Nonequilibrium Green Function Research Articles

Memory truncated Kadanoff-Baym equations

The Keldysh formalism for nonequilibrium Green's functions is a powerful theoretical framework for the description of the electronic structure, spectroscopy, and dynamics of strongly correlated systems. However, the underlying Kadanoff-Baym equations (KBE) for the two-time Keldysh Green's functions involve a memory kernel which results in a high computational cost for long simulation times $t_\text{max}$, with a cubic scaling of the computation time with $t_\text{max}$. Truncation of the memory kernel can reduce the computational cost to linear scaling with $t_\text{max}$, but the required memory times will depend on the model and the diagrammatic approximation to the self-energy. We explain how a truncation of the memory kernel can be incorporated into the time-propagation algorithm to solve the KBE, and investigate the systematic truncation of the memory kernel for the Hubbard model in different parameter regimes, and for different diagrammatic approximations. The truncation is easier to control within dynamical mean-field solutions, where it is applied to a momentum-independent self-energy. Here, simulation times up to two orders of magnitude longer are accessible both in the weak and strong coupling regime, allowing for a study of long-time phenomena such as the crossover between pre-thermalization and thermalization dynamics.

Investigating electronic properties and electron transport in molecular junctions of new model CoNiO2 doped with Li and Na via extended tight-binding based on DFT computational method

In this study, Li and Na atoms were doped in CoNiO2 transition metal to determine the band structure and density of states (DOS) by using density functional theory (DFT). The extended tight binding DFT (GFN1-xTB) was adopted to investigate the compounds for figuring out the geometry optimization process. The transmission spectra and electric current passing through metal-molecular junctions were calculated by using nonequilibrium Green’s-function (NEGF) method. The results obtained after DFT calculations indicated that while Li-doped CoNiO2 exhibited a semiconductor behaviour, Na-doped CoNiO2 exhibited a metallic behaviour.

Time-linear scaling nonequilibrium Green's function methods for real-time simulations of interacting electrons and bosons. I. Formalism

Simulations of interacting electrons and bosons out of equilibrium, starting from first principles and aiming at realistic multiscale scenarios, is a grand theoretical challenge. Here, using the formalism of nonequilibrium Green's functions and relying in a crucial way on the recently discovered time-linear formulation of the Kadanoff-Baym equations, we present a versatile toolbox for the simulation of correlated electron-boson dynamics. A large class of methods are available, from the Ehrenfest to the dressed $GD$ for the treatment of electron-boson interactions in combination with perturbative, i.e., Hartree-Fock and second-Born, or nonperturbative, i.e., $GW$ and $T$-matrices either without or with exchange effects, for the treatment of the Coulomb interaction. In all cases the numerical scaling is linear in time and the equations of motion satisfy all fundamental conservation laws.

Investigating magneto-resistance in transition metals doped silicene nanoribbons

The Magneto Resistance (MR) is investigated in magnetic impurity-doped Zigzag Silicene Nano-Ribbons (ZSiNR) through Density Functional Theory (DFT) and Non-Equilibrium Green's Functions (NEGF) formalism using different spin configurations (Ferromagnetic (FM) and Antiferromagnetic (AFM)) in electrodes. The MR is calculated, and results show a GMR of about 2 ​× ​104% at 0.01V bias voltage for up-spin electrons in Co@ZSiNR structure and a total GMR of 2.6 ​× ​104% at 0.02V, which makes the proposed structures as possible candidates in spintronic device applications. The structures are modified by adding the impurities to the electrodes region, and removing the Hydrogen passivation. The GMR effect is investigated in these structures with different widths, which improves GMR ratios in certain widths.

Time-linear scaling nonequilibrium Green's function method for real-time simulations of interacting electrons and bosons. II. Dynamics of polarons and doublons

Nonequilibrium dynamics of the open chain Holstein-Hubbard model is studied using the linear time-scaling GKBA+ODE scheme developed in the preceeding paper. We focus on the set of parameters relevant for photovoltaic materials, i.\,e., a pair of electrons interacting with phonons at the cross-over between the adiabatic and anti-adiabatic regimes and at moderately large electron-electron interaction. By comparing with exact solutions for two corner cases, we demonstrate the accuracy of the $T$-matrix (in the $pp$ channel) and the second-order Fan ($GD$) approximations for the treatment of electronic ($e$-$e$) and electron-phonon ($e$-ph) correlations, respectively. The feedback of electron on phonons is consistently included and is shown to be mandatory for the total energy conservation. When two interactions are simultaneously present, our simulations offer a glimpse into the dynamics of doublons and polarons unveiling the formation, propagation and decay of these quasiparticles, energy redistribution between them and self-trapping of electrons.

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.

Improving the Performance of a Doping-Less Carbon Nanotube FET with Dual Junction Source and Drain Regions: Numerical Studies

In this paper, a new structure using dual junctions in the source and drain regions is proposed to improve the electronic characteristics of the doping less carbon nanotube FET. To simulate the proposed structure, the Poisson–Schrodinger equation self-consistent solution method through the nonequilibrium Green’s function (NEGF) procedure has been used. The proposed structure is formed by dividing the source and drain regions into two parts with the same lengths and different work function for each part. The simulation results show that the short channel effects (SCEs) including the sub-threshold swing (SS) and the drain-induced barrier lowering (DIBL) are improved. Also, OFF current has a significant reduction, which in turn increases the current ratio. Besides, improving the ambipolar behavior and leakage current are other benefits of the proposed structure.

Optimization of gain region in mid-IR ( ≈ 5 μm) QCL.

Non-equilibrium Green's function (NEGF) formalism is used to optimize the gain region of a quantum cascade laser (QCL) tailored to emit radiation at ∼5 µm wavelength, originally designed by Evans et al. [Appl. Phys. Lett., 88,051105(2006)10.1063/1.2171476]. The optimization strategy uses electron-photon selfenergies to find characteristics of devices under the "operating conditions," i.e., interacting with the laser field. These conditions can be quite different from the one when the device is in no-lasing state and the unsaturated gain is being optimized. The saturation caused by the optical field can push the structure from strong to weak coupling conditions, what changes laser parameters in a non-linear manner. Moreover, the NEGF method does not require any phenomenological parameters (such as, e.g., the phase relaxation times), so the quantities dependent on these parameters are determined solely on physical grounds. The use of the above procedure for the structure under investigation shows that the increase of the quantum efficiency by 24% and the output power by 83% in comparison to the original design can be achieved when the widths of injection and extraction barriers are changed to their optimal values.

Electron mobility distribution in FD-SOI MOSFETs using a NEGF-Poisson approach

Modern electronic devices consist of several semiconductor layers where each layer exhibits unique carrier transport properties that can be represented by a unique mobility characteristic. To date, the mobility spectrum analysis technique is the main approach that has been developed and applied to the analysis of conductivity mechanisms of multi-carrier semiconductor structures and devices. Currently, there are no theoretical calculations of the mobility distribution in semiconductor structures or devices and specifically in MOSFET devices. In this article, we present a theoretical study of the electron mobility distribution in planar fully-depleted silicon-on-insulator (FD-SOI) transistors employing quantum mechanical modelling. The simulation results indicate that electronic transport in the 10 nm thick Si channel layer at room-temperature is due to two distinct and well-defined electron species for channel length varying from 50 nm to 200 nm. The two electron mobility distributions provide clear evidence of sub-band modulated transport in 10-nm thick Si planar FD-SOI MOSFETs that are associated with primed and non-primed valleys of silicon. The potential of the top gate electrode has been modulated, and thus only the top channel inversion-layer electron population transport parameters have been investigated employing self-consistent non-equilibrium Green’s function (NEGF)–Poisson numerical calculations. The numerical framework presented can be used to interpret experimental results obtained by magnetic-field dependent geometrical magnetoresistance measurements and mobility spectrum analysis, and provides greater insight into electron mobility distributions in nanostructured FET devices.

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.

Investigation of pristine and B/N/Pt/Au/Pd doped single-walled carbon nanotube as phosgene gas sensor: A first-principles analysis

The present work investigates a novel CNT based sensor for detection of highly toxic phosgene gas, which is widely used as a chemical agent. The investigation is performed via first-principle means in the vicinity of Density Functional Theory (DFT) and Non-Equilibrium Green’s Function (NEGF) formalisms. Though the pristine CNT is less reactive towards the phosgene gas, the near valence (B and N) and transition metal (Pt/Au/Pd) doped CNTs show very good reactivity, and the sensing mechanism is guided by Van der Waals interaction based physisorption phenomenon. The sensing mechanism is assessed with the help of various properties/parameters which include adsorption energies, electronic nature, density of states, charge transfer, electrical transport (I-V), recovery time and sensitivity. Unlike the pristine CNT, the doped CNTs exhibited significant sensing behaviour towards the phosgene, especially the transition metal dopants as witnessed from the I-V characteristics. Au-doped CNT offers tremendous reduction in the drive current on adsorption of phosgene gas, which can be used as an electronic fingerprint of detection. In particular, the Pd-doped CNT exhibits superior sensing behaviour in comparison to the other considered dopants, owing to its significant variation in the I-V, a Mulliken charge transfer of 0.24e, excellent sensitivity of 134.75 %, and very less recovery time of 8.9 ns. Thus, the transition metals (in particular Pd) doped CNTs could prove useful for the design of CNT based physical sensors to detect phosgene gas in various industries such as defence sector where phosgene is used as a chemical warfare agent, and in chemical sector where it is used as a precursor to synthesize several chemicals.

Non-Maxwell–Boltzmann dependence of channel carrier concentration in a quasi one dimensional charge density wave channel in the ballistic transport regime

With the aid of a coherent transport model utilizing the non-equilibrium Green function approach, a three terminal device with metallic gate, source and drain and a quasi one dimensional charge density wave (CDW) channel is simulated focussing on the transistor behaviour brought about by a sweep of the channel potential or equivalently the chemical potential in the channel. The channel is strongly insulating only at half-filling and moving to lower and higher carrier concentrations both incur a mean field phase transition to a conducting state. With the aid of conductance calculations for a pinned CDW condensate, we present calculations for the sub-threshold slope in terms of the hopping parameter or equivalently the width of the tight-binding chain. The effects of source to drain bias and length are examined. The conductance profiles are analyed in relation to transmission profiles. The observed CDW profiles are explained in terms of filling and Fermi surface nesting. Boundary conditions, gap equations and response functions are shown to reveal the commensurability conditions and size of the transport gap. The channel carrier concentration is modulated in an athermal (non-Maxwellian–Boltzmann) fashion, thereby making it an interesting prospect for steep transistors.

Numerical analysis of electronic state of CNT/BNNT heterojunction

The I-V characteristics of CNT/BNNT heterojunctions, which comprise quantum double barrier systems (DBS), were numerically investigated using density functional theory (DFT) calculations and the non-equilibrium Green’s function (NEGF) method. First, we performed a structural optimisation of the DBS of CNT/BNNT. Next, the I-V characteristics of the heterojunctions were calculated using the SIESTA package. The I-V characteristics remain flat up to a certain bias voltage, and then the current increases with increasing voltage. Our CNT/BNNT double-barrier heterojunctions have resonant states inside the barriers as semiconductor-based double-barrier diodes; these states determine the transmission function. An energy gap was centred at the Fermi energy and sharp peaks in the gap were observed. It was also found that the energy interval of the peaks near the Fermi energy was dependent on the separation length of the two BNNT barriers. Using projected density of states (PDOS) analysis, we also investigated the local behaviour of electrons in the devices.

DFT-NEGF Study of Electron Transfer Property in Graphene Nanoribbon Double Barrier System

Current-voltage (I – V) characteristics in armchair graphene nanoribbon (AGNR) double barrier systems (DBSs) are investigated by the density functional theory (DFT) and the non-equilibrium Green’s function (NEGF) methods. The hexagonal boron nitride (h-BN) which has an analogous structure with a graphene shows wide bandgap and becomes an electrical barrier in DBSs. The heterojunction structure of AGNR and two h-BNs with one six-membered ring width has properties as a resonant tunnelling device. Due to the resonance, sharp peaks are formed in the transmission function around the Fermi energy EF . Our previous studies have explained the origins of some peaks by solving the one-dimensional Dirac equation for massless particles. Here, by using the two-dimensional Dirac equation, we show that the remaining sharp peaks depend on the transverse nodal pattern of the wavefunction as well as the anisotropy of the Fermi velocity. Moreover, in the I – V characteristics, a flat part is formed without the negative differential resistance appearing. By partial density of state (PDOS) analysis, we reveal that the flat part in the current is caused by the fact that the peak positions in the resonance region are stable against the bias voltage.

Performance analysis of broadband Mid-IR graphene-phototransistor using strained black phosphorus sensing gate: DFT-NEGF investigation

In this work, a new high-performance broadband Infrared Optically Controlled Graphene Field-Effect Transistor (IR–OC–GFET) using strained black phosphorus sensing gate is proposed and investigated. The impact of the hydrostatic pressure on the optoelectronic properties of bulk Black Phosphorus (BP) is studied using density functional theory (DFT) calculations, including Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA) and the screened hybrid (YS-PBE0) function with van der Waals correction. It is revealed that the electronic and the optical properties of BP were substantially affected by the pressure effects, where the band gap energy decreases with increasing the hydrostatic pressure. The phototransistor drain current is calculated by self-consistently solving the Schrödinger/Poisson equations based on Non-Equilibrium Green's function (NEGF) approach. The impact of strained BP sensing gate material on the device sensing properties is investigated. It is found that the proposed device with strained sensing gate provides enhanced optical performances over the middle infrared (Mid-IR) spectral band, making it a new potential alternative photoreceiver for chip-level optical communications.

The piezoelectricity of 2D Janus ZnBrI: Multiscale prediction

The piezoelectric properties of two-dimensional (2D) Janus ZnBrI are predicted by the multiscale method. Based on the density functional theory, the electronic properties and piezoelectric coefficients of 2D Janus ZnBrI are calculated. It is found that the 2D Janus ZnBrI has a large in-plane piezoelectric coefficient d11, up to 145.85 pm/V, which is about 40 times that of 2D 2H-MoS2. Based on the finite element method and multi-physical field simulation, the large piezoelectric response of 2D Janus ZnBrI is shown intuitively. Based on the nonequilibrium Green's function, the device model of metal-semiconductor-metal is established, and the I-V response of the device model is calculated, which shows the modulation of piezoelectric effect on tunneling current. The simulation results show that 2D Janus ZnBrI is a 2D piezoelectric material with application potential. This multiscale prediction method can comprehensively predict other 2D piezoelectric materials.

Electronic phase transition, spin filtering effect, and spin Seebeck effect in 2D high-spin-polarized VSi2X4 (X = N, P, As)

Spin gapless semiconductors (SGSs) and bipolar magnetic semiconductors (BMSs) are highly desirable for advanced spintronic applications due to their unique electronic and magnetic properties. Being inspired by the recent synthesis of the 2D MA2X4 family with various electronic properties, including nontrivial topological properties, 2D ferromagnetism, Ising superconductivity, and robust valley polarization, we systematically studied the electron structure, magnetism, and spin transport properties for VSi2X4 (X = N, P, As) monolayers by the density functional theory combined with the nonequilibrium Green's function method. The results show that VSi2X4 monolayers are ferromagnetic type-II SGS and BMS with high Curie temperatures 230–250 K. The interesting electronic phase transitions of type-II-SGS-to-metal-to-type-I-SGS for VSi2P4 and BMS-to-type-I-SGS-to-type-II-SGS for VSi2As4 can be achieved by applying the biaxial strain. Furthermore, the VSi2P4 monolayer exhibits the bias-dependent spin filtering effect and the temperature-driven spin Seebeck effect. The high Curie temperature, the versatile strain-tuned electronic phase transitions, and the excellent spin transport characteristics make 2D VSi2X4 the promising candidate for spintronic applications and will stimulate intensive studies on this class of high-spin-polarized 2D systems.

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.

Doublon Production in Correlated Materials by Multiple Ion Impacts

Recently, it was demonstrated that ions impacting a correlated finite graphene‐type honeycomb cluster can excite strong nonequilibrium states. In particular, this can lead to an enhanced population of bound pairs of electrons with opposite spin—doublons—where the doublon number can be increased further via multiple ion impacts. These predictions were made based on nonequilibrium Green functions (NEGF) simulations allowing for a time‐dependent non‐perturbative study of the energy loss of charged particles penetrating a strongly correlated system. Herein, these simulations are extended to larger clusters and longer simulation times, utilizing the recently developed G1–G2 scheme, which allows for a dramatic speedup of NEGF simulations. Furthermore, the dependence of the energy and doublon number on the time interval between ion impacts and on the impact point is investigated.

Local equilibrium charge and spin currents in two-dimensional topological systems

We study the equilibrium and nonequilibrium electronic transport properties of multiprobe topological systems using a combination of the Landauer-B\"uttiker approach and nonequilibrium Green's functions techniques. We obtain general expressions for both nonequilibrium and equilibrium local electronic currents that, by suitable projections, allow one to compute charge, spin, valley, and orbital currents. We show that external magnetic fields give rise to equilibrium charge currents in mesoscopic system and study the latter in the quantum Hall regime. Likewise, a spin-orbit interaction leads to local equilibrium spin currents, that we analyze in the quantum spin Hall regime. We show that an accurate theoretical assessment of the equilibrium currents is quite challenging and propose a two-measurement protocol that facilitates a comparison between experiment and theory.