Nonequilibrium Green Function Method Research Articles

Transport properties and typical electron orbits in 2DEG under a local gradient magnetic field

Local current information in nanodevices is very helpful for good understanding the fundamental physical phenomena. In this work, the electron motion in 2DEG under a local gradient magnetic field is studied by the recursive non-equilibrium green's function method. The considered structure is a sandwiched configuration, lead-device-lead. For clearly observing the electron trajectories, we use the wide and narrow leads to contact the device respectively. The results show that in wide lead case, the quantized conductance plateaus with oscillations are found. These oscillations might be related to the resonant transmission in longitudinal electronic states. We also observe several typical snake electron orbits when the corresponding incident energy locates at the odd conductance plateaus, because there is exactly one opened transport channel around the horizontal middle line of device. When we shrink the width of leads and let electrons inject only from the lower half of device, the conductance suppression is very significant due to the energy level broadening in center device. In this case, we can find clear S-shape and 8-shape electron orbits at a very wide range of energy. These results can help us to understand the basic physics in 2D electron systems and may guide the design of future photoelectric device.

Graphenylene nanoribbons: electronic, optical and thermoelectric properties from first-principles calculations

Recently synthesized two-dimensional graphene-like material referred to as graphenylene is a semiconductor with a narrow direct bandgap that holds great promise for nanoelectronic applications. The significant bandgap increase can be provided by the strain applied to graphenylene crystal lattice or by using nanoribbons instead of extended layers. In this paper, we present the systematic study of the electronic, optical and thermoelectric properties of graphenylene nanoribbons using calculations based on the density functional theory. Estimating the binding energies, we substantiate the stability of nanoribbons with zigzag and armchair edges passivated by hydrogen atoms. Electronic spectra indicate that all considered structures could be classified as direct bandgap semiconductors. From the calculated dependence of bandgap on nanoribbon width we observe the identical scaling rule for armchair and zigzag graphenylene ribbons. A family-based classification used for the electronic structure of armchair graphene nanoribbons can not be extended to the case of graphenylene ones. The absorption coefficient, optical conductivity, and complex refractive index are calculated by means of the first-principles methods and the Kubo–Greenwood formula. It has been shown that graphenylene ribbons effectively absorb visible-range electromagnetic waves. Due to this absorption the conductivity is noticeably increased in this range. The transport coefficients and thermoelectric figure of merit are calculated by the nonequilibrium Green functions method. Summarizing the results, we discuss the possible use of graphenylene films and nanoribbons in nanoelectronic devices.

Large rectification ratio induced by nitrogen and boron doping in adjacent armchair graphene nanoribbons

The rectification behavior of two and three separated armchair graphene nanoribbons (AGNRs) which one of them plays the role of channel and others play the role of side gates is investigated. The tight-binding density functional approach (DFTB) and non-equilibrium Green function method (NEGF) are employed for calculations. The nitrogen (N) and boron (B) atoms are used to dope the side gates. The results show that these devices have low threshold voltage and their rectification ratios are higher than one doped AGNR device, with the maximum value of 3.18 × 108. Also, without doping the channel, it can become to n-type or p-type semiconductor, only by doping the side gates. The current-voltage characteristics of these devices are similar and have little dependence on side gates width.

Anisotropy of the spin-polarized edge current in monolayer transition metal dichalcogenide zigzag nanoribbons

We report anisotropic spin polarization of edge currents in ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$ monolayer zigzag nanoribbons (ZNRs) deposited on either nonmagnetic or ferromagnetic insulator substrates. We employ an 11-band tight-binding model to calculate the electronic band structures of transition metal dichalcogenide (TMDC) monolayers and their corresponding nanoribbons in the presence of Rashba spin-orbit coupling (RSOC) and magnetic proximity effect produced by ferromagnetic substrate. We adopt the nonequilibrium Green's function method together with Landauer-B\uttiker formalism to study the quantum transport behavior stemming from the edge states of ZNRs. We demonstrate that the spin-polarized edge current can be generated in both ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$ ZNRs with RSOC. We find that the spin polarization spreads out in all three directions. This is in stark contrast to what occurs in zigzag graphene nanoribbons, for which the polarization only exists in the transverse direction (across the width of ribbons). In addition, the spin polarization direction strongly depends on the strength of the intrinsic SOC component. The interplay of Rashba and intrinsic SOC is crucial for the spin polarization of the currents in any spatial direction. For TMDCs with stronger intrinsic SOC such as in ${\mathrm{WS}}_{2}$ monolayer ZNRs, we observe that the spin polarization along the perpendicular direction to the plane of the ZNR can be as large as $90%$. Moreover, the unusual anisotropy of the spin polarization can be further enhanced by the magnetic proximity effect. These results open up possibilities for the generation of tunable high-spin polarization currents in ZNRs without application of an external magnetic field.

Electronic Properties of the Interface Between Metallic Doped Zigzag Graphene and Pristine Graphene Nanoribbons

The electronic properties of the interface between metallic doped zigzag graphene nanoribbons and perfect graphene nanoribbons are investigated. The density functional based tight-binding approach and a non-equilibrium Green function method are employed for our calculations. The atoms of Ni, Co, and Fe are used as doping atoms. The graphs of current–voltage, density of states, electron density, transmission spectrum, and rectification ratio are obtained. The results show that the bond lengths around the atoms of Ni, Co, and Fe are increased. Moreover, it is observed that for the graphene-based nanodevice the curves of current–voltage are linear and symmetric when no impurity exists and with the effect of impurities the curves are non-linear and asymmetric. While Ni, Co, and Fe impurities are applied into the systems we found that the maximum electron densities are located around the impurities of Ni and the current is decreased. The density of states and transmission spectrum are also examined for different systems. It is found that for certain amount of energies some resonances occur for the current, and the atoms at the edge of nanoribbon are mostly responsible for the transfer of the electrons. The obtained results can be of interest for the construction of nanoelectronic devices and can have practical applications.

Angular momentum radiation from current-carrying molecular junctions

We consider the radiation of angular momentum (AM) from current-carrying molecular junctions. Using the nonequilibrium Green's function method, we derive a convenient formula for the AM radiation and apply it to a prototypical benzene molecule junction. We discuss the selection rules for inelastic transitions between the molecular angular momentum eigenstates due to a 6-fold rotational symmetry. Our study provides important insights into the generation of light with AM from DC-biased molecular junctions.

Influence of edge passivation on the transport properties of the zigzag phosphorene nanoribbons

By using first-principles calculation based on density functional theory and non-equilibrium Green's function method, we investigate the transport properties of zigzag phosphorene nanoribbons (ZPNRs). The edges of the ZPNRs can be passivated in three ways named W1, W2, W3. These calculated results show that the electronic transport properties of the ZPNRs can be seriously influenced by the edge passivation ways, and the transport is determined by both the two edges and the interaction between them. Moreover, we find the width of the ZPNR can switch on or switch off the transport channel of the W3-type ZPNR. Furthermore, we present the transmission spectra, the band structures of both left and right electrodes, the molecular energy levels, and transmission eigenstates of the H-S-passivated W3-type ZPNRs to uncover the transport mechanism. This study provides a theoretical support for designing the related nanodevices by changing the passivation ways, which is an effective route for tuning the electronic structures and the transport properties of the phosphorene nanoribbons.

Stacking order driving bandgap and conductance of graphene/C3B (C3N) van der Waals heterostructures

Constructing a van der Waals (vdW) heterostructure is a promising approach to tackle the bandgap hurdle of graphene meanwhile preserving its excellent electronic properties. The unique symmetry of atomic arrangement in C3B and C3N monolayers could induce interesting properties in the graphene/C3B (C3N) heterostructure. Based on the density functional theory calculation, we demonstrate that the bandgap near the Dirac cone depends sensitively on the stacking order in the graphene/C3B (C3N) vdW heterostructure. The bandgap is opened in the AB stacking order, but is completely closed in the AA stacking order. The bandgap closing is protected by the AA stacking order even under vertical strain, while the bandgap increases in the AB stacking order with a positive vertical strain. Based on the tight-binding model, the origin of the bandgap opening and closing is explained by the stacking-order dependent charge transfer. Moreover, the transport properties of the devices assembled by the graphene/C3B vdW heterostructures are investigated based on the non-equilibrium Green's function method plus the density functional theory. The conductance can be turned on or off depending on the stacking order. This finding is useful for the future design of on/off devices based on graphene/C3B (C3N) heterostructures.

Effect of wave versus particle phonon nature in thermal transport through nanostructures

Comprehensive understanding of thermal transport in nanostructured materials needs large scale simulations bridging length scales dictated by different physics related to the wave versus particle nature of phonons. Yet, available computational approaches implicitly treat phonons as either just waves or as particles. In this work, using a full wave-based Non-Equilibrium Green's Function (NEGF) method, and a particle-based ray-tracing Monte Carlo (MC) approach, we investigate the qualitative differences in the wave and particle-based phonon transport at the vicinity of nanoscale features. For the simple example of a nanoporous geometry, we show that phonon transmission agrees very well for both methods with an error margin of ±15%, across phonon wavelengths even for features with sizes down to 3–4 nm. For cases where phonons need to squeeze in smaller regions to propagate, we find that MC underestimates the transmission of long wavelength phonons whereas wave treatment within NEGF indicates that those long wavelength phonons can propagate more easily. We also find that particle-based simulation methods are somewhat more sensitive to structural variations compared to the wave-based NEGF method. The insight extracted from comparing wave and particle methods can be used to provide a better and more complete understanding of phonon transport in nanomaterials.

Theoretical study of nitrogen, boron, and co-doped (B, N) armchair graphene nanoribbons.

The electronic properties of the graphene nanoribbons (GNR) with armchair chirality were studied using the density functional theory (DFT) combined with non-equilibrium green's function method (NEGF) formalism. The role of donor and acceptor dopants of nitrogen and boron was studied separately and also in the situation of co-doping. The charge density, electronic density of states (DOS), and transmission coefficient at different bias voltages are presented for comparison between pure and doped states. It was found that this doping plays the main role in the distortion of the GNR lattices for cases of B and N as it affects straightly on the DOS and transmission coefficient of the systems under study. The band structure of edge was engineered by differently selecting the doping positions of B, N, and B-N hexagonal rings and it was found that there are significant changes in the electronic properties of these systems due to doping. This study can be used for developing GNR device based on doping B and N atoms.

Non-Equilibrium Green’s Function Modeling of the Transport Characteristics of Magnetic Tunnel Junctions Containing Anderson Impurities

A nonequilibrium Green’s function (NEGF) formulation to study the transport characteristics of magnetic tunnel junctions (MTJs) that contains impurities at the barrier region and many-body interaction at the electrode region is presented. The formulation makes use of the already developed NEGF method for MTJs without any impurities and the impurity Green’s function is obtained using Haldane’s approach that explicitly takes into account the on-site Coulomb interaction ([Formula: see text]) of the impurity. The formulation is used to obtain the spin-dependent tunnel current of model MTJs as a function of the applied bias for different values of [Formula: see text] corresponding to both the parallel and antiparallel configuration of the end electrodes of the MTJ. The tunnel currents are used to obtain the tunnel magnetoresistance (TMR) of the MTJ and we observed from our study that the TMR is strongly influenced by the impurities. The TMR is found to be reduced compared to that of the MTJ without impurities and the bias dependence of the TMR is found to be strongly influenced by [Formula: see text]. Our studies revealed that the MTJ can exhibit almost completely spin polarized current at certain values of [Formula: see text].

Achieving the Scaling Limit for Nonequilibrium Green Functions Simulations.

The dynamics of strongly correlated fermions following an external excitation reveals extremely rich collective quantum effects. Examples are fermionic atoms in optical lattices, electrons in correlated materials, and dense quantum plasmas. Presently, the only quantum-dynamics approach that rigorously describes these processes in two and three dimensions is the nonequilibrium Green functions (NEGF) method. However, NEGF simulations are computationally expensive due to their T^{3} scaling with the simulation duration T. Recently, T^{2} scaling was achieved with the generalized Kadanoff-Baym ansatz (GKBA), for second-order Born (SOA) selfenergies, which has substantially extended the scope of NEGF simulations. Here we demonstrate that GKBA-NEGF simulations can be performed with order T^{1} scaling, both for SOA and GW selfenergies, and point out the remarkable capabilities of this approach.

Plateaus of quantized conductance with high steps in topological nodal-line semimetals

Topological nodal-line semimetals (TNLSMs) are a new state of quantum matter. It has one-dimensional energy band intersections where the conduction band and valence band cross to form one or more nodal rings. Here, based on the Landauer-B\"uttiker formula combined with the nonequilibrium Green's function method, we investigate the electron transport through TNLSM nanowires under a perpendicular magnetic field. We theoretically calculate the conductance of TNLSM nanowires and find that it is essentially different from other topological and trivial materials. The conductance of TNLSM nanowires presents the high plateaus with high steps in multiples of ${e}^{2}/h$. The height of the steps is determined by the size of the nanowire and for a TNLSM nanowire with the size of several tens of nanometers, its conductance plateaus can reach hundreds of ${e}^{2}/h$ with the step height being tens of ${e}^{2}/h$. The unique phenomenon of the high-stepped conductance plateaus originates from the peculiar energy spectrum of the TNLSMs, that is, there are a lot of subbands having the same energy minimum value. Furthermore, the high conductance plateaus and the high steps can well remain in the existence of magnetic fields, the moderate disorder, and the PT-symmetry-breaking mass term. This unique conductance characteristic can serve as a reliable signature of TNLSMs in experimental detections.

Electron-Transport Characteristics through Aluminum Oxide (100) and (012) in a Metal-Insulator-Metal Junction System: Density Functional Theory-Nonequilibrium Green Function Approach.

Al2O3 is commonly used in modern electronic devices because of its good mechanical properties and excellent electrical insulating property. Although fundamental understanding of the electron transport in Al2O3 is essential for its use in electronic device applications, a thorough investigation for the electron-transport mechanism has not been conducted on the structures of Al2O3, especially in nanometer-scale electronic device settings. In this work, electron transport via Al2O3 for two crystallographic facets, (100) and (012), in a metal–insulator–metal junction configuration is investigated using a density functional theory-based nonequilibrium Green function method. First, it is confirmed that the transmission function, T(E), decreases as a function of energy in (E – EF) < 0 regime, which is an intuitively expected trend. On the other hand, in the (E – EF) > 0 regime, Al2O3(100) and Al2O3(012) show their own characteristic behaviors of T(E), presenting that major peaks are shifted toward lower energy levels under a finite bias voltage. Second, the overall conductance decay rates under zero bias are similar regardless of the crystallographic orientation, so that the contact interface seemingly has only a minor contribution to the overall conductance. A noteworthy feature at the finite bias condition is that the electrical current drastically increases as a function of bias potential (>0.7 V) in Al2O3(012)-based junction compared with the Al2O3(100) counterpart. It is elucidated that such a difference is due to the well-developed eigenchannels for electron transport in the Al2O3(012)-based junction. Therefore, it is evidently demonstrated that at finite bias condition, the contact interface plays a key role in determining insulating properties of Al2O3–Pt junctions.

Influence of antichiral edge states on Andreev reflection in graphene-superconductor junction

Using the tight binding model and the non-equilibrium Green function method, we study Andreev reflection in graphene-superconductor junction, where graphene has two nonequal Dirac Cones split in energy and therefore time reversal symmetry is broken. Due to the anti-chiral edge states of the current graphene model, an incident electron travelling along the edges makes distinct contribution to Andreev reflections. In a two-terminal device, because Andreev retro-reflection is not allowed for just the anti-chiral edges, in this case the mutual scattering between edge and bulk states is necessary, which leads that the coefficient of Andreev retro-reflection is always symmetrical about the incident energy. In a four-terminal junction, however, the edges are parallel to the interface of superconductor and graphene, so at the interface an incident electron travelling along the edges can be retro-reflected as a hole into bulk modes, or specularly reflected as a hole into anti-chiral edge states again. It is noted that, the coefficient of specular Andreev reflection keeps symmetric as to the incident energy of electron which is consistent with the reported results before, however the coefficient of Andreev retro-reflection shows an unexpected asymmetrical behavior due to the presence of anti-chiral edge states. Our results present some new ideas to study the anti-chiral edge modes and Andreev reflection for a graphene model with the broken time reversal symmetry.

The spin-dependent transport properties of endohedral transition-metal-fullerene X@C66H4 (X=Fe, Co, Mn, Ni)

Inspired by recent experiments on the successful synthesis of hydrofullerene C66H4 in Tian et al. (2019) [12] with two negatively curved heptagons. Based on the density functional theory and nonequilibrium Green's function method, we report the spin-dependent transport through transition-metal-atom-encapsulated C66H4 hydrofullerene, i.e., X@C66H4(X=Fe, Co, Mn, Ni), contacted by single gold atoms via semi-infinite non-magnetic Au electrodes. It is found that, Mn- and Fe-doped systems show highly spin-polarized transmission as well as considerable magnetic moments whereas Ni-doped systems show completely spin-unpolarized transmission and nonmagnetic. Interestingly, Co-doped systems show two spin states, i.e., spin-polarized and spin-unpolarized ones. Further analysis shows that, for Mn-, Fe- and Ni-doped systems, the spin-polarized/unpolarized state is caused by the finite/(nearly-)zero magnetism of the encapsulated metal atom. While the magnetism in Co-doped systems is quenched for the top hexagonal doping case, but not for the side heptagonal doping one, which induces the spin-unpolarized/spin-polarized state. And the screening effect of electrodes on the magnetism of Co is the underlying physical mechanism. Our findings would be beneficial to the design of spintronics devices.

Об особенностях электронного транспорта в наноустройстве на основе молекулы, содержащей окислительно-восстановительный центр нитроамина

Within the framework of the density functional theory in the local density approximation and the nonequilibrium Green function method (DFT + NEGF), electron transport was studied in a nanodevice consisting of a 2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol molecule placed between gold electrodes. Current-voltage, dI/dV-characteristics, transmission spectrum and electron density of a nanodevice are calculated. It is shown that the current-voltage characteristic of the considered nanodevice in the voltage range of -0.8÷0.9 V acquires an N-shape and appears on it a section with negative differential resistance due to resonant tunneling of quasiparticles. The same changes are observed on the dI/dV-characteristic. The results obtained may be useful for calculating new promising electronic switching devices.

Analytical Current Transport Modeling of Monolayer Molybdenum Disulfide-Based Dual Gate Tunnel Field Effect Transistor

In this paper, we present a current transport model of MoS2-based dual gate tunnel field effect transistor from both analytical and numerical techniques. In analytical modeling, two-dimensional (2D) model of potential distribution with its corresponding energy level for the hetero-junction has been solved and implemented with Landauer current transport mechanism. The output characteristics have been compared with the quantum mechanical solution obtained from non-equilibrium Green's function (NEGF) formalism. The sub-threshold slope (SS), obtained from analytical and NEGF methods are 10 mV/decade and 12.6 mV/decade with on/off current ratio of 107 and 106, respectively at VDS = 0.5 V. We have considered scattering mechanism into our calculation and have shown that carrier transportation for 20 nm channel length is diffusive. In addition, scattering reduces the on current by an order of magnitude, and resembles good agreement with the atomistic simulation. Lastly, mobility and ionized impurity concentration for our proposed structure have been calculated as 27.2 cm2/Vs and 1.17 × 1013/cm2.

The reservoir area dependent thermal transport at the nanoscale interface.

The reservoir area dependent thermal transport at nanoscale two-dimensional and one-dimensional interfaces is investigated by the non-equilibrium Green's function method. For the two-dimensional nanoscale interface composed of graphene sheets, the reservoir area is identical to the contact area S at the interface. As S increases from few atoms, the interfacial thermal conductance σ per S (Λ = σ/S) is negatively dependent on S due to the decrease of phonon transmission per S. With S increasing to several square nanometers, Λ converges to a constant value. However, for the one-dimensional nanoscale interface composed of nested carbon nanotubes (NCNTs), it is σ instead of Λ that converges to a constant value because the reservoir in one-dimensional nanoscale NCNTs has a fixed area, which can only provide finite transport channels. There are two competitive factors influencing the thermal transport at the interface in the NCNT model. One is phonon mode coupling and the other is phonon scattering. These two factors lead to an interesting trend of σ that as the overlap between NCNTs increases, σ increases at first and then decreases and converges to a constant value. These findings indicate that the thermal transport behavior has a strong dependence on the contact details and reservoir area at the nanoscale interface.

Enhanced carrier separation in ferroelectric In2Se3/MoS2 van der Waals heterostructure

Enhanced carrier separation in ferroelectric In2Se3/MoS2 van der Waals heterostructure and external electric field effects.