Quantum interference in carbon-nanotube electron resonators

A new mechanism is proposed to explain the slow conductance fluctuations in the conductance-gate voltage plot observed in the nanotube electron resonators. It is found that the slow conductance fluctuation is an intrinsic quantum interference phenomenon and exists in all metallic nanotube resonators except zigzag ones. Analytical expressions for both slow and rapid oscillation periods of the conductance fluctuations have been derived, which are well consistent with the existing experiments. It is predicted that the ratio of the slow oscillation period to the rapid one is independent of the gate-voltage efficiency, and determined only by the nanotube length used in experiments.     

管长和管径对单壁碳纳米管电导的影响

基于紧束缚模型，发展转移矩阵方法研究了单壁碳纳米管的导电性质.研究表明，由于卷曲效应，锯齿型(3k,0)管(k为整数)出现窄的电导沟，其大小与能隙一致.在费米能附近，电子输运不仅与管径和管长紧密相关，而且电子在不同能量下可能出现弹道的、扩散的和经典的三种不同输运特征. 关键词： 碳纳米管 转移矩阵 电导     

Physical mechanism of negative differential conductance in substrateless metallic carbon nanotubes

The existence of pronounced negative differential conductance at room temperature in suspended metallic carbon nanotubes was recently proven. We investigate here the physical nature of this phenomenon, which is of considerable importance for high-frequency devices, such as oscillators working up to few hundreds of GHz. Besides previous explanations, we find a new physical mechanism that explains the negative differential conductivity at room temperature. The entire suspended metallic carbon nanotube behaves as a very large quantum well, the negative differential conductance occurring due to the depletion of carriers on high-energy resonant levels.     

Differential conductance of armchair single-wall carbon nanotubes due to presence of electron–phonon interaction

We have theoretically investigated the first correction to conductance of armchair single wall carbon nanotubes (SWCNTs) with finite length, embedded between two electrodes, due to the presence of electron–transversal phonon interaction. The perturbative scheme has been used with finite length real space nearest neighbors tight binding method. Both radial breathing and tangential modes are investigated separately. It is found that not only the conductance correction crucially depends on source-drain voltage but also it strongly depends on the length and diameter of SWCNT. So, this work opens up opportunities to control the electrical conductance of SWCNT and increases yield of micro or nanodevices based on carbon nanotube.     

Electrodynamic dip in the local density of states of a metallic wire

We have measured the differential conductance of a tunnel junction between a thin metallic wire and a thick ground plane, as a function of the applied voltage. We find that near zero voltage, the differential conductance exhibits a dip, which scales as 1/square root of [V] down to voltages V approximately 10k(B)T/e. The precise voltage and temperature dependence of the differential conductance is accounted for by the effect on the tunneling density of states of the macroscopic electrodynamics contribution to electron-electron interaction, and not by the short-ranged screened-Coulomb repulsion at microscopic scales.     

Temperature dependence of conductance character in nanotube peapods

Recent electrical transport measurements on metallofullerene-doped nanotube peapods are reviewed. In temperature-dependent conductance measurements, it was found that the temperature plays a crucial role in charge transfer between the nanotube and entrapped metallofullerenes and it is shown that the metallofullerenes can function as electron donors and transfer charge to the carbon nanotube host. The amount of charge transferred varies with temperature. At room temperature, the doped nanotube shows p-type conduction. As the temperature decreases, the conductance becomes n-type and even metallic behavior is observed at still lower temperatures, indicating the degenerate state caused by doping. Received: 4 November 2002 / Accepted: 7 November 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. E-mail: chiu@fkf.mpg.de     

Conductance of metallic single-wall nanotubes with single magnetic impurities

Using a model of conducting cylinder with a few number of impurities on its surface, we investigate the effects of magnetic impurity scattering on the conductance of metallic single-wall carbon nanotubes. The nonlinear part of conductance, which is due to the interaction of conduction electrons with impurities, is obtained. The signature of Kondo anomaly is found in the nonlinear conductance and it is shown that its amplitude strongly depends on the position of impurities and diameter of nanotube.     

Simple and accurate model for voltage-dependent resistance of metallic carbon nanotube interconnects: An ab initio study

In this work, development of a voltage dependent resistance model for metallic carbon nanotubes is aimed. Firstly, the resistance of metallic carbon nanotube interconnects are obtained from ab initio simulations and then the voltage dependence of the resistance is modeled through regression. Self-consistent non-equilibrium Green's function formalism combined with density functional theory is used for calculating the voltage dependent resistance of metallic carbon nanotubes. It is shown that voltage dependent resistances of carbon nanotubes can be accurately modeled as a polynomial function which enables rapid integration of carbon nanotube interconnect models into electronic design automation tools.     

Effect of optical phonons scattering on electronic current in metallic carbon nanotubes

The effect of optical phonons scattering on electronic current has been studied in metallic carbon nanotubes. The current has been calculated self-consistently by total voltage equation and the heat transport equation. The total voltage equation consists of three terms, optical phonons collision term, acoustic phonon scattering term, and contact resistance one. Including LO, A₁, and E₁(2) phonons in collision term, we can reproduce the experimental I-V curves displaying negative differential conductance. Furthermore, one conclusion is made that the more optical phonons are scattered by electron, the lower current is in metallic carbon nanotubes. By comparing the current under different conditions, we can make another conclusion that there should be nonequilibrium optical phonons under high bias in spite of whether the metallic nanotube is suspended or not. This result agrees well with the others [M. Lazzeri, F. Mauri, Phys. Rev. B 73 (2006) 165419]. Based on these results, we do not only explain the experiment, but also propose to design a heat-controlling electronic transistor with metallic carbon nanotubes as its channel, in which the electronic current can be controlled by optical phonons.     

Boron carbon nanotube superlattices: Geometry, electronic structure and quantum conductance

The stable boron carbon nanotube superlattices (BCNTSLs) that are constructed by periodically connecting carbon nanotube (CNT) and boron nanotube (BNT) with different lengths and diameters are predicted by employing the density functional first-principles calculations. The geometrical and electronic structures as well as quantum conductance of BCNTSLs are studied. It is found that the superlattices can be metallic or semiconducting depending on tube diameters and the ratio of BNT to CNT segments in a periodic unit. The confined states in the superlattice are observed. The present study could offer a useful way for designing some functional nanodevices.     

Carbon nanotube based magnetic tunnel junctions

Spin-coherent quantum transport in carbon nanotube magnetic tunnel junctions is investigated theoretically. A spin-valve effect is found for metallic, armchair tubes, with a magnetoconductance ratio ranging up to 20%. Because of the finite length of the nanotube junctions, transport is dominated by resonant transmission. The magnetic tunnel junctions are found to have distinctly different transport behavior depending on whether or not the length of the tubes is commensurate with a 3N+1 rule, with N the number of basic carbon repeat units along the nanotube length.     

Electromagnetic scattering of the carbon nanotubes excited by an electric line source

An analytical solution is presented for the electromagnetic scattering from an infinite-length metallic carbon nanotube and a carbon nanotube bundle. The scattering field and scattering cross section are predicted using a modal technique based on a Bessel and Hankel function for the electric line source and a quantum conductance function for the carbon nanotube. For the particular case of an isolated armchair (10, 10) carbon nanotube, the scattered field predicted from this technique is in excellent agreement with the measured result. Furthermore, the analysis indicates that the scattering pattern of an isolated carbon nanotube differs from that of the carbon nanotube bundle of identical index (m, n) metallic carbon nanotubes.     

A scheme for a topological insulator field effect transistor

We propose a scheme for a topological insulator field effect transistor. The idea is based on the gate voltage control of the Dirac fermions in a ferromagnetic topological insulator channel with perpendicular magnetization connecting to two metallic topological insulator leads. Our theoretical analysis shows that the proposed device displays a switching effect with high on/off current ratio and a negative differential conductance with a good peak to valley ratio.     

Quantum interference in nanotube electron waveguides

We have calculated the quantum conductance of single-walled carbon nanotube (SWNT) waveguide by using a tight binding-based Greens function approach. Our calculations show that the slow conductance oscillations as well as the fast conductance oscillations are manifestations of the intrinsic quantum interference properties of the conducting SWNTs, being independent of the defect and disorder of the SWNTs. And zigzag type tubes do not show the slow oscillations. The SWNT electron waveguide is also found to have distinctly different transport behavior depending on whether or not the length of the tube is commensurate with a (3N+1) rule, with N the number of basic carbon repeat units along the nanotube length.     

Combined effect of Aharonov-Bohm effect and uniaxial strain on quantum conductance oscillations of carbon nanotube resonators

Using the π orbital tight-binding model and the multi-channel Laudauer-Büttiker formula, the combined effect of Aharonov-Bohm effect (induced by an axial magnetic field) and uniaxial strain on quantum conductance oscillations of the electronic Fabry-Perot resonators composed of armchair and metallic zigzag single-walled carbon nanotubes (SWNTs) has been studied. It is found that, for the case of the armchair SWNT, conductance oscillations near the band gap are dominated by Aharonov-Bohm effect, while the conductance oscillations in other regions are dominated by the uniaxial strains. The combined effect of Aharonov-Bohm effect and uniaxial strains on quantum conductance oscillations is not obvious. But, for the case of the metallic zigzag SWNTs, obvious single-channel transport and one or two conductance oscillations existing in two different gate voltage ranges were found by the combined effect of uniaxial strain and axial magnetic field.     

Influence of boron distribution on the transport of single-walled carbon nanotube

Using density functional theory combined with nonequilibrium Green’s functions, we investigated and found that boron substitutional doping affects the transport properties of single-walled carbon nanotubes with different distribution. The results reveal that the semiconducting nanotube transits to the quasi-metallic state with nonlinear current–voltage curve after boron doping. Some regular regions of total transmission coefficient with integral values appear with the varying of electron energy and bias voltage. The transport properties of the doped tubes are affected remarkably by the impurity states of quasi-bound defect states which are tuned by the distance between the boron atoms. The current of metallic nanotube is reduced by the impurities and changed with doping patterns. PACS 72.80.Rj; 73.22.2f; 73.61.Wp; 73.63.Rt     

Out of equilibrium Anderson model: Conductance and Kondo temperature

We calculate the conductance through a quantum dot weakly coupled to metallic contacts by means of the Keldysh out of equilibrium formalism. We model the quantum dot with the SU(2) Anderson model and consider the limit of infinite Coulomb repulsion. The interacting system is solved with the numerical diagrammatic Non-Crossing Approximation (NCA) and the conductance is obtained as a function of temperature and gate voltage from differential conductance (dI/dV) curves. We discuss the results in comparison with those from the linear response approach which can be performed directly in equilibrium conditions. Comparison shows that out of equilibrium results are in good agreement with the ones from linear response supporting reliability of the method employed. The last discussion becomes relevant when dealing with general transport models through interacting regions. We also analyze the evolution of conductance vs gate voltage with temperature. While at high temperatures the conductance is peaked, when the Fermi energy coincides with the localized level it presents a plateau at low temperatures as a consequence of the Kondo effect. We discuss different ways to determine Kondo's temperature.     

Hybridization Induced Competitive Scanning Tunneling Interference Process into a Heavy Fermion System

We theoretically present the results for a scanning interference tunneling process between a metallic tip and a heavy fermion system. The density of states(DOS) and the differential conductance at zero temperature under different c-f band hybridizations, as well as the interference Fano ratio strength in the heavy fermion system,are calculated. It is found that the hybridization strength gives rise to the splitting effect in the DOS around the Fermi energy. Also the interference Fano ratio strength makes the differential conductance characteristics strongly asymmetric.     

Photon-Assisted Electron Transport for a λ-shaped Carbon Nanotube Junction

We theoretically study the electron transport properties for two coupled single-walled caxbon nanotube quantum dots connected to metallic electrodes under the irradiation of an external electromagnetic field at low tempera- tures. Using the standaxd nonequilibrium Green＇s function techniques, we examine the time-averaged transmission coefficient and linear conductance. It is shown that by some numerical examples, the photon-assisted inter-dot coupling causes Fano resonance and the conductance of the system is sensitive to the external field parameters. The transport dependence on the external field parameters may be used to detect the high-frequency microwave irradiation.     

Observation of peak splitting in the Coulomb blockade regime of a carbon nanotube rope

Electronic transport measurements have been carried out on a single-walled carbon nanotube (SWCN) rope contacted to a 4-probe Au/Pd electrode in the Coulomb blockade regime. With varying substrate backgate voltage, the observed Coulomb blockade peaks exhibit interesting three-way splitting. We find that this peak splitting can be attributed to a contribution from resonant tunnelling through discrete energy levels of a finite length metallic SWCN within the rope. We also consider the role that interactions between `quantum dot' (Q-dot) regions within the rope can play in causing the peak splitting.