Multichannel Transport Research Articles

Some peculiarities of magnetoresistance and Hall resistance of Sb2Te3 nanoplates

Antimony telluride (Sb2Te3) nanoplates of various thickness were grown by the vapor phase deposition method. The Hall resistance and magnetoresistance of the samples were measured in magnetic fields up to 9 T at temperatures from 2 to 300 K. Temperature dependence of the magnetoresistance and Hall resistance of the nanoplates shows a strong dependence on the thickness of the samples. Relatively thick samples show a nonlinear dependence of the Hall resistance on magnetic field. The measurement data are analyzed within the model of multi-channel transport. The difference in behavior is attributed to the existence of two channels of charge transfer with high and low mobility.

Shape-Controlled Metal-Free Catalysts: Facet-Sensitive Catalytic Activity Induced by the Arrangement Pattern of Noncovalent Supramolecular Chains.

Metal-free catalytic materials have recently received broad attention as promising alternatives to metal-involved catalysts. This is owing to their inherent capability to overcome the inevitable limitations of metal-involved catalysts, such as high sensitivity to poisoning, the limited reserves, high cost and scarcity of metals (especially noble metals), etc. However, the lack of shape-controlled metal-free catalysts with well-defined facets is a formidable bottleneck limiting our understandings on the underlying structure-activity relationship at atomic/molecular level, which thereby restrains their rational design. Here, we report that catalytically active crystals of a porphyrin, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, could be shaped into well-defined cubes and sheet-like tetradecahedrons (TDHD), which are exclusively and predominantly enclosed by {101} and {001} facets, respectively. Fascinatingly, compared to the cubes, the TDHDs display substantially enhanced catalytic activity toward water decontamination under visible-light irradiation, although both the architectures have identical crystalline structure. We disclose that such interesting shape-sensitive catalytic activity is ascribed to the distinct spatial separation efficiency of photogenerated electrons and holes induced by single-channel and multichannel charge transport pathways along noncovalent supramolecular chains, which are arranged as parallel-aligned and 2D network patterns, respectively. Our findings provide an ideal scientific platform to guide the rational design of next-generation metal-free catalysts of desired catalytic performances.

Ultra-low threshold field emission from amorphous BN nanofilms

Abstract In this work, Field emission (FE) properties of mixed phase amorphous boron nitride (BN) nanofilms prepared by plasma enhanced chemical vapor deposition (PECVD) were thoroughly investigated. The results showed that, for mixed amorphous BN nanofilms with a little cubic boron nitride (c-BN) and wurtzite boron nitride (w-BN) nanocrystalline, the turn on field of the nanofilms is as low as ∼0.5 V/μm and the FE current density can reach ∼12 mA/cm2 when the electrical field is only 0.75 V/μm. However, for primary cubic phase nanocrystalline BN films, the FE current density is only 500 μA/cm2; at as high as about 2.5 V/μm. Moreover, the field emission mechanism of the phase amorphous BN nanofilms was further discussed. Field emission enhancement for mixed phase amorphous BN may be originated from electron step-transport in different energy states and multi-channel electron transport in nanocrystalline interface.

Nonlinear transport in ionic liquid gated strontium titanate nanowires

Measurements of the current-voltage (I–V) characteristics of ionic liquid gated nanometer scale channels of strontium titanate have been carried out. At low gate voltages, the I–V characteristics exhibit a large voltage threshold for conduction and a nonlinear power law behavior at all temperatures measured. The source-drain current of these nanowires scales as a power law of the difference between the source-drain voltage and the threshold voltage. The scaling behavior of the I–V characteristic is reminiscent of collective electronic transport through an array of quantum dots. At large gate voltages, the narrow channel acts as a quasi-1D wire whose conductance follows Landauer's formula for multichannel transport.

Topological edge states in single- and multi-layer Bi4Br4

Topological edge states at the boundary of quantum spin Hall (QSH) insulators hold great promise for dissipationless electron transport. The device application of topological edge states has several critical requirements for QSH insulator materials, e.g. a large band gap, appropriate insulating substrates, and multiple conducting channels. In this paper, based on first-principles calculations, we show that Bi4Br4 is a suitable candidate. Single-layer Bi4Br4 was recently demonstrated to be a QSH insulator with sizable gap. Here we find that, in multilayer systems, both the band gaps and low-energy electronic structures are only slightly affected by the interlayer coupling. On the intrinsic insulating substrate of bulk Bi4Br4, the single-layer Bi4Br4 preserves its topological edge states well. Moreover, at the boundary of multilayer Bi4Br4, the topological edge states stemming from different single-layers are weakly coupled, and can be fully decoupled by constructing a stair-stepped edge. The decoupled topological edge states are very suitable for multi-channel dissipationless transport. Our work indicates that an ideal QSH insulator can be prepared by nano-fabricaton on the cleaved surface of layered Bi4Br4 single crystal.

A comparative study of size-dependent magnetoresistance and Hall resistance of Sb2Te3 nanoflakes

Single crystal Sb2Te3 topological insulator nanoflakes with the thickness of 25nm and 456nm were synthesized via vapor phase deposition method. The Hall resistance and magnetoresistance of the nanoflakes have been measured at temperatures 2K and 300K in the fields up to 9T. The magnetoresistance and Hall resistance of the nanoflakes demonstrate significant differences, so that despite ordinary magnetoresistance and Hall effect obtained for 25nm sample 450nm nanoflake demonstrates unusual magnetoresistance and nonlinear Hall resistance. The sense of curvature of both Rxx(B) and Rxy(B) dependences is inversed at high temperature. The experimental data have been analyzed in the frame of a multichannel transport model. The difference in the behavior is attributed to the existence of the charge carriers with high and low mobility, as well as to their relative contribution (which varies depending on the temperature) to the magneto-transport of the nanoflakes.

Multi-channel Andreev reflection in Co–W nanocontacts fabricated using focused electron/ion beam induced deposition

We report multi-channel electron transport in nano-contacts fabricated using focused electron beam induced deposited (FEBID) cobalt and focused ion beam induced deposited (FIBID) tungsten. Anomalous Andreev reflection (AR) effect is observed to which the conventional Blonder–Tinkham–Klapwijk (BTK) fit cannot be applied. In specific, we have observed multiple number of shoulders near the AR peak, whose origin is unknown in literature. We explain this effect based on a simple model that takes into account the material properties of the FIBID grown W superconductor, as well as the specific interface properties that are an outcome of using FEBID/FIBID as a fabrication technique. We show that numerical calculations using the BTK approximation based on the consideration of multiple channels generate similar shoulders as we observed in the AR experiments. Electrical measurements and x-ray photoemission spectroscopy carried out on FIBID W deposits puts additional evidence towards multi-channel current transport occuring at the interface of the nanocontacts.

Impact of lattice strain on the tunnel magnetoresistance in Fe/insulator/Fe and Fe/insulator/La0.67Sr0.33MnO3magnetic tunnel junctions

The objective of this work is to describe the tunnel electron current in single barrier magnetic tunnel junctions within a new approach that goes beyond the single-band transport model. We propose a ballistic multi-channel electron transport model that can explain the influence of in-plane lattice strain on the tunnel magnetoresistance as well as the asymmetric voltage behavior. We consider as an example single crystal magnetic Fe(110) electrodes for Fe/Insulator/Fe and Fe/Insulator/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ tunnel junctions, where the electronic band structures of Fe and La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ are derived by \it{ab-initio} calculations.

Multi-channel transport experiments at Alcator C-Mod and comparison with gyrokinetic simulations

Multi-channel transport experiments have been conducted in auxiliary heated (Ion Cyclotron Range of Frequencies) L-mode plasmas at Alcator C-Mod [Marmar and Alcator C-Mod Group, Fusion Sci. Technol. 51(3), 3261 (2007)]. These plasmas provide good diagnostic coverage for measurements of kinetic profiles, impurity transport, and turbulence (electron temperature and density fluctuations). In the experiments, a steady sawtoothing L-mode plasma with 1.2 MW of on-axis RF heating is established and density is scanned by 20%. Measured rotation profiles change from peaked to hollow in shape as density is increased, but electron density and impurity profiles remain peaked. Ion or electron heat fluxes from the two plasmas are the same. The experimental results are compared directly to nonlinear gyrokinetic theory using synthetic diagnostics and the code GYRO [Candy and Waltz, J. Comput. Phys. 186, 545 (2003)]. We find good agreement with experimental ion heat flux, impurity particle transport, and trends in the fluctuation level ratio (T̃e/Te)/(ñe/ne), but underprediction of electron heat flux. We find that changes in momentum transport (rotation profiles changing from peaked to hollow) do not correlate with changes in particle transport, and also do not correlate with changes in linear mode dominance, e.g., Ion Temperature Gradient versus Trapped Electron Mode. The new C-Mod results suggest that the drives for momentum transport differ from drives for heat and particle transport. The experimental results are inconsistent with present quasilinear models, and the strong sensitivity of core rotation to density remains unexplained.

Shot noise of multichannel transport in mesoscopic junction systems

Shot noise, which accompanies the emergence of the quantization nature (discreteness) of electrons, indicates dynamic fluctuations in electrical current and further reveals underlying operations in mesoscopic applications. This work presents a multichannel junction device with which to study the shot noise of coupled-channel transports as in normal nanocircuits, and it describes the theoretical algorithms. The numerical results show that, in contrast to the indistinguishable results of average current on direct-current measurements, the shot noise related to a dynamic current definitely exhibits significant differences among devices with diverse multichannel set-ups and presents super-Poissonian behaviors associated with the splitting of tunneling states.

A Multichannel Model for the Self-Consistent Analysis of Coherent Transport in Graphene Nanoribbons

In this contribution, we analyze the multichannel coherent transport in graphene nanoribbons (GNRs) by a scattering matrix approach. We consider the transport properties of GNR devices of a very general form, involving multiple bands and multiple leads. The 2D quantum transport over the whole GNR surface, described by the Schrödinger equation, is strongly nonlinear as it implies calculation of self-generated and externally applied electrostatic potentials, solutions of the 3D Poisson equation. The surface charge density is computed as a balance of carriers traveling through the channel at all of the allowed energies. Moreover, formation of bound charges corresponding to a discrete modal spectrum is observed and included in the model. We provide simulation examples by considering GNR configurations typical for transistor devices and GNR protrusions that find an interesting application as cold cathodes for X-ray generation. With reference to the latter case, a unified model is required in order to couple charge transport and charge emission. However, to a first approximation, these could be considered as independent problems, as in the example.

Multi-channel magnetotransport in Co2FeSi/(Al,Ga)As spin-LEDs

We have performed Hall effect measurements on Co2FeSi/(Al,Ga)As spin light emitting diodes and have found unique field dependencies that differ strongly from the expected behaviors for both the ferromagnetic Co2FeSi layer and the underlying semiconductor structure. To understand such unique field dependencies, we have developed a multi-channel transport model for parallel transport through a ferromagnet and a semiconductor. By applying this model to our data for the Hall and sheet resistance, we extract values for the carrier density and mobility in the semiconductor layer. We find that these values decrease with increasing growth temperature of the Co2FeSi layer, presumably due to stronger in-diffusion of Co and Fe impurities, which compensate the n-type dopants in the underlying n-(Al, Ga) As layer. Despite such compensation, spin-LEDs with the Co2FeSi layer grown at the relatively high temperature of 280 °C exhibit the highest spin injection efficiencies of more than 50%, hence calling into question the requirement of electron tunneling through the ferromagnet/semiconductor Schottky barrier for efficient spin injection.

Analyzing the measured phase in the multichannel Aharonov–Bohm interferometer

We address the quantum dot phase measurement problem in an open Aharonov–Bohm interferometer, assuming multiple transport channels. In such a case, the quantum dot is characterized by more than one intrinsic phase for the electrons transmission. It is shown that the phase which would be extracted by the usual experimental method (i.e. by monitoring the shift of the Aharonov–Bohm oscillations, as in Schuster et al., Nature 385 (1997) 417) does not coincide with any of the dot intrinsic phases, but is a combination of them. The formula of the measured phase is given. The particular case of a quantum dot containing a S = 1 2 spin is discussed and variations of the measured phase with < π are found, as a consequence of the multichannel transport.

First-Principle Calculations of Multi-Channel Spin-Polarized Transport in Atomic-Size Barrier

A multi-channel tunneling theory through a realistic hetero-junction barrier was developed using a mixed representation and studied within a few modes model. The transfer-matrix technique was employed to calculate spin-polarized transmission and reflection amplitudes between Bloch states in the semi-infinite leads. The transmission coefficients were used in Fisher and Lee’s formula for conductance. The multi-tunneling theory was applied within a real Fe/MgO/Fe hetero-junction barrier.

First principle calculation of potential and multi-channel transport in Fe/MgO/Fe hetero-junction

In this work we studied the behavior of multi-channel tunneling through a realistic Fe/MgO/Fe hetero-junction ab initio potential. For this purpose we obtained Fe/MgO/Fe(001) hetero-junction effective potentials using the full potential-linear augmented plane wave WIEN2k code together with the generalized gradient approximation for the exchange-correlation potential. Multi-channel tunneling through the Fe/MgO/Fe hetero-junction barrier was studied within a few modes model. The transfer matrix technique was used to calculate the transmission and reflection amplitudes between Bloch states in the semi-infinite leads. The transmission coefficients were used in Fisher and Lee formula for conductance, an expression derived from Kubo formula and valid for any number of scattering channels.

Coherent carrier transport and scattering by lattice defects in single- and multibranch carbon nanoribbons

We introduce a theoretical model for the investigation of the scattering properties by defects in semiconducting carbon nanoribbons. The analysis of multichannel coherent transport in the presence of discontinuities is performed by means of a numerical solver, which was previously shown to correctly predict the electronic properties of carbon nanotubes, in some respects analogous to the nanoribbon. A scattering-matrix representation has been used to characterize the propagation and scattering of the periodic wave functions of carriers traveling along the carbon nanoribbon. Results show that reflection of charge is important for the defect types considered, i.e., atomic vacancy, potential deformation, and discontinuity caused by a $120\ifmmode^\circ\else\textdegree\fi{}$ bifurcation. Reflection may be enhanced by interaction (resonance) between successive or periodic discontinuities. The dependence of scattering on carrier energy, referred to the band edge, has also been investigated: Generally, reflection decreases as carrier energy increases. The analysis suggests that the operation of devices employing carbon nanoribbons, such as nanotransistors, may be quite affected by the presence of lattice defects.

Fluctuation Theorem for Channel-Facilitated Membrane Transport of Interacting and Noninteracting Solutes

In this paper, we discuss the fluctuation theorem for channel-facilitated transport of solutes through a membrane separating two reservoirs. The transport is characterized by the probability, P(n)(t), that n solute particles have been transported from one reservoir to the other in time t. The fluctuation theorem establishes a relation between P(n)(t) and P-(n)(t): The ratio P(n)(t)/P-(n)(t) is independent of time and equal to exp(nbetaA), where betaA is the affinity measured in the thermal energy units. We show that the same fluctuation theorem is true for both single- and multichannel transport of noninteracting particles and particles which strongly repel each other.

Multi-channel spintronic transistor design based on magnetoelectric barriers andspin–orbital effects

We present a spin transistor design based on spin–orbital interactions in a two-dimensionalelectron gas, with magnetic barriers induced by a patterned ferromagnetic gate. Theproposed device overcomes certain shortcomings of previous spin transistor designs such aslong device length and degradation of conductance modulation for multi-channel transport.The robustness of our device for multi-channel transport is unique in spin transistordesigns based on spin–orbit coupling. The device is more practical in fabricationand experimental respects compared to previously conceived single-mode spintransistors.

A simple drain current model for Schottky-barrier carbon nanotube field effecttransistors

We report on a new computational model to efficiently simulate carbon nanotube-basedfield effect transistors (CNT-FET). In the model, a central region is formed by asemiconducting nanotube that acts as the conducting channel, surrounded by a thin oxidelayer and a metal gate electrode. At both ends of the semiconducting channel, twosemi-infinite metallic reservoirs act as source and drain contacts. The current–voltagecharacteristics are computed using the Landauer formalism, including the effect of theSchottky barrier physics. The main operational regimes of the CNT-FET are described,including thermionic and tunnel current components, capturing ambipolar conduction,multichannel ballistic transport and electrostatics dominated by the nanotubecapacitance. The calculations are successfully compared to results given by moresophisticated methods based on non-equilibrium Green’s function formalism (NEGF).

Spin polarization in a T-shaped conductor induced by strong Rashba spin-orbit coupling

We investigate numerically the spin polarization of the current in the presence of Rashba spin-orbit interaction in a T-shaped conductor proposed by A.A. Kiselev and K.W. Kim (Appl. Phys. Lett. {\bf 78} 775 (2001)). The recursive Green function method is used to calculate the three terminal spin dependent transmission probabilities. We focus on single-channel transport and show that the spin polarization becomes nearly 100 % with a conductance close to $e^{2}/h$ for sufficiently strong spin-orbit coupling. This is interpreted by the fact that electrons with opposite spin states are deflected into an opposite terminal by the spin dependent Lorentz force. The influence of the disorder on the predicted effect is also discussed. Cases for multi-channel transport are studied in connection with experiments.