structural imperfections such as the Stone-Wales lattice distortion. Interactions can lead to different phases of the electron liquid, depending on their range and strength, and can produce quasi-localized ground states of e.g. the Mott insulator type or a charge density wave. Here we discuss a systematic approach using the density-matrix renormalization group (DMRG) method to treat a recently derived lattice model for a single-wall armchair CNT with short-range interactions and a Stone-Wales impurity. We show interaction driven modifications to the expected density patterns that can lead to anomalous Friedel oscillations around the impurity. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D36.00009: Zero-bias anomalies in multi-section carbon nanotube FETs Yanfei Yang, Georgy Fedorov, Serhii Shafraniuk, Rupert Lewis, Benjamin Cooper, Christopher Lobb, Paola Barbara Carbon nanotube field effect transistors (CNFETs) with high transparency contacts show maxima of differential conductance at zero bias voltage [1]. These zero-bias anomalies (ZBAs) occur at large negative gate voltages and in narrow gate voltage ranges (about 1 V wide). Our proposed explanation is superconductivity in the nanotubes, occurring when the gate voltage shifts the Fermi energy into van Hove singularities of the electronic density of states. Here we probe this scenario using 3 FETs fabricated from different sections of one semiconducting carbon nanotube. Source and drain electrodes were patterned by e-beam lithography to achieve FET lengths of 500 nm, 1500 nm and 7000 nm, respectively. All devices showed high transparency contacts to their Pd electrodes. We report the observation of pronounced ZBAs in the multi-section CNFETs, their magnetic field (up to 7 T) and temperature evolution, and the modulation on the ZBAs by Fabry-Perot oscillation. [1] J. Zhang et al., Zero-bias anomaly and possible superconductivity in single-walled carbon nanotubes, Phys. Rev. B, 74, 155414 (2006). [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D36.00010: Interacting resonant level side-coupled to a Luttinger liquid: Duality to resonant tunneling Moshe Goldstein, Richard Berkovits We study a model of a single level quantum dot side-coupled to a Luttinger liquid wire by both hopping and interactions. By canonical transformations and a Coulomb gas mapping, we prove a duality between this problem and that of resonant tunneling through a level connecting the edges of two wires with the inverse Luttinger liquid parameter $g$. The two systems thus have complementary transport properties: when one is conducting the other is insulating, and vice-versa. Using this result, as well as an exact solution at $g=2$ and Monte-Carlo simulations on the Coulomb gas, we fully characterize the system's conductance. It exhibits an anti-resonance as a function of the level energy, whose width vanishes (enhancing transport) as a power law at low temperatures and bias voltages for $g>1$, while diverging (suppressing transport) for $g<1$. Level population is shown to be either a linear, a power law, or a discontinuous function of a small level energy, depending on the parameters. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D36.00011: Wigner crystal \emph{vs.} Friedel oscillations in the 1D Hubbard model Stefan Soeffing, Michael Bortz, Sebastian Eggert We investigate the ground state density distribution of the Hubbard model in a finite one-dimensional wire. For weak interactions we find the expected Friedel oscillations, but for low filling a destinct Wigner crystal state can be observed. Although there cannot be a phase transition in a 1D system we observe a well-defined crossover into a Wigner crystal region with different physical behavior even for relatively weak short range interactions. The combination of Luttinger liquid theory and numerical Density Matrix Renormalization Group (DMRG) calculations allows a quantitative analysis of the crossover as a function of system length, lattice filling, and interaction strength. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D36.00012: Mott Insulating State in Ultra-clean Carbon Nanotubes Vikram Deshpande, Bhupesh Chandra, Robert Caldwell, Dmitry Novikov, James Hone, Marc Bockrath The Mott insulating state is a manifestation of strong electron interactions in nominally metallic systems. Using transport spectroscopy, we show that an energy gap exists in nominally metallic carbon nanotubes, and occurs in addition to the band-gap in small-band-gap nanotubes, indicating that carbon nanotubes are never metallic. This gap has a magnitude $\sim $10-100 meV and nanotube radius dependence $\sim $1/r, in good agreement with predictions for a nanotube Mott insulating state. We also observe neutral excitations within the gap, as predicted for this state. Our results underscore nanotubes' exceptional capabilities for studying correlated electron phenomena in 1D.\\ \\ Ref: V. V. Deshpande et al, \textit{Science} (in press) [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D36.00013: Superconductivity in thin films of boron-doped carbon nanotubes J. Haruyama, N. Murata, J. Reppert, A. Rao, T. Koretsune, S. Saito It is well known that the small mass of carbon can promote high transition temperature (T$_{c})$ in BCS-type superconductivity (SC). Recently, new carbon-based superconductors with order of T$_{c}$ of $\sim $10K [1, 2] were discovered and higher T$_{c}$ has been expected. In particular, the SC in a carbon nanotube (CNT) is attracting considerable attention [3]. We reported that entirely end-bonded multi-walled CNTs could show SC with T$_{c}$ = 12K, previously [4]. In contrast, it had problem in reproducibility, because correlation with carrier doping was not clarified. Moreover, none has succeeded substitutional carrier doping into CNTs and also revealed the correlation with SC. Here, we report on the Meissner effect found in thin films consisting of assembled boron-doped single-walled CNTs [5]. We reveal that only highly homogeneous CNT films consisting of low boron concentration leads to evident Meissner effect with T$_{c}$ = 12 K. The first-principles electronic-structure study of the $B$-SWNT strongly supports these results. [1] T. E. Weller et al., \textit{Nature Physics} 1, 39 (2005), [2] E. A. Ekimov et al., \textit{Nature} 428, 542 (2004), [3] M. Kociak et al., \textit{Phys. Rev. Lett.} 86, 2416 (2001), [4] I. Takesue, J.Haruyama, et al., \textit{Phys. Rev. Lett.} 96, 057001(2006), [5] N.Murata, J.Haruyama, et al., \textit{Phys.Rev.Lett.} 101, 027002 (2008) [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D36.00014: Fabrication and transport properties of size tunable single-walled carbon nanotube quantum dots Paul Stokes, Yodchay Jompol, Saiful I. Khondaker Single electron transistors (SETs) have attracted considerable attention because of their potential as a building block for quantum based nanoelectronic devices. However fabrication of reproducible and controllable quantum dot sizes that can operate at high temperature is challenging. We developed a novel technique for the fabrication of size tunable and controllable quantum dot using single-walled carbon nanotube (SWNT) [1]. Our technique is based on the formation of two tunnel barriers of controllable separation by naturally bending SWNT at the edges of a raised local gate. A SWNT is placed on a local Al/Al$_{2}$O$_{3}$ bottom gate of width $L$, and then contacted with Pd source and drain electrodes of 1 $\mu $m separation on Si/SiO$_{2}$ substrates. The Al gate serves three purposes: (i) it acts as a ``mechanical template'' to define two tunnel barriers at the edges by naturally bending the nanotube due to van der Walls interactions with the substrate, (ii) the width of the gate defines the size ($L)$ of the quantum dot, and (iii) it acts as a local bottom gate to control the operation of the SET device. Using this approach we fabricated SETs of different sizes down to 50 nm. We present detailed fabrication procedures and low temperature transport studies of these SET devices. [1] P. Stokes and S. I. Khondaker, APL \textbf{92}, 262107 (2008). [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D36.00015: Conductance of a Conjugated Molecule with Carbon Nanotube Contacts Nicolas Bruque, Khalid Ashraf, Thomas Helander, Roger Lake Quantitative predictions of the energy levels is a well-known weakness of density functional theory (DFT). To understand the HOMO level alignment of a $\pi $-cruciform molecule [1] with the Fermi level of a carbon nanotube (CNT) contact, we have performed quantum chemical calculations of the adiabatic ionization potential (IP) of the central molecule. The adiabatic IP of the molecule is -5.86 eV. The image charge potential, calculated using our fully self-consistent DFT - Recursive Green Function (RGF) approach, is 0.7 eV. Treating the image potential as a self-energy correction to the IP, the HOMO energy level is at -5.16 eV which is comparable to the intrinsic CNT Fermi level at -5 eV. The above considerations of the energy level alignments, combined with the DFT-RGF analysis of the molecular orbitals and transmission spectrum, indicate that the HOMO resonance lies within the 50 meV energy window created by the experimental source-drain bias. This appears to be the most likely scenario that would give rise to the relatively small resistance of 6 M$\Omega $. \newline 1. X. Guo, et. al. Science, \textbf{311}, 356 (2006). [Preview Abstract] |
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