Showing papers on "Ballistic conduction published in 2010"
TL;DR: In this article, the electron transport properties of linear atomic carbon wire−graphene junctions were investigated by combining nonequilibrium Green's function with density functional theory, and it was shown that for short wires, linear ballistic transport is observed in wires consisting of odd numbers of carbon atoms but not in those consisting of even numbers of atoms.
Abstract: Long, stable, and free-standing linear atomic carbon wires (carbon chains) have been carved out from graphene recently [Meyer et al. Nature (London) 2008, 454, 319; Jin et al. Phys. Rev. Lett. 2009, 102, 205501]. They can be considered as extremely narrow graphene nanoribbons or extremely thin carbon nanotubes. It might even be possible to make use of high-strength and identical (without chirality) carbon wires as a transport channel or on-chip interconnects for field-effect transistors. Here we investigate electron transport properties of linear atomic carbon wire−graphene junctions by combining nonequilibrium Green’s function with density functional theory. For short wires, linear ballistic transport is observed in wires consisting of odd numbers of carbon atoms but not in those consisting of even numbers of carbon atoms. For wires longer than 2.1 nm as fabricated above, however, the ballistic conductance of carbon wire−graphene junctions is independent of the structural distortion, structural imperfect...
180 citations
TL;DR: In this paper, non-equilibrium molecular dynamics (NEMD) simulations were performed on Au-SAM (self-assembly monolayer) and Au-Au junctions to study the thermal energy transport across the junctions.
110 citations
Journal Article•
TL;DR: The yield is low with relatively low stability of the graphene solution, in which the graphene sheets have a tendency to settle down, but the third process, which is based on onsubstrate deposition from a graphene suspension, has several advantages including the large-scale production of reduced graphene oxide (RGO) and easy-to-apply chemical and physical manipulations for functionalization and directed deposition.
Abstract: Graphene is a single-atom-thick two-dimensional macromolecule with sp-bound carbon atoms arranged in a honeycomb lattice. Recently, graphene has emerged as an attractive candidate for several applications, including ultrafast nanoelectronic devices, tunable spintronics, ultracapacitors, transparent conducting electrodes, single-molecule chemical sensors, ultrasensitive biodevices, and nanomechanical devices. These applications have evolved from its atypical properties, such as weakly scattered ballistic transport of charge carriers behaving as massless fermions at room temperature, magneto-sensitive transport, tunable bandgap, quantum Hall effect at room temperature, tunable optical transitions, exceptional mechanical strength, megahertz characteristic frequency, carrier collimation, and ultrahigh stiffness. Graphene can be 1) synthesized on-substrate, 2) deposited on-substrate via mechanical processes, or 3) deposited onsubstrate from solution. On-substrate synthesis includes hightemperature (>1000 8C) epitaxial growth on SiC, ruthenium or chemical vapor deposition on nickel and copper, while mechanical deposition includes adhesive-tape exfoliation of highly oriented pyrolytic graphite (HOPG) and the ensuing transfer. The third process, which is based on onsubstrate deposition from a graphene suspension, has several advantages including the large-scale production of reduced graphene oxide (RGO) and easy-to-apply chemical and physical manipulations for functionalization and directed deposition. Graphene suspension synthesis methods include 1) p–p intercalation or graphite intercalation compound (GIC)-based exfoliation of graphite flakes into graphene sheets, and 2) in-solution reduction of graphite oxide prepared by Hummers method with hydrazine. The p–p intercalation and GIC-based methods produce highquality graphene; however, the yield is low with relatively low stability of the graphene solution, in which the graphene sheets have a tendency to settle down. The graphene suspension
103 citations
TL;DR: This work demonstrates experimentally the efficient tuning of the energy relaxation that limits quantum coherence and permits the return toward equilibrium, and elucidates the inelastic mechanisms at work at ν(L)=2, informing us, in particular, that those within the outer edge channel are negligible.
Abstract: The chiral edge channels in the quantum Hall regime are considered ideal ballistic quantum channels, and have quantum information processing potentialities. Here, we demonstrate experimentally, at a filling factor of ν(L)=2, the efficient tuning of the energy relaxation that limits quantum coherence and permits the return toward equilibrium. Energy relaxation along an edge channel is controllably enhanced by increasing its transmission toward a floating Ohmic contact, in quantitative agreement with predictions. Moreover, by forming a closed inner edge channel loop, we freeze energy exchanges in the outer channel. This result also elucidates the inelastic mechanisms at work at ν(L)=2, informing us, in particular, that those within the outer edge channel are negligible.
102 citations
TL;DR: Graphene is a two-dimensional macromolecule with sp-bound carbon atoms arranged in a honeycomb lattice as discussed by the authors, and it can be synthesized on-substrate, synthesized via mechanical processes, or deposited onsubstrate from solution.
Abstract: Graphene is a single-atom-thick two-dimensional macromolecule with sp-bound carbon atoms arranged in a honeycomb lattice. Recently, graphene has emerged as an attractive candidate for several applications, including ultrafast nanoelectronic devices, tunable spintronics, ultracapacitors, transparent conducting electrodes, single-molecule chemical sensors, ultrasensitive biodevices, and nanomechanical devices. These applications have evolved from its atypical properties, such as weakly scattered ballistic transport of charge carriers behaving as massless fermions at room temperature, magneto-sensitive transport, tunable bandgap, quantum Hall effect at room temperature, tunable optical transitions, exceptional mechanical strength, megahertz characteristic frequency, carrier collimation, and ultrahigh stiffness. Graphene can be 1) synthesized on-substrate, 2) deposited on-substrate via mechanical processes, or 3) deposited onsubstrate from solution. On-substrate synthesis includes hightemperature (>1000 8C) epitaxial growth on SiC, ruthenium or chemical vapor deposition on nickel and copper, while mechanical deposition includes adhesive-tape exfoliation of highly oriented pyrolytic graphite (HOPG) and the ensuing transfer. The third process, which is based on onsubstrate deposition from a graphene suspension, has several advantages including the large-scale production of reduced graphene oxide (RGO) and easy-to-apply chemical and physical manipulations for functionalization and directed deposition. Graphene suspension synthesis methods include 1) p–p intercalation or graphite intercalation compound (GIC)-based exfoliation of graphite flakes into graphene sheets, and 2) in-solution reduction of graphite oxide prepared by Hummers method with hydrazine. The p–p intercalation and GIC-based methods produce highquality graphene; however, the yield is low with relatively low stability of the graphene solution, in which the graphene sheets have a tendency to settle down. The graphene suspension
102 citations
TL;DR: It is demonstrated that chevron-type GNRs recently synthesized should display superior thermoelectric properties and that increasing the number of interfaces in a single GNR system increases the peak ZT values that are thus maximized in a periodic superlattice.
Abstract: Using model interaction Hamiltonians for both electrons and phonons and Green's function formalism for ballistic transport, we have studied the thermal conductance and the thermoelectric properties of graphene nanoribbons (GNR), GNR junctions and periodic superlattices. Among our findings we have established the role that interfaces play in determining the thermoelectric response of GNR systems both across single junctions and in periodic superlattices. In general, increasing the number of interfaces in a single GNR system increases the peak ZT values that are thus maximized in a periodic superlattice. Moreover, we proved that the thermoelectric behavior is largely controlled by the width of the narrower component of the junction. Finally, we have demonstrated that chevron-type GNRs recently synthesized should display superior thermoelectric properties.
100 citations
TL;DR: In this paper, the problem of spin-resolved scattering through spin-orbit nanostructures in graphene is addressed, where the authors discuss the phenomenon of spin double refraction and its consequences on the spin polarization.
Abstract: We address the problem of spin-resolved scattering through spin-orbit nanostructures in graphene, ie, regions of inhomogeneous spin-orbit coupling on the nanometer scale We discuss the phenomenon of spin-double refraction and its consequences on the spin polarization Specifically, we study the transmission properties of a single and a double interface between a normal region and a region with finite spin-orbit coupling, and analyze the polarization properties of these systems Moreover, for the case of a single interface, we determine the spectrum of edge states localized at the boundary between the two regions and study their properties
99 citations
TL;DR: In this paper, the authors provided experimental evidence for ballistic heat transport in a S = 1/2 Heisenberg chain and investigated high purity samples of the chain cuprate SrCuO2 and observed a huge magnetic heat conductivity.
Abstract: Fundamental conservation laws predict ballistic, i.e., dissipationless transport behaviour in one-dimensional quantum magnets. Experimental evidence, however, for such anomalous transport has been lacking ever since. Here we provide experimental evidence for ballistic heat transport in a S=1/2 Heisenberg chain. In particular, we investigate high purity samples of the chain cuprate SrCuO2 and observe a huge magnetic heat conductivity $\kappa_{mag}$. An extremely large spinon mean free path of more than a micrometer demonstrates that $\kappa_{mag}$ is only limited by extrinsic scattering processes which is a clear signature of ballistic transport in the underlying spin model.
98 citations
TL;DR: In this article, the impact of surface states, diameter, lateral and vertical fields, as well as crystal structure, on electron transport and transport coefficient calculation was analyzed for InAs nanowires.
Abstract: The vapor--liquid--solid growth of semiconductor nanowires led to the implementation of engineered electronic and optoelectronic one-dimensional nanostructures with outstanding promise for device applications. To realize this promise, detailed understanding and control over their growth, crystal structure, and transport properties and their combined impact on device performance is vital. Here, we review our work on electron transport in InAs nanowires in a variety of device schemes. First, we provide a brief introduction and historical perspective on growth and transport studies in InAs NWs. Second, we discuss and present experimental measurements of ballistic transport in InAs nanowires over ~200 nm length scale, which indicates a large electron mean free path and correlates with the high electron mobility measured on similar nanowires. Third, we devise a device model that enables accurate estimation of transport coefficients from field-effect transistor measurements by taking into account patristic device components. We utilize this model to reveal the impact of surface states, diameter, lateral and vertical fields, as well as crystal structure, on electron transport and transport coefficient calculation. We show in these studies that electron transport in InAs nanowires is dominated by surface state effects that introduce measurement artifacts in parameter extraction, reduce electron mobility for smaller diameters, and degrade the subthreshold characteristics of transistors made of Zinc Blende InAs nanowires. This device model is also used for isolating vertical and lateral field effects on electron transport in nanowire transistor channels and explaining observed negative differential conductance and mobility degradation at high injection fields, which is supported by electro-thermal simulations and microstructure failure analysis. We adopt the concept of lack of inversion symmetry in polar III-V materials and the resultant spontaneous polarization charges perpendicular to the electron transport trajectory in twinned Wurtzite nanowires to explain compensation of surface charges for this type of nanowires and their enhanced subthreshold characteristics over transistors made of Zinc Blende ones. Fourth, we discuss the combined effects of surface states and field variations in InAs nanowire transistor channels to shed light on the local electrostatic behavior in 1D channels studied by scanning probe measurements. Fifth, we survey and benchmark results on nanowire transistor performance and demonstrate the superiority of InAs nanowires for high-on currents and high-speed applications. Finally, we implement a novel integration scheme for InAs nanowires on Si substrates that enables vertical alignment and electrical isolation between nanowires which is necessary for achieving multifunctional devices per single chip.
95 citations
TL;DR: Coherent transport involving more than a single one-dimensional mode transport was observed in the experiment and manifested by Fabry-Perot conductance oscillations, implying nearly ballistic electron transport through the nanowire.
Abstract: We report on observation of coherent electron transport in suspended high-quality InAs nanowire-based devices. The InAs nanowires were grown by low-temperature gold-assisted vapor-liquid-solid molecular-beam-epitaxy. The high quality of the nanowires was achieved by removing the typically found stacking faults and reducing possibility of Au incorporation. Minimizing substrate-induced scattering in the device was achieved by suspending the nanowires over predefined grooves. Coherent transport involving more than a single one-dimensional mode transport was observed in the experiment and manifested by Fabry-Perot conductance oscillations. The length of the Fabry-Perot interferometer, deduced from the period of the conductance oscillations, was found to be close to the physical length of the device. The high oscillations visibility imply nearly ballistic electron transport through the nanowire.
94 citations
TL;DR: In this paper, the conductance of mesoscopic graphene rings in the presence of a perpendicular magnetic field was studied by means of numerical calculations based on a tight-binding model. But the results for both clean (ballistic) and disordered (diffusive) rings are presented.
Abstract: We study the conductance of mesoscopic graphene rings in the presence of a perpendicular magnetic field by means of numerical calculations based on a tight-binding model. First, we consider the magnetoconductance of such rings and observe the Aharonov-Bohm effect. We investigate different regimes of the magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm oscillations are suppressed. Results for both clean (ballistic) and disordered (diffusive) rings are presented. Second, we study rings with smooth mass boundary that are weakly coupled to leads. We show that the valley degeneracy of the eigenstates in closed graphene rings can be lifted by a small magnetic flux, and that this lifting can be observed in the transport properties of the system.
TL;DR: The transport properties of the anisotropic Heisenberg model in a disordered magnetic field and in the presence of dephasing due to external degrees of freedom were studied in this article.
Abstract: In this work, we study the transport properties of the anisotropic Heisenberg model in a disordered magnetic field and in the presence of dephasing due to external degrees of freedom Without dephasing, the model can display, depending on parameter values, the whole range of possible transport regimes: ideal ballistic conduction, diffusive, or ideal insulating behavior We show that the presence of dephasing induces normal diffusive transport in a wide range of parameters We also analyze the dependence of spin conductivity on the dephasing strength In addition, by analyzing the decay of the spin–spin correlation function, we find a long-range order for finite chain sizes All our results for a one-dimensional spin chain at infinite temperature can be equivalently rephrased for strongly interacting disordered spinless fermions
TL;DR: In this article, an efficient carbon nanotube (CNT) transistor modeling technique based on cubic spline approximation of the nonequilibrium mobile charge density is presented. But the model is not suitable for the CNT drain-source current.
Abstract: This paper presents an efficient carbon nanotube (CNT) transistor modeling technique that is based on cubic spline approximation of the nonequilibrium mobile charge density. The approximation facilitates the solution of the self-consistent voltage equation in a CNT so that calculation of the CNT drain-source current is accelerated by at least two orders of magnitude. A salient feature of the proposed technique is its ability to incorporate both ballistic and nonballistic transport effects without a significant computational cost. The proposed models have been extensively validated against reported CNT ballistic and nonballistic transport theories and experimental results.
TL;DR: In this paper, it was shown that the thermoelectric efficiency is directly related to the geometry of the diameter modulation, and that geometry optimization can lead to efficient devices based on modulated nanowires.
Abstract: High optimal thermoelectric efficiencies are theoretically demonstrated in ballistic nanowires with diameter modulation. The physics underlying the good thermoelectric performance of diameter-modulated nanowires is the strong energy dependence of their transmission coefficients. It is shown that the thermoelectric efficiency is directly related to the geometry of the diameter modulation. It becomes evident that geometry optimization can lead to efficient thermoelectric devices based on modulated nanowires.
TL;DR: In this article, the authors investigated the application of one-dimensional fluid model in modeling of electron transport in carbon nanotubes and equivalent circuits for interconnections and compared the performances with the currently used copper interconnects in very large-scale integration (VLSI) circuits.
Abstract: We investigated the application of one-dimensional fluid model in modeling of electron transport in carbon nanotubes and equivalent circuits for interconnections and compared the performances with the currently used copper interconnects in very-large-scale integration (VLSI) circuits. In this model, electron transport in carbon nanotubes is regarded as quasi one-dimensional fluid with strong electron-electron interaction. Verilog-AMS in Cadence/Spectre was used in simulation studies. Carbon nanotubes of the types single-walled, multiwalled and bundles were considered for ballistic transport region, local and global interconnections. Study of the S-parameters showed higher transmission efficiency and lower reflection losses. Theoretical modeling and computer-aided simulation studies through a complimentary CNT-FET inverter pair, interconnected through a wire, exhibited reduced delays and power dissipations for carbon nanotube interconnects in comparison to copper interconnects in 22 nm and lower technology nodes. The performance of CNT interconnects was shown to be further improved with increase in number of metallic carbon nanotubes. Our study suggests the replacement of copper interconnect with the multiwalled and bundles of single-walled carbon nanotubes for the sub-nanometer CMOS technologies.
TL;DR: In this article, the authors show that the ultrashort decoherence time of superpositions of valence-and conduction-band states plays a crucial role for the efficiency of the tunneling process.
Abstract: Ultrafast high-field transport is studied in $n$-type GaAs with ultrashort terahertz (THz) pulses of an electric field amplitude of up to 300 kV/cm. At lattice temperatures between $T=300$ and 80 K, we observe coherent ballistic transport of electrons over a major part of the first Brillouin zone. At $T=300\text{ }\text{K}$, ballistic transport occurs at a constant electron density whereas at lower temperatures, the THz pulses generate additional electron-hole pairs by field-induced tunneling between valence and conduction bands. We show that the ultrashort decoherence time of superpositions of valence- and conduction-band states plays a crucial role for the efficiency of the tunneling process. The extremely fast interband decoherence at room temperature results in a negligible tunneling rate.
TL;DR: In this article, the electronic structures of the doped graphene nanoribbons with eight-armchair edges containing nitrogen or boron substitutional impurity were calculated using ab initio density functional theory.
Abstract: Calculation of electronic structures has been performed for graphene nanoribbons with eight-armchair edges containing nitrogen or boron substitutional impurity by using ab initio density functional theory. It is found that the electronic structures of the doped graphene nanoribbon are different from those of doped carbon nanotubes. The impurity levels are autoionized, so that the relevant charge carriers occupy the conduction or valence bands. The donor and acceptor levels are derived mainly from the lowest unoccupied orbital and highest occupied orbital of pristine graphene nanoribbon, respectively. N introduces an impurity level above the donor level, while an impurity level introduced by B is below the acceptor level. The doped graphene nanoribbons with armchair edges are inactive compared to the doped carbon nanotubes around the impurity site, which may indicate that the doped graphene nanoribbons with armchair edges could be more stable than the doped carbon nanotubes at the ambient.
TL;DR: In this paper, a performance analysis of field effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene and theoretically predicted semihydrogenated graphene (i.e., graphone) is presented.
Abstract: In this work, we present a performance analysis of field-effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semihydrogenated graphene (i.e., graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor $s{p}^{3}$ tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large ${I}_{\text{on}}$ and ${I}_{\text{on}}/{I}_{\text{off}}$ ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.
TL;DR: In this paper, the authors studied the electronic band structure of monolayer graphene when Rashba spin-orbit coupling is present and showed that the low-energy bands undergo trigonal-warping deformation and for energies smaller than the Lifshitz energy, the Fermi circle breaks up into separate parts.
Abstract: We study the electronic band structure of monolayer graphene when Rashba spin-orbit coupling is present. We show that if the Rashba spin-orbit coupling is stronger than the intrinsic spin-orbit coupling, the low-energy bands undergo trigonal-warping deformation and that for energies smaller than the Lifshitz energy, the Fermi circle breaks up into separate parts. The effect is very similar to what happens in bilayer graphene at low energies. We discuss the possible experimental implications, such as threefold increase in the minimal conductivity for low electron densities, anisotropic, wave-number-dependent spin splitting of the bands, and the spin-polarization structure.
TL;DR: In this article, a detailed simulation study on the currentvoltage characteristics of ballistic graphene nanoribbon (GNR) tunneling FETs of different widths with varying temperatures and channel length is presented.
Abstract: We present a detailed simulation study on the current-voltage characteristics of ballistic graphene nanoribbon (GNR) tunneling FETs of different widths with varying temperatures and channel length. Our model uses the self-consistent nonequilibrium Green's function and the quasi-2-D Poisson solver with the material details of the GNRs modeled by the uncoupled mode space Dirac equation. We find that, in general, the GNR tunneling FETs from the 3p + 1 family have better ION/IOFF characteristics than those from the 3p family due to smaller effective masses of the former. A lower drain doping concentration relative to that of the source enhances the ION/IOFF. Most significantly, we find that a higher doping concentration at the source enhances ION but degrades the subthreshold swing (SS). As a function of temperature, the SS shows highly nonlinear behaviors. In terms of intrinsic delay and power-delay product, the GNR tunneling FETs show very promising scaling behaviors and can be optimized to meet the International Technology Roadmap for Semiconductors roadmap requirements through adjustment in doping concentrations and other parameters.
TL;DR: In this article, a comparative study of the density dependence of the conductivity of graphene sheets calculated in the tight-binding (TB) Landauer approach and on the basis of the Boltzmann theory is presented.
Abstract: We present a comparative study of the density dependence of the conductivity of graphene sheets calculated in the tight-binding (TB) Landauer approach and on the basis of the Boltzmann theory. The ...
TL;DR: In this article, the thermoelectric properties of silicon nanowires with different shapes, sizes, and orientations are theoretically investigated using sp3d5s∗ tight-binding model coupled with ballistic transport approach.
Abstract: The thermoelectric properties of silicon nanowires with different shapes, sizes, and orientations are theoretically investigated using sp3d5s∗ tight-binding model coupled with ballistic transport approach. We found that the thermoelectric properties significantly depend on nanowire geometry. Compared to [111] and [100] nanowires, n-doped and p-doped [110] nanowires show the worst performance in terms of power factor per cross-section area and figure of merit (ZT). As nanowire size decreases, thermoelectric properties of nanowires can be enhanced. As a result, triangular nanowires with side length of 1 nm have the best results of ZT and it can be enhanced to 1.5 and 0.85 for an n-type nanowire along [111] orientation and a p-type nanowire along [100] orientation, respectively. For extremely narrow nanowires, thermoelectric properties are only dependent on the number of the transmission modes instead of material properties such as carrier effective mass. Moreover, cross-section shape and thermal conductance...
TL;DR: In this paper, the magnetoconductance of an open carbon nanotube (CNT)-quantum wire was measured in pulsed magnetic fields and it was found that the current in the peak regions is highly spin polarized, which calls for application in future CNT-based spintronic devices.
Abstract: The magnetoconductance of an open carbon nanotube (CNT)-quantum wire was measured in pulsed magnetic fields. At low temperatures, we find a peculiar split magnetoconductance peak close to the chargeneutrality point. Our analysis of the data reveals that this splitting is intimately connected to the spin-orbit interaction and the tube chirality. Band-structure calculations suggest that the current in the peak regions is highly spin polarized, which calls for application in future CNT-based spintronic devices.
TL;DR: In this paper, the defect levels of surface dangling bonds and Au impurities in the Si shell were investigated and it was shown that dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one component Si nanowires.
Abstract: The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.
TL;DR: In this paper, conductance oscillations with the flux piercing the disk area Φd, characterized by the period Φ0=2(h/e)ln(Ro/Ri), where Ro(Ri) is the outer (inner) disk radius.
Abstract: Electron transport through the Corbino disk in graphene is studied in the presence of uniform magnetic fields. At the Dirac point, we observe conductance oscillations with the flux piercing the disk area Φd, characterized by the period Φ0=2(h/e)ln(Ro/Ri), where Ro(Ri) is the outer (inner) disk radius. The oscillations magnitude increase with the radii ratio and exceed 10% of the average conductance for Ro/Ri≥5 in the case of the normal Corbino setup or for Ro/Ri≥2.2 in the case of the Andreev-Corbino setup. At a finite but weak doping, the oscillations still appear in a limited range of |Φd|≤Φdmax, away from which the conductance is strongly suppressed. At large dopings and weak fields we identify the crossover to a normal ballistic transport regime.
TL;DR: In this article, the authors theoretically investigate the time-dependent ballistic transport in metallic graphene nanoribbons of width $W$ after the sudden switching of a bias voltage and show that the potential drop is linear across a central part of length $L$ where the current is calculated.
Abstract: We theoretically investigate the time-dependent ballistic transport in metallic graphene nanoribbons of width $W$ after the sudden switching of a bias voltage. The potential drop is linear across a central part of length $L$ where the current is calculated. During the early transient time the current does not grow linearly in time but remarkably reaches a temporary plateau. Such behavior allows us to define a transient conductivity, the value of which coincides with the minimal conductivity of two-dimensional graphene. At time $L/{v}_{F}$ (${v}_{F}$ being the Fermi velocity) a crossover takes place: the current changes abruptly and saturates at its final steady-state value (second plateau). We show that the two plateaus develop with damped oscillations of totally different nature and demonstrate that the occurrence of the first plateau is independent of the boundary conditions. The transition from quasi-one-dimensional to bulk behavior $(W\ensuremath{\rightarrow}\ensuremath{\infty})$ is also analyzed.
TL;DR: In this paper, first principles calculations of currentvoltage characteristics (IVC) and conductance of Au(111):S-2-cumulene-S 2:Au(111) molecular wire junctions with realistic contacts are presented.
Abstract: We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S-2-cumulene-S-2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio nonequilibrium Green's function density-functional theory methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metalliclike ballistic transport behavior. The reason is the high density of states in broad lowest unoccupied molecular orbital levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region of +/- 1 V. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.
TL;DR: In this paper, a model for 1-D field effect transistors is presented, taking into account on equal footing both Schottky barriers and FFE transport regime, and intermediate transport is introduced within the Buttiker's probe approach to dissipative transport, in which a nonballistic transistor is seen as a suitable series of individually ballistic channels.
Abstract: Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, which cannot be described neither by drift diffusion nor by purely ballistic models. In carbon-based nanotransistors, source and drain contacts are often characterized by the formation of Schottky barriers (SBs), with strong influence on transport. In this paper, we present a model for 1-D field-effect transistors, taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Buttiker's probe approach to dissipative transport, in which a nonballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport and, in particular, of the transition between SB- and dissipation-limited transports.
TL;DR: In this article, the scattering between a beam of electrons and a two-dimensional electron gas (2DEG) as a function of the beam's injection energy, and distance from the injection point was investigated.
Abstract: Using scanning gate microscopy (SGM), we probe the scattering between a beam of electrons and a two-dimensional electron gas (2DEG) as a function of the beam's injection energy, and distance from the injection point. At low injection energies, we find electrons in the beam scatter by small angles, as has been previously observed. At high injection energies, we find a surprising result: placing the SGM tip where it backscatters electrons increases the differential conductance through the system. This effect is explained by a nonequilibrium distribution of electrons in a localized region of 2DEG near the injection point. Our data indicate that the spatial extent of this highly nonequilibrium distribution is within $\ensuremath{\sim}1\text{ }\ensuremath{\mu}\text{m}$ of the injection point. We approximate the nonequilibrium region as having an effective temperature that depends linearly upon injection energy.
TL;DR: In this paper, the spin polarized transport properties through a ballistic graphene-based quantum tunneling junctions with the spin-orbit interaction have been investigated, and it is found that the magnetoresistance (MR) oscillates with the Rashba spinorbit interaction (RSOI) and the intrinsic spinorbit interactions (ISOI), whereas for the ISOI alone no such negative MR can be found.
Abstract: Based on the transfer-matrix method, the spin polarized transport properties through a ballistic graphene-based quantum tunneling junctions with the spin-orbit interaction have been investigated. It is found that the magnetoresistance (MR) oscillates with the Rashba spin-orbit interaction (RSOI) and the intrinsic spin-orbit interaction (ISOI). In addition, when the RSOI is present, the negative MR can be observed due to the spin-flip effect, whereas for the ISOI alone no such negative MR can be found. It is anticipated to apply such a phenomenon to design the electron devices based on the graphene.