Transport properties of AB-stacked bilayer graphene nanoribbons in an electric field

The electronic and thermal properties of AB-stacked bilayer graphene nanoribbons subject to the influences of a transverse electric field are investigated theoretically, including their transport properties. The dispersion relations are found to exhibit a rich dependence on the interlayer interactions, the field strength, and the geometry of the layers. The interlayer coupling will modify the subband curvature, create additional band-edge states, change the subband spacing or energy gap, and separate the partial flat bands. The bandstructures will be symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The inclusion of a transverse electric field will further alter the bandstructures and lift the degeneracy of the partial flat bands. The chemical-potential-dependent electrical and thermal conductance exhibit a stepwise increase behavior. Variations in the electronic structures with field strength will be reflected in the electrical and thermal conductance. Prominent peaks, as well as single-shoulder and multi-shoulder structures in the electrical and thermal conductance are predicted when varying the electric field strength. The features of the conductance are found to be strongly dependent on the field strength, the geometry, interlayer interactions and temperature.     

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.     

Electron transport in graphene nanoribbons under a central potential modulation

We study electron transport in graphene nanoribbons (GNs) under a one-dimensional central potential. Two kinds of structures are considered: armchair-edged GN (AGN) where a central potential is applied on the central dimer chain, and zigzag-edged GN (ZGN) where a potential is applied on the central zigzag chain. Both nanoribbons show unique electronic structure and interesting transport properties under potential modulation. Without considering edge effect, a metallic AGN is still metallic after potential modulation. However, if the edge effect is considered, an AGN under potential modulation can be either semiconducting or metallic depending on the potential strength. The AGN transits from semiconducting to metallic and then to semiconducting again with the potential increase. As to ZGN, the potential on the central zigzag chain also greatly affect the quantum conductance. A ZGN will transit from metallic to semiconducting as the potential strength exceeds critical value. All transitions of AGN and ZGN are correlated with the localized state on the central potential chain which induces a quantum channel along the chain.     

Magnetoconductance of graphene nanoribbons

The electronic and transport properties of monolayer and AB-stacked bilayer zigzag graphene nanoribbons subject to the influences of a magnetic field are investigated theoretically. We demonstrate that the magnetic confinement and the size effect affect the electronic properties competitively. In the limit of a strong magnetic field, the magnetic length is much smaller than the ribbon width, and the bulk electrons are confined solely by the magnetic potential. Their properties are independent of the width, and the Landau levels appear. On the other hand, the size effect dominates in the case of narrow ribbons. In addition, the dispersion relations rely sensitively on the interlayer interactions. Such interactions will modify the subband curvature, create additional band-edge states, change the subband spacing or the energy gap, and separate the partial flat bands. The band structures are symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The chemical-potential-dependent electrical and thermal conductance exhibits a stepwise increase behaviour. The competition between the magnetic confinement and the size effect will also be reflected in the transport properties. The features of the conductance are found to be strongly dependent on the field strength, number of layers, interlayer interactions, and temperature.     

Effect of tubular chiralities of single-walled ZnO nanotubes on electronic transport

The electronic transport properties of single-walled ZnO nanotubes with different chiralities are investigated by nonequilibrium Green's function combined with density functional theory. In this paper we consider three representative ZnO nanotubes, namely (3, 3) armchair, (5, 0) zigzag, and (4, 2) chiral, with a similar diameter of about 5.4 Å. Short nanotubes exhibit good conductance behavior. As the tube length increases, the conductance decreases at low bias and the nanotubes indicate semiconducting behavior. The current-voltage characteristics of the nanotubes longer than 3 nm depend weakly on the length of the tubes. The armchair and chiral ZnO nanotubes with the same length and diameter have almost overlapped current-voltage curves. The electron transport behaviors are analyzed in terms of the transmission spectra, density of states and charge population of these nanotubes. The results indicate that the resonant peaks above the Fermi level are responsible for electric currents. However, the zigzag ZnO nanotubes exhibit asymmetric current-voltage curves attributed to the built-in polarization field and give larger current than the armchair and chiral nanotubes at the same bias. The features explored here strongly suggest that the ZnO nanotubes are stable, flexible structures, which are valuable in Nano-Electromechanical System.     

曲率效应对超小碳纳米管电子性质的影响

在考虑曲率效应的情况下,在螺旋坐标系下解析地推导了非手性的碳纳米管(SWNTs)(包括扶手椅型和锯齿型)的能量色散关系,并分析了曲率效应对超小扶手椅型SWNTs的能带、能隙和导电能力及其对超小锯齿型SWNTs(包括扶手椅型和锯齿型)的能隙的影响.     

Stochastic method for in-situ damage analysis

We are going to apply dissipative energy method, considered as perturbation method, in order to investigate the effects of electron-optical phonon coupling on the electronic transport of armchair single wall carbon nanotubes. This method almost deals with the modeling of the behavior of electrons near ballistic regime. The results of calculations indicate that this model can be applied in estimating the current and the differential conductance of the armchair single-wall carbon nanotubes at low bias; however the perturbation method fails to reproduce the current and differential conductance at high voltages. Furthermore, this approach suggests a method that the observation of phonon energy modes involved in electron-phonon coupling becomes possible experimentally at low temperature.     

Strain effect on transport properties of hexagonal boronben nitride nanoribbons

The transport properties of hexagonal boron--nitride nanoribbons under the uniaxial strain are investigated by the Green's function method. We find that the transport properties of armchair boron--nitride nanoribbon strongly depend on the strain. In particular, the features of the conductance steps such as position and width are significantly changed by strain. As a strong tensile strain is exerted on the nanoribbon, the highest conductance step disappears and subsequently a dip emerges instead. The energy band structure and the local current density of armchair boron--nitride nanoribbon under strain are calculated and analysed in detail to explain these characteristics. In addition, the effect of strain on the conductance of zigzag boron--nitride nanoribbon is weaker than that of armchair boron nitride nanoribbon.     

Electronic properties of Z-shaped graphene nanoribbon under uniaxial strain

Based on tight-binding approximation and a generalized Green's function method, the effect of uniaxial strain on the electron transport properties of Z-shaped graphene nanoribbon (GNR) composed of an armchair GNR sandwiched between two semi-infinite metallic armchair GNR electrodes is numerically investigated. Our results show that the increase of uniaxial strain enhances the band gap and leads to a metal-to-semiconductor transition for Z-shaped GNR. Furthermore, in the Landauer–Büttiker formalism, the current–voltage characteristics, the noise power resulting from the current fluctuations and Fano factor of strained Z-shaped GNR are explored. It is found the threshold voltage for the current and the noise power increased so that with reinforcement of the uniaxial strain parameter strength, the noise power goes from the Poisson limit to sub-Poisson region at higher bias voltages.     

Electronic transport properties of metallic graphene nanoribbons with two vacancies

We studied theoretically the electronic transport of metallic graphene nanoribbons (GNRs) with two vacancies using the tight-binding model and Green’s function method. The results show that the conductance of zigzag GNR (ZGNR) varies with the relative position of two vacancies. However, when two vacancies reside on the edges, the conductance remain unchanged compared to that of perfect GNRs due to the interaction between vacancy state and edge state. Moreover, the conductance at the Fermi level for armchair GNR (AGNR) can be zero or finite depending on the position of vacancies on the GNRs. The demonstrated features of electronic transport open extremely attractive perspectives for designing well-defined GNR-based nanoelectronic devices.     

Band structures of carbon nanotube with spin-orbit coupling interaction

We explore the band structures of single-walled carbon nanotubes (SWCNTs) with two types of spin-orbit couplings. The obtained results indicate that weak Rashba spin-orbit coupling interaction can lead to the breaking of four-fold degeneracy in all tubes even though without the intrinsic SO coupling. The asymmetric splitting between conduction bands and valence bands is caused by both SO couplings at the same time. When the ratio of Rashba spin-orbit coupling to the intrinsic spin-orbit coupling is larger than 3, metallic zigzag nanotube is always metallic conductor, on the contrary it becomes semiconducting properties. However, only when this ratio is equal to about 3 or the intrinsic spin-orbit coupling is much weak, the metallic armchair nanotube still holds the metallic behavior in transport.     

Equipartition of current in metallic armchair nanoribbon of graphene-based device

We numerically investigate the mesoscopic electronic transport properties of Bernal-stacked bilayer/trilayer graphene connected with four monolayer graphene terminals. In armchair-terminated metallic bilayer graphene, we show that the current from one incoming terminal can be equally partitioned into other three outgoing terminals near the charge-neutrality point, and the conductance periodically fluctuates, which is independent of the ribbon width but influenced by the interlayer hopping energy. This finding can be clearly understood by using the wave function matching method, in which a quantitative relationship between the periodicity, Fermi energy, and interlayer hopping energy can be reached. Interestingly, for the trilayer case, when the Fermi energy is located around the charge-neutrality point, the fractional quantized conductance 1/(4e²h) can be achieved when system exceeds a critical length.     

Role of symmetry in the transport properties of graphene nanoribbons under bias

The intrinsic transport properties of zigzag graphene nanoribbons (ZGNRs) are investigated using first-principles calculations. It is found that although all ZGNRs have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.     

Electronic properties of MoS2 nanoribbons with disorder effects

A theoretical study of electronic properties on MoS₂ nanoribbon is made on focusing the calculation of zero bias transport in the presence of disorders. Disorders including intrinsic and extrinsic vacancies and also weak uniform scatter defects are considered. The calculations are based on the tight-binding Green's function formalism by including an iterative procedure. The Slater–Koster transformations are used to determine the parameters. This model reduces the numerical calculation time. The unsaturated atoms at the edge of armchair (zigzag) ribbon induce some mid-gap states with nearly high (low) localization, which act as scattering centers. The antiresonances of created quasi-localized states due to vacancy cause the conductance of the armchair nanoribbon to decrease. Finally, the zigzag ribbon provides the highest sensitivity as well as selectivity between the smaller energy range, in the presence of the single weak scatter with potential value of 2 eV at the edge of the ribbon.     

外场作用下铁电/铁磁双层膜的极化磁化性质

建立了铁电/铁磁双层膜模型,铁电层的电矩用连续标量描述,而铁磁层的自旋应用经典矢量描述.利用蒙特卡罗方法模拟了体系的热力学性质和极化、磁化行为.给出了零场下体系的内能、比热、极化和磁化随温度变化的关系,并分别研究了体系在外磁场和外电场下的极化和磁化行为.模拟结果表明,双层膜体系的内能、比热、极化和磁化性质因层间耦合系数的不同而明显不同,当界面耦合较弱时,双层膜表现出各自的热力学性质,当层间耦合增强到一定程度时,双层膜耦合为一个整体,表现出统一的热力学性质.该双层膜在外场中形成电滞回线和磁滞回线,并表现出偏置特性,界面耦合强度和温度影响滞后回线和偏置现象.     

Resonance Transport of Graphene Nanoribbon T-Shaped Junctions

We investigate the transport properties of T-shaped junctions composed of armchair graphene nanoribbons of different widths. Three types of junction geometries are considered. The junction conductance strongly depends on the atomic features of the junction geometry. When the shoulders of the junction have zigzag type edges, sharp conductance resonances usually appear in the low energy region around the Dirac point, and a conductance gap emerges. When the shoulders of the junction have armchair type edges, the conductance resonance behavior is weakened significantly, and the metal-metal-metal junction structures show semimetallic behaviors. The contact resistance also changes notably due to the various interface geometries of the junction.     

Spin-dependent delay time and Hartman effect in asymmetrical graphene barrier under strain

We study the spin-dependent tunneling time, including group delay and dwell time, in a graphene based asymmetrical barrier with Rashba spin–orbit interaction in the presence of strain, sandwiched between two normal leads. We find that the spin-dependent tunneling time can be efficiently tuned by the barrier width, and the bias voltage. Moreover, for the zigzag direction strain although the oscillation period of the dwell time does not change, the oscillation amplitude increases by increasing the incident electron angle. It is found that for the armchair direction strain unlike the zigzag direction the group delay time at the normal incidence depends on the spin state of electrons and Hartman effect can be observed. In addition, for the armchair direction strain the spin polarization increases with increasing the RSOI strength and the bias voltage. The magnitude and sign of spin polarization can be manipulated by strain. In particular, by applying an external electric field the efficiency of the spin polarization is improved significantly in strained graphene, and a fully spin-polarized current is generated.     

Conductance of metallic nanoribbons with defects

The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green’s function method. The weak scattering impurities, U ～ 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons (150 nm). For the narrow armchair graphene nanoribbons, the conductance at the Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U > 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.     

Theory of activated transport in bilayer quantum Hall systems

We analyze the transport properties of bilayer quantum Hall systems at total filling factor nu=1 in drag geometries as a function of interlayer bias, in the limit where the disorder is sufficiently strong to unbind meron-antimeron pairs, the charged topological defects of the system. We compute the typical energy barrier for these objects to cross incompressible regions within the disordered system using a Hartree-Fock approach, and show how this leads to multiple activation energies when the system is biased. We then demonstrate using a bosonic Chern-Simons theory that in drag geometries current in a single layer directly leads to forces on only two of the four types of merons, inducing dissipation only in the drive layer. Dissipation in the drag layer results from interactions among the merons, resulting in very different temperature dependences for the drag and drive layers, in qualitative agreement with experiment.     

Conductance of bilayer graphene nanoribbons with different widths

The electronic and transport properties of bilayer graphene nanoribbons with different width are investigated theoretically by using the tight-binding model. The energy dispersion relations are found to exhibit significant dependence on the interlayer interactions and the geometry of the bilayer graphene nanoribbons. The energy gaps are oscillatory with the upper ribbon displacement. For all four types of bilayer graphene nanoribbons, the bandgaps touch the zero value and exhibit semiconductor–metal transitions. Variations in the electronic structures with the upper ribbon displacement will be reflected in the electrical and thermal conductance. The chemical-potential-dependent electrical and thermal conductances exhibit a stepwise increase and spike behavior. These conductances can be tuned by varying the upper ribbon displacement. The peak and trench structures of the conductance will be stretched out as the temperature rises. In addition, quantum conductance behavior in bilayer graphene nanoribbons can be observed experimentally at temperature below 10 K.