Nonequilibrium Green Function Formalism Research Articles

Study of the transport properties of cobalt atomic contact under mechanical strain in a nitrogen atmosphere

Achieving an efficient spin-filter effect and large magneto-resistance in molecular junctions is very important in spintronics. By employing the non-equilibrium Green's function formalism combined with the density functional theory, this study investigated the atomic structures and spin-resolved transport properties of molecular junctions consisting of nitrogen (N2) sandwiched between two cobalt (Co) electrodes with different crystal lattices as a function of stretching distance. In the case of face-centred cubic Co electrodes, our calculations showed that the transport properties depend strongly on the stretching process, which changes the contact between the N2 and Co electrodes. Specifically, the maximum spin polarization is due to the configuration of N2 molecules sitting in parallel with the protruding Co atoms. Furthermore, the Co dxz,yz orbitals interact with the N px,y orbitals, which contribute to the high transmission coefficient at the Fermi level for spin-down electrons. Meanwhile, the Co s-dz2 hybridization orbitals, which interact weakly with the N p orbitals, contribute to the small transmission coefficient at the Fermi level. This can also be verified by the molecular junction model adopting simple monatomic Co chain electrodes, which also show a perfect spin-filter effect and achieve an ideal magnetoresistance. In contrast, in the case of hexagonal Co electrodes, the spin-filter effect disappears. However, the effect appears again by adding one more Co atom to the central point contact. Our findings show that the spin-resolved electronic transport of molecular junctions depends on the stretching process and the nature of the electrodes, which can help in the future design of molecular devices based on the spin degree of freedom.

Controllable sensitivity mechanism in an energetic compound of [FeII(Rtrz)6] as a molecular switch

Transition metal iron complexes can be designed for spin crossover compounds with low spin and high spin bistable (LS and HS) states, spin transition can be triggered by external excitation, which regulates stability (sensitivity) and magnetism of complexes. [FeII(Rtrz)6] systems are bulit with the nitrogen-rich group triazole as the ligand, potentially making them high-energy compounds that can be applied in pyrotechnics and especially good candidates as green initiator for weapon systems. In this paper, two kinds of mononuclear complexes [FeIIN6-1,2,4-triazole] (1) and [FeIIN6-1,2,3-triazole] (2) are constructed, and LS ⇌ HS transition occurs by altering the bond lengths of metal ligand at room temperature; therefor, complex 1 and 2 may be spin crossover energetic materials. The ground state of both complexes is the LS state (low sensitivity), and the stable trend is 1 > 2. The spin transport mechanism is calculated by the non-equilibrium Green's function formalism combined with density functional theory. The theoretical results show that both 1 and 2 can be used as molecular switches in a small bias range, but 2 cannot be used when the bias voltage is high. In short, complex 1 is an ideal molecular switch from HS state (high-sensitivity) to LS state (low-sensitivity), acting as a reversible on ⇌ off switch. The spin state transition of complex 1 can be induced with an electric field; therefore, it is expected to control the safety of primary explosives precisely in the long term.

Calculated characterisation of a sensitive gas sensor based on PEDOT:PSS

The interactions between poly (3,4-ethylene dioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) and small gas molecules are studied using non-equilibrium Green's function formalism based on the density functional theory. The proposed method is implemented in the Tran SIESTA code to benefit from the potential application of PEDOT:PSS as a gas sensor. The results show that doping with nanoparticles can drastically improve the sensitivity of polymer-based chemical gas sensors. Moreover, among various PEDOT:PSS doping materials, silver nanoparticles have an appropriate response to ammonia, while platinum shows the best response to carbon dioxide. The numerical results can be useful to design PEDOT:PSS-based gas sensors.

Electron Transport Through Experimentally Controllable Parabolic Bubbles on Graphene Nanoribbons

We present a theoretical study of electron transport properties through experimentally controllable graphene nanobubbles [P. Jia et al., Nat. Commun. 10 (2019) 1] employing a tight-binding model and the non-equilibrium Green's function formalism. Sharp conductance peaks are observed at low energy region which signifies the emergence of quasi-bound states caused by pseudomagnetic field in the strained nanobubbles. Analysis based on local density of states reveals the nature of electron transmission at peak energies. Our results also show that the emergence of quasi-bound states and its role in electron transport depend on both strain strength and bubble size: when the strain or size of the bubble increases, more quasi-bound states emerge and resonant tunnelling assisted by these quasi-bound states becomes dominant.

Nonlinear Hall effect induced by internal Coulomb interaction and phase relaxation process in a four-terminal system with time-reversal symmetry

We numerically investigate the second-order nonlinear Hall transport properties of a four-terminal system with time-reversal symmetry and broken inversion symmetry. Within the nonequilibrium Green's function formalism, the second-order nonlinear conductances are derived, where the internal Coulomb potential in response to external voltages is explicitly included to guarantee the gauge invariance. For the system with single mirror symmetry ${\mathcal{M}}_{x}$, nonlinear Hall properties are only observable in the $y$ direction and contributed solely from the second-order nonlinear effect. From the symmetry point of view, the observed nonlinear Hall transport phenomena have one-to-one correspondence with the Berry curvature dipole induced nonlinear Hall effect semiclassically obtained for the same Hamiltonian. In addition to the nonlinear Hall effect originated from symmetries of the system, it is found that the internal Coulomb potential has the same symmetry of the four-terminal system, which gives rise to an extra nonlinear Hall response. Moveover, the phase relaxation mechanism modeled by virtual probes leads to the dephasing-induced nonlinear Hall effect.

High‐Performance Monolayer SiMe‐Graphene n‐Type Field‐Effect Transistors with Low Supply Voltage and High On‐State Current in Sub‐5 nm Gate Length

AbstractWith the development of device miniaturization, traditional Si‐based field effect transistors (FETs) are approaching their scaling limits. It is necessary to find more alternative materials to improve the performance of FETs. 2D semiconductors attract great attention for channel material due to their atomic‐scale smoothness and dangling‐bond‐free surface, and the gate electrostatic properties and charge carrier mobility are better than traditional 3D Si‐based semiconductor. In this paper, using density functional theory combined with nonequilibrium Green's function formalism, the electronic properties and device performance of sub‐5 nm monolayer SiMe‐graphene n‐type field‐effect transistors are evaluated. Remarkably, the on‐state current value of the FET can reach 3070 µA µm−1, which greatly surpasses most of reported FETs based on other 2D semiconductors. In addition, the on‐state current, power dissipation, and delay time of the SiMe‐graphene FETs satisfy the high‐performance requirements of the 2020 International Roadmap for Device and Systems under different supply voltages, which indicates that the FETs can work at different operating frequencies and power consumption. Moreover, via controlling electrode doping concentration, the channel length can be reduced to 4 nm. In total, the SiMe‐graphene is a favorable channel material for future FETs devices.

First-principles mobility prediction for amorphous semiconductors

Carrier mobility in amorphous semiconductors remained unpredictable due to random electronic states in the absence of the long-range order in a lattice structure, although amorphous semiconductors have been investigated over several decades and widely used in diverse electronic devices. In this work, we develop a method to predict mobility of disordered systems by virtue of the first-principles calculation without using any empirical parameters. Quantum transport modeling based on the nonequilibrium Green's function formalism enables us to establish a formula to connect first-principles results with amorphous-phase mobility. Finally, the developed approach is quantitatively validated by comparing the theoretical predictions with previously measured mobilities of amorphous metal oxides $({\mathrm{SnO}}_{2},{\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ZnO) and amorphous silicon. Localization analysis provides further physical insight into a distinct feature between the amorphous metal oxides and amorphous silicon.

Band offset and leakage current in fluorine doped Si/HfO2/SiO2 gate stack of metal oxide semiconductor field effect transistors: An ab initio investigation

Ab initio calculations were employed to investigate the influence of Fluorine doping in the gate dielectric stacks of state-of-the-art metal-oxide-semiconductor field-effect transistors. Different configurations of F dopants as well as their interactions with O vacancies at different layers of a Si/SiO2/HfO2 gate stack were considered. The calculated formation energies reveal that F atoms prefer to occupy O vacancy sites, particularly at the SiO2/HfO2 interface. It is shown that the instability of the threshold voltage due to the high concentration of O vacancies can be compensated by F doping in the HfO2 layer. Moreover, non-equilibrium Green's function formalism was employed for the ab initio calculations of the gate leakage current. For applied gate voltages between 0.8 V and 1.0 V, F-doped gate stacks show a lower gate leakage current compared to the stacks with O vacancies. In general, F doping seems to be an effective way to improve the quality of Si/SiO2/HfO2 gate stacks.

A Polynomial Approximation to Self Consistent Solution for Schrödinger–Poisson Equations in Superlattice Structures

The paper deals with a new approach to iterative solving the Schrödinger and Poisson equations in the first type of semiconductor superlattice. Assumptions of the transfer matrix method are incorporated into the approach, which allows to take into account the potential varying within each single layer of bias voltage superlattice. The key process of the method is to approximate the charge density and wave functions with polynomials. It allows to obtain semi-analytical solutions for the Schrödinger and Poisson equations, which in turn have significant impact on the accuracy and speed of superlattice simulations. The presented procedure is also suifihue for finding eigenstates extended over relatively large superlattice area, and it can be used as an effective pro-gram module for a superlattice finite model. The obtained quantum states are very similar to the Wannier-Stark functions, and they can serve as the base under non-equilibrium Green’s function formalism (NEGF). Exemplary results for Schrödinger and Poisson solutions for superlattices based on the GaAs/AlGaAs heterostructure are presented to prove all the above.

Coherent Modulation of Quasiparticle Scattering Rates in a Photoexcited Charge-Density-Wave System.

We present a complementary experimental and theoretical investigation of relaxation dynamics in the charge-density-wave (CDW) system TbTe_{3} after ultrafast optical excitation. Using time- and angle-resolved photoemission spectroscopy, we observe an unusual transient modulation of the relaxation rates of excited photocarriers. A detailed analysis of the electron self-energy based on a nonequilibrium Green's function formalism reveals that the phase space of electron-electron scattering is critically modulated by the photoinduced collective CDW excitation, providing an intuitive microscopic understanding of the observed dynamics and revealing the impact of the electronic band structure on the self-energy.

Giant Magnetoresistance and Rectification Behavior in Fluorinated Zigzag Boron Nitride Nanoribbon for Spintronic Nanodevices

In the present work, the spin-polarized structural and electronic properties of fluorine (F) passivated zigzag boron nitride nanoribbons (ZBNNRs) at the selective boron (B) and nitrogen (N) edge atoms are investigated. This study is based on the density functional theory (DFT) along with non-equilibrium Green function (NEGF) formalism. Our study predicts that half-metallic property can be obtained in ZBNNRs via F passivation at selective edges. The F-passivated ZBNNRs are found to be structurally stable in both non-magnetic as well as magnetic ground states irrespective of their width. Hence, the transport properties of F-passivated ZBNNRs are also studied as fluorinated structures are reported to be more stable. The current-voltage characteristic of F-passivated ZBNNRs based devices exhibit the perfect spin-filter characteristics with magnificently high spin-filtering efficiency (SFE) even under a low bias. It is worth mentioning here that giant magnetoresistance resistance (GMR), and rectification ratio of the order of 10⁸and 10⁵ have also been observed for F-BN-F devices. This is because dangling bonds break the edge states' symmetry and induce some localized states, which suppress the electron transmission and reduce the current. The observed perfect spin-filtering characteristics, GMR, and rectifying characteristics suggest that Fpassivated ZBNNRs have immense potentials to be deployed for nanoscale spintronic devices.

Electronic and transport property of two-dimensional boron phosphide sheet

Using density functional theory (DFT) approach, we have investigated the effect of strain on the electronic properties of two-dimensional (2D) boron phosphide (BP) sheet. With the increase in uniaxial and biaxial tensile strain band gap increases while band gap decreases and becomes metallic with the increase in uniaxial and biaxial compressive strain. Electrical and thermal transport properties of zigzag and armchair 2D BP sheets have been explored using nonequilibrium Green's function formalism (NEGF) and the changes in the nature of I–V characteristics with the application of strain have been reported. The magnitude of the current decreases with the increase of strain value along transport direction for both zigzag and armchair 2D BP sheets. For unstrained systems, the magnitude of current is nearly same for both zigzag and armchair 2D BP sheets. However, for a particular strain value, magnitude of current is more for zigzag sheet compared to armchair sheet. Though both zigzag and armchair 2D BP sheets have reasonably high ZTe which confirms its potentiality for designing efficient thermoelectric material but zigzag sheet is more preferable for thermoelectric application compared to armchair sheet due to its higher ZTe in comparison to armchair sheet.

A tight-binding study of the electron transport through single-walled carbon nanotube-graphene hybrid nanostructures.

Hybrid carbon nanostructures based on the sp2 hybridized allotropes of carbon, such as graphene and single-walled carbon nanotubes (SWCNTs), hold vast potential for applications in electronics of various forms. Electronic properties of such hybrid structures are modified due to the interaction between atoms of the components, which can be utilized to tailor the properties of the hybrid structures to suite the application. In this study, we have explored charge (electron) transport through the hybrid structures of single-layer graphene (SLG) and SWCNTs (both metallic and semiconducting) using the nonequilibrium Green's function formalism within the framework of tight-binding density functional theory. Our calculations show that the electronic transport in hybrid nanostructures is affected by the interactions between SWCNT and SLG in comparison to the individual components. The changes in the electronic structure and the transport properties with increasing interaction in hybrids (captured by decreasing the separation between SWCNT and SLG) are discussed, and it is demonstrated from this analysis that the hybrids with semiconducting SWCNTs and metallic SWCNTs show different behavior in the low bias regime while they show similar behavior at higher biases. The difference in the transport properties of hybrids with semiconducting and metallic SWCNTs is explained in terms of changes in the electronic structure, the local density of states, and the energy dispersion for electrons due to the interaction between atoms of the two components.

Half-metallic porphyrin-based molecular junctions for spintronic applications

Single molecular magnets (SMMs) have become promising paradigms to develop novel spintronics for futuristic information technologies such as high-density information and quantum computing. The efficiency and characteristics of SMM devices are determined by the intrinsic nature of the molecular magnets placed in the spin transport pathway. In this work, to understand the role of the central magnetic ion on the performance of SMM devices, we screen the spin-conductance properties of whole $3d$ and $4d$ metalloporphyrins using the nonequilibrium Green's function formalism in conjunction with density functional theory. Our results show that the investigated SMMs according to spintronic conductance behavior can be categorized into three groups: type I non-spin-polarized, type ${\mathrm{II}}^{\ensuremath{'}}$ minor spin-polarized current, and type ${\mathrm{II}}^{\ensuremath{''}}$ major spin-polarized current devices. Type-${\mathrm{II}}^{\ensuremath{'}}$ and type-${\mathrm{II}}^{\ensuremath{''}}$ molecular systems show perfect spin filtering and spin-dependent negative differential resistance. The optimal energy alignment of spin-polarized molecular orbitals with gold electrodes results in one-channel spin transport (minor for type ${\mathrm{II}}^{\ensuremath{'}}$ and major for type ${\mathrm{II}}^{\ensuremath{''}}$). Thus type-II junctions are half-metal. The type-${\mathrm{II}}^{\ensuremath{''}}$ junctions also show a voltage-induced spin switchability at low bias voltages. In this regard, type-II molecular systems are promising candidates for a low-power consumption spin filter, spin switch, memory, to name just a few. Our results highlight the practical applications of metalloporphyrin for the development of multipurpose miniature spintronic devices.

All-electric electron spin resonance studied by means of Floquet quantum master equations

We present a theoretical framework to describe experiments directed to controlling single-atom spin dynamics by electrical means using a scanning tunneling microscope. We propose a simple model consisting of a quantum impurity connected to electrodes where an electrical time-dependent bias is applied. We solve the problem in the limit of weak coupling between the impurity and the electrodes by means of a quantum master equation that is derived by the non-equilibrium Green's function formalism. We show results in two cases. The first case is just a single atomic orbital subjected to a time-dependent electric field, and the second case consists of a single atomic orbital coupled to a second spin-1/2. The first case reproduces the main experimental features Ti atoms on MgO/Ag (100) while the second one directly addresses the experiments on two Ti atoms. These calculations permit us to explore the effect of different parameters on the driving of the atomic spins as well as to reproduce experimental fingerprints.

Impact of Kondo correlations and spin–orbit coupling on spin-polarized transport in carbon nanotube quantum dot

Spin polarized transport through a quantum dot coupled to ferromagnetic electrodes with noncollinear magnetizations is discussed in terms of nonequilibrium Green functions formalism in the finite-U slave boson mean field approximation. The difference of orientations of the magnetizations of electrodes opens off-diagonal spin–orbital transmission and apart from spin currents of longitudinal polarization also spin-flip currents appear. We also study equilibrium pure spin current at zero bias and discuss its dependence on magnetization orientation, spin–orbit coupling strength and gate voltage. Impact of these factors on tunneling magnetoresistance (TMR) is also undertaken. In general spin–orbit coupling weakens TMR, but it can change its sign.

Thermal Transport in Disordered Harmonic Chains Revisited: Formulation of Thermal Conductivity and Local Temperatures

In this paper, thermal transport in bond-disordered harmonic chains is revisited in detail using a nonequilibrium Green's function formalism. For strong bond disorder, thermal conductivity is independent of the system size. However, kinetic temperatures described by the local number of states coupling to external heat reservoirs are anomalous since they form a nonlinear profile in the interior of the system. Both results are accounted for in a unified manner in terms of the frequency-dependent localization length. From this argument, we derive a generic formula describing the asymptotic profile of local temperatures in a disordered harmonic chain. A linear temperature profile obeying Fourier's law is recovered by contact with a self-consistent reservoir of Ohmic type even in the limit of weak system-reservoir coupling. This verifies that mechanisms leading to local thermal equilibrium and breaking total momentum conservation are essential for Fourier transport in low dimensions.

Green's function formalism for nonlocal elliptical magnon transport

We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.

Nonequilibrium Green’s Function Modeling of Trap-Assisted Tunneling in InxGa1−xN /GaN Light-Emitting Diodes

This work presents an investigation of carrier transport in $\mathrm{Ga}\mathrm{N}$-based light-emitting diodes in the subthreshold forward-bias regime where tunneling processes are relevant. A quantum kinetic theory of trap-assisted tunneling is developed within the framework of the nonequilibrium Green's function formalism. Based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure, the model provides access to spectral quantities, such as the local density of states and the current density, which are essential to understand the nature of the tunneling process. The quantum nonradiative recombination rates can be reproduced by the conventional Shockley-Read-Hall theory, provided that the classical charge is replaced with the correct quantum charge, which means that trap-assisted tunneling can be described with drift-diffusion solvers complemented with appropriate quantum corrections for the calculation of the local density of states. The subthreshold $I$-$V$ characteristics and ideality factors predicted by the quantum kinetic model are in agreement with measurements.

Design and Self-Consistent Schrodinger-Poisson Model Simulation of Ultra-Thin Si-Channel Nanowire FET

Since at the regime of nanometer, the quantum confinement effects are observed and the wave nature of electrons is more dominant. Therefore, the classical approach of current formulation in mesoelectonics and nanoelectronics results in inaccuracy as it does not consider the quantum effect, which is only applicable for the bulk electronic device. For accurate modeling and simulation of nanoelectronics, device atomic-level quantum mechanical models are required. In this work, an ultra-thin (2 nm diameter) Silicon- channel Cylindrical Nanowire FET (CNWFET) is designed and simulated by invoking non-equilibrium green function (NEGF) formalism and self-consistent Schrodinger-Poisson’s equation model. Then impact variation of temperature, oxide thickness, and metal work function variation in the NWFET is investigated to analyze the distinct performance parameters of the device e.g. threshold voltage (Vth) drain induced barrier lowering (DIBL), sub-threshold swing (SS), and ION/IOFF ratio. The designed device exhibits reliable results and shows a SS of 57.8 mV/decade and ION to IOFF ratio of order 109 at room temperature.