Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L31: Carbon Nanotubes: Superconductivity |
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Sponsoring Units: DCMP Chair: Stefano Curtarolo, Duke University Room: Colorado Convention Center 401 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L31.00001: Carbon Nanotube Superconducting Quantum Interference Device. Vincent Bouchiat, Jean-Pierre Cleuziou, Thierry Ondarcuhu, Marc Monthioux, Wolfgang Wernsdorfer We report on the study of a superconducting quantum interference device (SQUID) with Josephson junctions made of portions of metallic single-walled carbon nanotube [1]. Quantum confinement in each nanotube junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with a lateral electrostatic gate. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting to change the hybridization of the QD states with the superconducting contacts [2]. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of a 6nA supercurrent current with magnetic flux is achieved when both QD junctions are in the ``on'' or ``off'' state. Futhermore, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled $\pi $-junctions. This allow to verify that the sign of the current-phase relation across a proximity coupled Qdot can be reversed with a gate voltage. Noise studies shows that the noise figure of the nanotube SQUID together with the size of the junction should allow the detection of a single molecule magnet. [1] J-P. Cleuziou et al. Nature Nanotec., \textbf{1}, 53, (2006). [2] J-P. Cleuziou et al. cond-mat/0610622. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L31.00002: Nonequilibrium giant loop currents and orbital magnetism in carbon nanotubes Naoto Tsuji, Shigehiro Takajo, Hideo Aoki Recent experiments have shown that carbon nanotubes can have large orbital magnetic moments ($\sim 10\mu_B$). Although isolated carbon nanotubes in equilibrium in external magnetic fields have been theoretically studied, nonequilibrium transport of nanotubes attached to electrodes has yet to be established. Based on Keldysh formalism, we analyze currents flowing in carbon nanotubes attached to electrodes with finite bias voltages. We show that large magnetic moments are generated from giant loop currents circulating around the tube, which makes carbon nanotubes ``molecular solenoids''. While this is an example of the quantum loop current when incident electrons are resonant to degenerate levels of molecules as proposed by Nakanishi and Tsukada [Surf. Sci. {\bf 438}, 305 (1999)], a speciality of the nanotubes is that they have inherent doubly-degenerate states (propagating clockwise and anticlockwise around the tube). We have further identified the full conditions for large loop currents that include the position of the electrodes and the chirality of the tube. The current-voltage characteristic and effects of external magnetic fields are also discussed. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L31.00003: Zero-bias anomaly and possible superconductivity in single-walled carbon nanotubes Jian Zhang, Alexander Tselev, Yanfei Yang, Kyle Hatton, Paola Barbara, Serhii Shafraniuk We report measurements of field-effect transistors made of isolated single-walled carbon nanotubes contacted by superconducting electrodes. For large negative gate voltage, we find a dip in the low-bias differential resistance. Remarkably, this dip persists well above the superconducting transition temperature of the electrodes, indicating that it is {\em not} caused by superconducting proximity effect from the electrodes. This conclusion is supported by measurements on carbon nanotubes contacted by normal electrodes showing similar features. One possible 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. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L31.00004: Interplay between induced superconductivity and Luttinger liquid behavior in carbon nanotubes Gang Liu, Yong Zhang, Chun Ning Lau Superconductors and Luttinger Liquids (LL) are two prototypical strongly correlated electron systems. In a Josephson junction where the normal metal is a LL rather than a Fermi Liquid, the Cooper pairs are expected to decay as a power law the distance between the superconductors. Here we experimentally investigate the interplay between LL and superconductivity by coupling individual single-walled carbon nanotubes to superconducting leads. Low bias conductance peaks induced by proximity effects are observed, and induced superconductivity in nanotubes are examined as a function of inter-electrode spacing. Latest experimental results will be discussed in terms of various theoretical models. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L31.00005: Double-gap proximity effect in nanotubes Smitha Vishveshwara, Karyn Le Hur We study the properties of a single-walled metallic carbon nanotube placed on a superconducting substrate. Given that the nanotube possesses two bands in its excitation spectrum, we find a novel proximity effect which allows the existence of a ``double superconducting gap.'' We show that there is a critical experimentally-accessible interaction strength in the nanotube at which this proximity effect transitions from being suppressed to being enhanced. We also analyze the effect of possible phase fluctuations within the substrate on the induced superconductivity. We discuss the consequences of these features on the single-particle tunneling density-of-states of the nanotube. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L31.00006: Pressure-Induced Metal-Insulator Transition in Single-Walled Carbon Nanotubes Li Lu, J. Z. Cai, W. J. Kong, H. W. Zhu, C. Zhang, B. Q. Wei, D. H. Wu, Feng Liu The resistance of single-walled carbon nanotube (SWNT) bundles was studied under combined extreme conditions of high pressure (up to 10 GPa), low temperature (down to 2 K) and strong magnetic field (up to 12 T). A pressure-induced metal-insulator transition was found to occur at $\sim $ 1.5 GPa, across which the temperature and field-dependent functional forms of the resistance changes dramatically. The transition pressure of $\sim $ 1.5 GPa correlates closely with the structural phase transition of SWNT under pressure. In the insulator phase, the magnetoresistance of the samples shows typical behaviors of two-dimensional electron weak localization, presumably reflecting the coherent hopping processes of the electrons in the collectively flattened plane of the SWNTs bundles. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L31.00007: Thermoelectric transport in the vicinity of a superconductor-metal quantum phase transition in nanowires Adrian Del Maestro, Bernd Rosenow, Subir Sachdev We consider a model of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism, e.g. a magnetic field in the wire direction or disorder in an unconventional superconductor. The critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. In a large-N limit, we calculate the universal dependence of the electrical and thermal conductivity on both pair breaking strength and temperature. We find that the conductivity has a non-monotonic temperature dependence on the metallic side of the transition and that the Wiedemann-Franz law is obeyed at low temperatures. In the quantum critical region, we study the dynamics of a two-component order parameter field via the Langevin equation formalism and compare with the large-N result. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L31.00008: Superconductivity of nanowires in contact with bulk metals H. Liu, Z. Ye, H. Zhang, W. Wu A counter-intuitive anti-proximity effect (APE) was recently reported for Zn nanowires in contact with two superconducting bulk electrodes (PRL \textbf{95}, 076802 (2005)). It was observed that the Zn nanowires were superconducting when the bulk electrodes were normal. When bulk electrodes were superconducting, superconductivity in Zn nanowires appeared to be partially or fully suppressed. However, the resistance of the extrinsic contacts between the Zn nanowires and the bulk electrodes has raised questions about these experiments. To address this issue, we have fabricated Sn, Pb, and Zn single nanowires of various diameters and lengths in contact with a number of different bulk materials using an in-situ contact method develop by our group (APL \textbf{84}, 6996 (2004)) which eliminates any extrinsic contact resistance. Transport properties of the nanowires have been measured using a Physical Property Measurement System (PPMS). We have found that long ($\sim $60$\mu $m) nanowires of Sn and Pb demonstrate superconductivity as expected with either superconducting or normal bulk electrodes. However, short ($<$10$\mu $m) Sn and Pb nanowires demonstrate superconductivity only when the bulk electrodes are superconducting, such as Sn and Pb. Other samples with similar structures are being studied and will be used to clarify these results. We will discuss these results in the context of the proximity effect. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L31.00009: Magnetization drops in arrays of superconductive multi-walled carbon nanotubes J. Haruyama, N. Murata, E. Einarsson, S. Chiashi, S. Maruyama, N. Kishi, T. Sugai, H. Shinohara Superconductivity in CNT, which is an ideal one-dimensional (1D) conductor, is attracting significant attention, because it allows one to study how Cooper pairs can be generated and behave in 1D space within a ballistic charge transport regime. This study also reveals interplay between superconductive phase and phases of 1D quantum phenomena, which tend to prevent superconductivity from its appearance (e.g., Tomonaga-Luttinger liquid states and Pierls transition). We have recently reported finding of superconductivity with the highest Tc of 12 K for abrupt resistance drops in arrays of MWCNTs by entirely end-bonding those by gold electrode [1]. Here, I will report finding of magnetization drops with the highest Tc of 18K, which is greater than the above-mentioned Tc of 12K, in the arrays of MWCNTs. Because only the samples with showing resistance drops can exhibit this magnetization drips, we conclude that this is attributed to Meissner effect. Based on this observation, we clarify that contribution of graphite structure of a MWNT is a dominant mechanism for the present Meissner effect rather than influence of curvature. We also reveal contribution of intertube coupling in an array of MWCNTs. [1] I.Takesue, J.Haruyama. et al., Phys.Rev.Lett. 96, 057001 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L31.00010: Very Unusual Magnetic Properties in Multi-walled Carbon Mats Pieder Beeli, Guo-meng Zhao We report magnetic measurements up to 1100 K on a multi-walled carbon nanotube mat sample using a Quantum Design vibrating sample magnetometer. In an ultra-low field ($H$ = $-$0.02 Oe), we find a very large paramagnetic susceptibility (up to 12.7$\%$ of 1/4$\pi$) at 1100 K and a very large diamagnetic susceptibility (at least 8.4$\%$ of $-$1/4$\pi$) at 482 K. A small magnetic field (2.1 Oe) completely suppresses the diamagnetic susceptibility at 482 K and reduces the paramagnetic susceptibility at 1100 K by a factor of over 20. We rule out explanations based on magnetic contaminants, instrument artifacts, and the orbital diamagnetism. The magnetic data are inconsistent with any known physical phenomena except for granular superconductivity. The present results suggest the existence of an unknown new physical phenomenon or superconductivity with an ultra-high transition temperature. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L31.00011: Lifetime of a one-dimensional fermion Maxim Khodas, Iddo Ussishkin, Michael Pustilnik, Alex Kamenev, Leonid Glazman Interaction between fermions in one dimension is usually accounted for within the exactly solvable Tomonaga-Luttinger model. The crucial simplification made in this model is the linearization of the fermionic spectrum. That simplification leads to an infinite lifetime of a fermion at the mass shell, i.e., the corresponding Green function $G(\varepsilon,\xi_k)$ diverges at $\varepsilon=\xi_k$. We find that inclusion of the curvature of electron spectrum, $\xi_k=v_Fk+k^2/2m$, yields a finite decay rate of a fermion, $1/\tau(\xi_k)\propto \theta(k)k^8/m^3$; here for definiteness we consider right-moving particles, and $k$ is measured from the Fermi wave vector. The found finite lifetime allows one to assess the limitations of the Luttinger liquid paradigm. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L31.00012: Electron Transport and Tunneling in Single Walled Carbon Nanotube Devices Travis Dirks, Nadya Mason Carbon nanotubes remain a fertile ground for the exploration of interacting one-dimensional (1D) physics and Tomonaga-Luttinger liquid theory. Much is still unknown about the factors that influence the transport and tunneling properties of interacting 1D systems such as nanotubes. We report on experiments that use techniques such as multiple contacts on long nanotubes and tunable tunnel barriers to determine how the manifestations of electron-electron interactions, such as the zero-bias anomaly, depend on the length and defect strength in nanotubes. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L31.00013: Luttinger liquid parameters of carbon nanotubes from first-principles calculations Boris Kozinsky, Leonid Levitov, Nicola Marzari Electron interactions in carbon nanotubes are responsible for particle correlations which manifest themselves in a power-law suppression of the density of states observed in tunneling transport. The Tomonaga-Luttinger model, which describes the behavior of 1d metals at low energy, links the power law exponents to the microscopic parameters, interaction strength and Fermi velocity. These exponents have been measured in recent experiments [1]. Motivated by these findings, we employ density functional theory methods to estimate charge compressibility and Fermi velocity, and thereby obtain the Tomonaga-Luttinger model parameters in the charge sector [2]. Our calculations are in quantitative agreement with experimental results and previous RPA calculations [3]. [1] M. Bockrath et al., Nature 397, 598 (1999); Z. Yao et al., Nature 402, 273 (1999); H. Ishii et al. Nature 426, 540 (2003). [2] B. Kozinsky et al. (to be published). [3] R. Egger, A. O. Gogolin, Phys. Rev. Lett. 79, 5082 (1997). [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L31.00014: The Study of Electron-Electron Interactions in Semi-Conducting Carbon Nanotubes Using a Numerical Renormalization Group Robert Konik We present a non-perturbative, numerical renormalization group (NRG) based technique for the study of the spectrum of semi-conducting single-walled carbon nanotubes in the presence of electron-electron interactions. This technique permits a full many-body treatment of the system. As our starting point, we model a single walled semi-conducting carbon nanotube as four gapped Dirac fermions in the presence of interactions. Focusing on a poorly screened carbon nanotube where the interactions are strongest in the forward scattering direction, the nanotube can be equivalently modeled as four Luttinger liquids coupled together with a quadratic gap term. The NRG based technique then is readily able to determine non-perturbatively the effects of the gap term upon the four Luttinger liquids. Using this approach we are able to obtain results for both the excitonic and single particle spectra of the nanotube. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L31.00015: Observation of the Kondo effect in a carbon nanotube with asymmetric Schottky barriers Jerome Licini, Jeffrey Stephens, A. T. Charlie Johnson, Douglas Strachan, Sam Khamis, Danvers Johnston A CVD carbon nanotube sample measured at low temperature and high magnetic field was observed to show a substantial increase in differential conductivity ($\Delta $I/$\Delta $V) near zero voltage and a pronounced asymmetry with bias voltage that appear only below temperatures of 3.0K. The magnetic field peaks show complicated shifts and possible splits. The simplest interpretation that satisfies the data is that of a metallic tube whose Schottky barriers to the external contacts are asymmetric. The observation of a Kondo effect that varies with bias polarity dramatically illustrates the impact of coupling to the external leads. [Preview Abstract] |
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