We present measurements of tunneling magneto-resistance (TMR) in single-wall carbon nanotubes attached to ferromagnetic contacts in the Coulomb blockade regime. Strong variations of the TMR with gate voltage over a range of four conductance resonances, including a peculiar double-dip signature, are observed. The data is compared to calculations in the "dressed second order" (DSO) framework. In this non-perturbative theory, conductance peak positions and linewidths are affected by charge fluctuations incorporating the properties of the carbon nanotube quantum dot and the ferromagnetic leads. The theory is able to qualitatively reproduce the experimental data.
We observed the coulomb blockade phenomena in ferromagnetic contacting single wall semiconducting carbon nanotube devices. No obvious Coulomb peaks shift was observed with existing only the Zeeman splitting at 4K. Combining with other effects, the ferromagnetic leads prevent the orbital spin states splitting with magnetic field up to 2 Tesla at 4K. With increasing magnetic field further, both positive or negative coulomb peaks shift slopes are observed associating with clockwise and anticlockwise orbital state splitting. The strongly suppressed/enhanced of the conductance has been observed ass…
The intertube conductance $G$ of the parallel side contacted armchair nanotubes is calculated by Landauer's formula as a function of the Fermi level $E$. When the intertube difference in the dope strength is large enough, a resonant peak dominant over the others appears in the $E$-$G$ curve. The overlap length and the interlayer configuration do not influence the resonant energy. The intervalley transmission at the resonant peak works as a reverser and a filter of the valley current.
We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the de…
High-performance solution-processed short-channel carbon nanotube (CNT) thin film transistors (TFTs) are fabricated using densely aligned arrays of metallic CNTs (m-CNTs) as source and drain electrodes, and aligned arrays of semiconducting enriched CNTs (s-CNTs) as channel material. The electrical transport measurements at room temperature show that the m-CNT contacted s-CNT array devices with a 2 um channel length perform superiorly to those of control Pd contacted s-CNT devices. The m-CNT contacted devices exhibit a maximum (average) on-conductance of 36.5 uS (19.2 uS), transconductance of 2…
Recent conductance measurements on multi-wall carbon nanotubes (CNTs) reveal an effective behavior similar to disordered single-wall CNTs. This is due to the fact that electric current flows essentially through the outermost shell and is strongly influenced by inhomogeneous electrostatic potential coming from the inner tubes. Here, we present theoretical studies of spin-dependent transport through disorder-free double-wall CNTs as well as single-wall CNTs with Anderson-type disorder. The CNTs are end-contacted to ferromagnetic electrodes modelled as fcc (111) surfaces. Our results shed additio…
Hysteretic switching in the magnetoresistance of short-channel, ferromagnetically contacted individual single wall carbon nanotubes is observed, providing strong evidence for nanotube spin transport. By varying the voltage on a capacitively coupled gate, the magnetoresistance can be reproducibly modified between +10% and -15%. The results are explained in terms of wave vector matching of the spin polarized electron states at the ferromagnetic / nanotube interfaces.
We have contacted single-walled carbon nanotubes after aligning the tubes by the use of surface acoustic waves. The acoustoelectric current has been measured at 4.2 K and a probing of the low-dimensional electronic states by the surface acoustic wave has been detected. By decreasing the acoustic wavelength resulting in an adjustment to the length of the defined carbon nanotube constriction a quantization of the acoustoelectric current has been observed.
We address the electronic resonant transport in presence of a transverse magnetic field through the single level of a suspended carbon nanotube acting as a quantum oscillator. We predict a negative magneto-conductance with a magnetic-field induced narrowing of the resonance line and a reduction of the conductance peak when the nanotube is asymmetrically contacted to the leads. At finite bias voltage we study the threshold for phonon-assisted transport.
Centimeter scale aligned carbon nanotube arrays are grown from nanoparticle metal catalyst pads. We find the nanotubes grow both with and against the wind. A metal underlayer provides in-situ electrical contact to these long nanotubes with no post growth processing needed. Using the electrically contacted nanotubes, we study electrical transport of 0.4 cm long nanotubes. The source drain I-V curves are quantitatively described by a classical, diffusive model. Our measurements show that the outstanding transport properties of nanotubes can be extended to the cm scale and open the door to large …
We have reproducibly contacted gated single wall carbon nanotubes (SWCNT) to superconducting leads based on niobium. The devices are identified to belong to two transparency regimes: The Coulomb blockade and the Kondo regime. Clear signature of the superconducting leads is observed in both regimes and in the Kondo regime a narrow zero bias peak interpreted as a proximity induced supercurrent persist in Coulomb blockade diamonds with Kondo resonances.
Low-temperature measurements of asymmetric carbon nanotube (CNT) quantum dots are reported. The CNTs are end-contacted with one ferromagnetic and one normal-metal electrode. The measurements show a spin-dependent rectification of the current caused by the asymmetry of the device. This rectification occurs for gate voltages for which the normal-metal lead is resonant with a level of the quantum dot. At the gate voltages at which the current is at the maximum current, a significant decrease in the current shot noise is observed.
We present low-temperature electron transport measurements on a single-wall carbon nanotube quantum dot exhibiting Kondo resonances at low temperature. Contrary to the usual behavior for the spin-1/2 Kondo effect we find that the temperature dependence of the zero bias conductance is nonmonotonic. In nonlinear transport measurements low-energy splittings of the Kondo resonances are observed at zero magnetic field. We suggest that these anomalies reflect interactions between the nanotube and a magnetic (catalyst) particle. The nanotube device may effectively act as a ferromagnetically contacted…
We present a data-calibrated compact model of carbon nanotube (CNT) field-effect transistors (CNFETs) including contact resistance, direct source-to-drain and band-to-band tunneling currents. The model captures the effects of dimensional scaling and performance degradations due to parasitic effects and is used to study the trade-offs between the drive current and leakage current of CNFETs according to the selection of CNT diameter, CNT density, contact length, and gate length for a target contacted gate pitch. We describe a co-optimization study of CNFET device parameters near the limits of sc…
The electrical transport properties of anodically grown TiO2 nanotubes was investigated. Amorphous nanotubes were anodically grown on titanium foil and transformed through annealing into the anatase phase. Amorphous and polycrystalline single nanotubes were isolated and contacted for measurements of the electrical resistance. Non-linear current-voltage characteristics were explained using the fluctuation induced tunneling conduction model. A clear enhancement of the conductance was induced in an insulating anatase nanotube through low-energy Ar/H ion irradiation. Confocal Raman spectroscopy sh…
Carbon nanotube field-effect transistors (FETs) with passivated coaxial gate structures have been fabricated after growth of contacted suspended single wall nanotubes (SWNTs) and subsequent coating with gate dielectrics. Electron Fabry-Perot interferences are observed in the FETs indicating ballistic transport in the coated structures. Reliable production of high-performance SWNT FETs has been obtained by combining patterned growth of SWNT arrays with feed-back controlled electrical breakdown to select desired semiconducting SWNTs, making large-scale SWNT FET fabrication achievable. P-type and…
Based on a tight-binding model and a recursive Green's function technique, spin-depentent ballistic transport through tinny graphene sheets (flakes) is studied. The main interest is focussed on: electrical conductivity, giant magnetoresistance (GMR) and shot noise. It is shown that when graphene flakes are sandwiched between two ferromagnetic electrodes, the resulting GMR coefficient may be quite significant. This statement holds true both for zigzag and armchair chiralities, as well as for different aspect (width/length) ratios. Remarkably, in absolute values the GMR of the armchair-edge grap…
Effects of impurity on the spin-dependent transport in a single wall carbon nanotube spin-valve, as ferromagnetic electrode/carbon nanotube/ferromagnetic electrode model junction is numerically investigated. Using a generalized Green's function method and the Landauer-Buttiker formalism, the impurity conditions are determined by randomly substitution of carbon atoms in the honeycomb carbon nanotube lattice by nitrogen and boron atoms. We have found that transport characteristics, including the spin-dependent current and tunnel magnetoresistance are strongly influenced by the impurity effects. …
We use numerical simulations to investigate the properties of metal contacts to three-dimensional arrays of carbon nanotubes (CNTs). For undoped arrays top-contacted with high or low work function metals, electrostatic screening is very strong, resulting in a small Schottky barrier for current injection in the top layer and large Schottky barriers for current injection in the deeper layers. As a consequence, the majority of the current flows through the top layer of the array. Our simulations show that doping of the CNT array can alleviate this problem, even without direct contact to each tube…
We study theoretically nonlocal spin transport in a ballistic carbon nanotube contacted to two ferromagnetic leads and two normal-metal leads. When the magnetizations of the two ferromagnets are changed from a parallel to an antiparallel configuration, the circuit shows a hysteretic behavior which is specific to the few-channel regime. In the coherent limit, the amplitude of the magnetic signals is strongly enhanced due to resonance effects occurring inside the nanotube. Our calculations pave the way for experiments on low-dimensional nonlocal spin transport, which should give results remarkab…
We report the experimental observation of spin-induced magnetoresistance in single-wall carbon nanotubes contacted with high-transparency ferromagnetic electrodes. In the linear regime the spin-induced magnetoresistance oscillates with gate voltage in quantitative agreement with calculations based on a Landauer-Buttiker model for independent electrons. Consistent with this interpretation, we find evidence for bias-induced oscillation in the spin-induced magnetoresistance signal on the scale of the level spacing in the nanotube. At higher bias, the spin-induced magnetoresistance disappears beca…
We study the electrical transport properties of well-contacted ballistic single-walled carbon nanotubes in a three-terminal configuration at low temperatures. We observe signatures of strong electron-electron interactions: the conductance exhibits bias-voltage-dependent amplitudes of quantum interference oscillation, and both the current noise and Fano factor manifest bias-voltage-dependent power-law scalings. We analyze our data within the Tomonaga-Luttinger liquid model using the non-equilibrium Keldysh formalism and find qualitative and quantitative agreement between experiment and theory.
Carrier injection into carbon nanotubes and graphene nanoribbons, contacted by a metal coating over an arbitrary length, is studied by various means: Minimal models allow for exact analytic solutions which can be transferred to the original system with high precision. Microscopic ab initio calculations of the electronic structure at the carbon-metal interface allow us to extract -- for Ti and Pd as contacting materials -- realistic parameters, which are then used in large scale tight-binding models for transport calculations. The results are shown to be robust against nonepitaxially grown elec…
We present an investigation of different thin-film evaporated ferromagnetic materials for their suitability as electrodes in individual single-wall and multi-wall carbon nanotube-based spin devices. Various electrode shapes made from permalloy (Ni_{81}Fe_{19}), the diluted ferromagnet PdFe, and PdFe/Fe bilayers are studied for both their micromagnetic properties and their contact formation to carbon nanotubes. Suitable devices are tested in low-temperature electron transport measurements, displaying the typical tunneling magnetoresistance of carbon nanotube pseudo spin valves.
We report on two top-gate defined, coupled quantum dots in a semiconducting single wall carbon nanotube, constituting a tunable double quantum dot system. The single wall carbon nanotubes are contacted by titanium electrodes, and gated by three narrow top-gate electrodes as well as a back-gate. We show that a bias spectroscopy plot on just one of the two quantum dots can be used to extract the addition energy of both quantum dots. Furthermore, honeycomb charge stability diagrams are analyzed by an electrostatic capacitor model that includes cross capacitances, and we extract the coupling energ…
Using carbon nanotubes (CNTs) produced by chemical vapor deposition, we have explored different strategies for the preparation of carbon nanotube devices suited for electrical and mechanical measurements. Though the target device is a single small diameter CNT, there is compelling evidence for bundling, both for CNTs grown over structured slits and on rigid supports. Whereas the bundling is substantial in the former case, individual single-wall CNTs (SWNTs) can be found in the latter. Our evidence stems from mechanical and electrical measurements on contacted tubes. Furthermore, we report on t…
We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both these quantum mechanical degrees of freedom on the same footing.
We used tip-enhanced Raman spectroscopy to study the diameter-dependent Raman modes in a contacted carbon nanotube rope. We show that with the near-field tip enhancement a large number of nanotubes within a rope can be identified, even if the nanotube modes can not be distinguished in the far-field signal. Several metallic and semiconducting nanotubes can be identified and assigned to nanotube families. Additionally, we provide a tentative chiral index assignment.
We present a fabrication scheme called 'fork stamping' optimized for the dry transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic contact electrodes fabricated by standard lithography. We demonstrate the detailed recipes for a residue-free device fabrication and in-situ current annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py) contacts and report preliminary transport characterization and magnetoresistance experiments at cryogenic temperatures. This scheme can directly be used to implement more complex device structures, including multiple ga…
We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes with ultra small diameters. In particular, we consider the effect of the inter-wall interactions and the lead/nanotube coupling. Comparative studies have been performed to show that in the case when all walls are well coupled to the electrodes, the so-called inverse GMR can appear. The tendency towards a negative GMR depends on the inter-wall interaction and on the nanotube le ngth. If, however, the inner nanotubes are out of contact with one of the electrodes, the GMR remains positive even for relatively strong inter…
We investigate magnetoresistance in spin valves involving CoPd-contacted carbon nanotubes. Both temperature and bias voltage dependence clearly indicate tunneling magnetoresistance as the origin. We show that this effect is significantly affected by the tunnel barrier strength, which appears to be one reason for the variation between devices previously detected in similar structures. Modeling the data by means of the scattering matrix approach, we find a non-trivial dependence of the magnetoresistance on the barrier strength. Furthermore, analysis of the spin precession observed in a nonlocal …
We report the observation of two fundamental sub-gap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of $\sim145\,μ$eV, with a gate, bias and temperature dependence characteristic for boson-assisted, inelastic AT, …
We image the remnant magnetization configurations of CoFeB and permalloy nanotubes (NTs) using x-ray magnetic circular dichroism photo-emission electron microscopy. The images provide direct evidence for flux-closure configurations, including a global vortex state, in which magnetization points circumferentially around the NT axis. Furthermore, micromagnetic simulations predict and measurements confirm that vortex states can be programmed as the equilibrium remnant magnetization configurations by reducing the NT aspect ratio.
We develop an rf circuit model for single walled carbon nanotubes for both dc and capacitively contacted geometries. By modeling the nanotube as a nano-transmission line with distributed kinetic and magnetic inductance as well as distributed quantum and electrostatic capacitance, we calculate the complex, frequency dependent impedance for a variety of measurement geometries. Exciting voltage waves on the nano-transmission line is equivalent to directly exciting the yet-to-be observed one dimensional plasmons, the low energy excitation of a Luttinger liquid.
We study the effciency of the magnetoimpedance (MI) of thin-wall circumferentially ordered nanotubes in sub-GHz and GHz frequency regimes using micromagnetic simulations. We consider empty ferromagnetic tubes as well as tubes filled with non-magnetic conductors of circular cross-section (nanowire coverings) focusing on the low-field regime of MI (below the characteristic field of the low-frequency ferromagnetic resonance). In this field area, the effcient mechanism of MI is related to oscillations of the positions of (perpendicular to the tube axis) domain walls (DWs). Two mechanisms of drivin…
Possibility of a ferromagnetic semiconductor single wall carbon nanotube (SWCNT), where ferromagnetism is due to coupling between doped magnetic impurity on a zigzag SWCNT and electrons spin, is investigate. We found, in the weak impurity-spin couplings, at low impurity concentrations the spin up electrons density of states remain semiconductor while the spin down electrons density of states shows a metallic behavior. By increasing impurity concentrations the semiconducting gap of spin up electrons in the density of states is closed, hence a semiconductor to metallic phase transition is take p…
Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfac…
We report a study on the field-driven propagation of vortex-like domain walls in ferromagnetic nanotubes. This particular geometry gives rise to a special feature of the static wall configuration, which significantly influences its dynamics. Unlike domain walls in flat strips, the left-right symmetry of domain wall propagation is broken. Furthermore, the domain wall velocity is not limited by the Walker breakdown. Under sufficiently large magnetic fields, the domain wall velocity reaches the velocity of spin waves (about 1000 m/s) and is thereafter connected with a direct emission of spin wave…
We have measured the differential conductance of a parallel carbon nanotube (CNT) double quantum dot (DQD) with strong inter-dot capacitance and inter-dot tunnel coupling. Nominally, the device consists of a single CNT with two contacts. However, we identify two sets of Coulomb blockade (CB) diamonds that do not block transport individually, which suggest that two quantum dots (QDs) are contacted in parallel. We find strong and periodic anti-crossings in the gate and bias dependence, which are only possible if the QDs have similar characteristics. We discuss qualitatively the level spectrum an…
The reversal of a uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to nucleate and propagate through vortex domains forming at the ends. In dynamic cantilever magnetometry measurements of individual FNTs, we identify the entry of these vortices as a function of applied magnetic field and show that they mark the nucleation of magnetization reversal. We find that the entry field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of t…
We use a scanning nanometer-scale superconducting quantum interference device to map the stray magnetic field produced by individual ferromagnetic nanotubes (FNTs) as a function of applied magnetic field. The images are taken as each FNT is led through magnetic reversal and are compared with micromagnetic simulations, which correspond to specific magnetization configurations. In magnetic fields applied perpendicular to the FNT long axis, their magnetization appears to reverse through vortex states, i.e.\ configurations with vortex end domains or -- in the case of a sufficiently short FNT -- wi…
Carbon-based nanostructures and graphene, in particular, evoke a lot of interest as new promising materials for nanoelectronics and spintronics. One of the most important issue in this context is the impact of external electrodes on electronic properties of graphene nanoribbons (GNR). The present theoretical method is based on the tight-binding model and a modified recursive procedure for Green's functions. The results show that within the ballistic transport regime, the so called end-contacted geometry (of minimal GNR/electrode interface area), is usually more advantageous for practical appli…
The dynamical conductance of electrically contacted single-walled carbon nanotubes is measured from dc to 10 GHz as a function of source-drain voltage in both the low-field and high-field limits. The ac conductance of the nanotube itself is found to be equal to the dc conductance over the frequency range studied for tubes in both the ballistic and diffusive limit. This clearly demonstrates that nanotubes can carry high-frequency currents at least as well as dc currents over a wide range of operating conditions. Although a detailed theoretical explanation is still lacking, we present a phenomen…
We theoretically study the current-induced dynamics of a domain wall in a ferromagnetic nanotube by developing a theory for the orbital angular momentum of a domain wall and the current-induced torque on it. Specifically, a domain wall with nontrivial magnetization winding along the circumference is shown to possess finite orbital angular momentum, which is proportional to the product of its Skyrmion charge and position, and the current is shown to exert a torque changing the orbital angular momentum of the domain wall and thereby drives it. The current-induced torque is interpreted as the tra…
We report equilibrium electric resistance R and tunneling spectroscopy dI/dV measurements obtained on single multiwall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence and elastic-scattering lengths are deduced. Because the latter is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density-of-states (DOS) due to the one-dimensional subbands expected for a perfect single-wall…
During the last years great attention has been given to the encapsulation of magnetic nanoparticles. In this work we investigated the stability of small magnetic particles inside magnetic nanotubes. Multisegmented nanotubes were tested in order to optimize the stability of the particle inside the nanotubes. Our results evidenced that multisegmented nanotubes are more efficient to entrap the particles at temperatures up to hundreds of kelvins.
One of the actual challenges of spintronics is the realization of a spin-transistor allowing to control spin transport through an electrostatic gate. In this review, we report on different experiments which demonstrate a gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyze spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic c…
We have contacted single wall carbon nanotubes grown by chemical vapor deposition to superconducting Ti/Al/Ti electrodes. The device, we here report on is in the Kondo regime exhibiting a four-fold shell structure, where a clear signature of the superconducting electrodes is observed below the critical temperature. Multiple Andreev reflections are revealed by sub-gap structure and a narrow peak in the differential conductance around zero bias is seen depending on the shell filling. We interpret the peak as a proximity induced supercurrent and examine its interplay with Kondo resonances.
Controlling spin current and magnetic exchange coupling by applying an electric field and achieving high spin injection efficiency at the same time in a nanostructure coupled to ferromagnetic electrodes have been the outstanding challenges in nanoscale spintronics. A relentless quest is going on to find new low dimensional materials with tunable spin dependent properties to address these challenges. Herein, we predict, from first principles, the transverse electric field induced switching in the sign of exchange coupling and tunnel magnetoresistance in a boron nitride nanotube quantum dot atta…
The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger-liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically-contacted single-wall nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the in…
We present theoretical study of shot noise in single wall metallic carbon nanotubes weakly coupled to either nonmagnetic or ferromagnetic leads. Using the real-time diagrammatic technique, we calculate the current, Fano factor and tunnel magnetoresistance in the sequential tunneling regime. It is shown that the differential conductance displays characteristic four-fold periodicity, indicating single-electron charging. Such a periodicity is also visible in tunnel magnetoresistance of the system as well as in the Fano factor. The present studies elucidate the impact of ferromagnetic (vs. nonmagn…
A silicon micromechanical torsional oscillator is used to measure the hysteresis loops of two manganite La_0.67Ca_0.33MnO_3 nanotubes at different temperatures, applying an external field along its main axes. These structures are composed of nanograins with a ferromagnetic core surrounded by a dead layer. Micromagnetic calculations based on the stochastic Landau-Lifshitz-Gilbert equation, are performed to validate a simple model that allows for quantitatively describing the ferromagnetic behavior of the system. Further simulations are used to analyze the experimental data more in depth and to …
The modeling of carbon nanotube-metal contacts is important from both basic and applied view points. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free electron metal, which is side-contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of con…
Carbon nanotubes (CNT) have a very large application potential in the rapid developing field of molecular electronics. Infinite single-wall metallic CNTs have theoretically a conductance of 4e2/h because of the two electronic bands crossing the Fermi level. For finite size CNTs experiments have shown that other values are also possible, indicating a very strong influence of the contacts. We study electron transport in single- and double-wall CNTs contacted to metallic electrodes within the Landauer formalism combined with Green function techniques. We show that the symmetry of the contact regi…
In this study, a model of a Schottky-barrier carbon nanotube field- effect transistor (CNT-FET), with ferromagnetic contacts, has been developed. The emphasis is put on analysis of current-voltage characteristics as well as shot (and thermal) noise. The method is based on the tight-binding model and the non- equilibrium Green's function technique. The calculations show that, at room temperature, the shot noise of the CNT FET is Poissonian in the sub-threshold region, whereas in elevated gate and drain/source voltage regions the Fano factor gets strongly reduced. Moreover, transport properties …
We report on electrical spin injection measurements on MWNTs . We use a ferromagnetic alloy Pd$_{1-x}$Ni$_{x}$ with x $\approx$ 0.7 which allows to obtain devices with resistances as low as 5.6 $kΩ$ at 300 $K$. The yield of device resistances below 100 $kΩ$, at 300 $K$, is around 50%. We measure at 2 $K$ a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP) orientation and the parallel (P) orientation is about 2%.
The mechanism of carbon nanotube (CNT) nucleation and growth has been a mystery for over 15 years. Prior models have attempted the extension of older classical transport mechanisms. In July 2000, a more detailed and accurate nonclassical, relativistic mechanism was formulated considering the detailed dynamics of the electronics of spin and orbital rehybridization between the carbon and catalyst via novel mesoscopic phenomena and quantum dynamics. Ferromagnetic carbon was demonstrated. Here, quantum (Hall) effects and relativistic effects of intense many body spin-orbital interactions for novel…
This contribution reports on comparative studies on giant magnetoresistance (GMR) in carbon nanotubes (CNTs) and graphene nanoribbons of similar aspect ratios (i.e perimeter/length and width/length ratios, for the former and the latter, respectively). The problem is solved at zero temperature in the ballistic transport regime, by means of the Green's functions technique within the tight-binding model and with the so-called wide band approximation for electrodes. The GMR effect in graphene is comparable to that of CNTs, it depends strongly on the chirality and only slightly on the aspect ratio.…
