Nonequilibrium Green Function Formalism Research Articles

Numerical simulation of Rashba spin–orbit coupling effect on quantum Hall resistivity within tight-binding approximation

A numerical simulation of the effect of Rashba spin–orbit coupling (RSO) on quantum transport properties of electrons confined to a two-dimensional electron gas (2DEG) in the integer quantum Hall regime is presented. The mechanism of quantization and the dependence of the Hall resistivity (HR) on the system built-in and external parameters are studied using the non-equilibrium Green’s function (NEGF) technique. Relevant Green’s functions are computed numerically using spectral decomposition method in combination with recursive techniques. The tight-binding-like representation of the system Hamiltonian including magnetic field is obtained within the finite difference method, using Dirichlet boundary conditions. Corresponding formula linking the Hall resistivity with the transmission function of the system has been derived analytically within the NEGF formalism in a general form, in the sense that the formula can also be used beyond the linear response limit and for nonzero temperatures. In particular, it was shown explicitly that in the presence of the RSO coupling the even- and odd plateaus emerging in the magnetic-field dependence of the quantum Hall resistivity are related to different physical mechanisms.

Trade-off analysis between gm/ID and fT of GNR-FETs with single-gate and double-gate device structure

This study examines the operational parameters of field-effect transistors (FETs) using single-gate (SG) and double-gate (DG) graphene nanoribbons (GNRs) within the analog/RF domain. A detailed exploration is conducted through an atomistic pz orbital model, derived from the Hamiltonian of graphene nanoribbons, employing the nonequilibrium Green’s function formalism (NEGF) for analysis. The atomic characteristics of the GNRFETs channel are accurately described by utilizing a tight-binding Hamiltonian with an atomistic pz orbital basis set. The primary focus of the analysis revolves around essential analog/RF parameters such as transconductance, transconductance generation factor (TGF), output resistance, early voltage, intrinsic gain, gate capacitance, cut-off frequency, and transit time. Furthermore, the study assesses the gain frequency product (GFP), transfer frequency product (TFP), and gain transfer frequency product (GTFP) to evaluate the balance between transistor efficiency, gain, and cut-off frequency. The research outcomes indicate that double-gate GNRFETs exhibit superior analog/RF performance in comparison to their single-gate counterparts. However, both types of devices demonstrate cut-off frequencies in the gigahertz range. The extensive data presented in this study provides valuable insights into the characteristics of SG and DG GNRFETs, particularly in terms of the figure-of-merit (FoM) for analog/RF performance, offering a comprehensive analysis of the trade-offs in analog applications. In addition, the analysis has been extended be performing a high-performance hybrid 6T static random-access memory (SRAM) to get the impact in their circuit level variation as well as improvement in their circuit performance.

Jiezi: An open-source Python software for simulating quantum transport based on non-equilibrium Green's function formalism

We present a Python-based open-source library named Jiezi, which provides the means of simulating the electronic transport properties of nanoscaled devices on the atomistic level. The key feature of Jiezi lies in its core algorithm, i.e., self-consistent orchestration between the non-equilibrium Green's function (NEGF) method and a Poisson's equation solver. Beyond the construction of the tight-binding (TB) Hamiltonian with empirical parameters for conventional materials, the package offers a comprehensive framework for constructing the Wannier-based Hamiltonian matrix, enabling the investigation of novel materials and their heterostructures. To expedite the solution of NEGF systems, a methodology based on renormalization theory is proposed for reducing the dimension of the Hamiltonian matrix. Additionally, we adopt a non-linear Poisson equation solver with no analytical approximation in this software. The software facilitates seamless integration with external tools for geometry and mesh generation and post-processing. In this paper, we present the main capabilities and workflow by demonstrating with a simulation for the carbon nanotube field-effect transistor (CNTFET). Program summaryProgram Title: JieziCPC Library link to program files:https://doi.org/10.17632/nk79kbtww4.1Developer's repository link:https://github.com/Jiezi-negf/JieziLicensing provisions: GPLv3Programming language: PythonNature of problem: Simulates the quantum transport property of nano-scaled transistors based on the predefined device structure and the material composition.Solution method: Solves the coupled Schrödinger equation and Poisson equation by NEGF and finite element method.

Floquet non-equilibrium Green's function and Floquet quantum master equation for electronic transport: The role of electron-electron interactions and spin current with circular light.

The non-equilibrium Green's function (NEGF) and quantum master equation (QME) are two main classes of approaches for electronic transport. We discuss various Floquet variances of these formalisms for transport properties of a quantum dot driven via interaction with an external periodic field. We first derived two versions of the Floquet NEGF. We also explore an ansatz of the Floquet NEGF formalism for the interacting systems. In addition, we derived two versions of Floquet QME in the weak interaction regime. With each method, we elaborate on the evaluation of the expectation values of the number and current operators. We examined these methods for transport through a two-level system that is subject to periodic driving. The numerical results of all four methods show good agreement for non-interacting systems in the weak regime. Furthermore, we have observed that circular light can introduce spin current. We expect these Floquet quantum transport methods to be useful in studying molecular junctions exposed to light.

Nanotube ferroelectric tunnel junctions with an ultrahigh tunneling electroresistance ratio.

Low-dimensional ferroelectric tunnel junctions are appealing for the realization of nanoscale nonvolatile memory devices due to their inherent advantages of device miniaturization. Those based on current mechanisms have limitations, including low tunneling electroresistance (TER) effects and complex heterostructures. Here, we introduce an entirely new TER mechanism to construct a nanotube ferroelectric tunnel junction with ferroelectric nanotubes as the tunneling region. When rolling a ferroelectric monolayer into a nanotube, due to the coexistence of its intrinsic ferroelectric polarization with the flexoelectric polarization induced by bending, a metal-insulator transition occurs depending on the radiative polarization states. For the pristine monolayer, its out-of-plane polarization is tunable by an in-plane electric field, and the conducting states of the ferroelectric nanotube can thus be tuned between metallic and insulating states via axial electric means. Using α-In2Se3 as an example, our first-principles density functional theory calculations and nonequilibrium Green's function formalism confirm the feasibility of the TER mechanism and indicate an ultrahigh TER ratio that exceeds 9.9 × 1010% of the proposed nanotube ferroelectric tunnel junctions. Our findings provide a promising approach based on simple homogeneous structures for high density ferroelectric microelectric devices with excellent ON/OFF performance.

Enhancing Spin-Transport Characteristics, Spin-Filtering Efficiency, and Negative Differential Resistance in Exchange-Coupled Dinuclear Co(II) Complexes for Molecular Spintronics Applications.

Single-molecule spintronics, where electron transport occurs via a paramagnetic molecule, has gained wide attention due to its potential applications in the area of memory devices to switches. While numerous organic and some inorganic complexes have been employed over the years, there are only a few attempts to employ exchange coupled dinuclear complexes at the interface, and the advantage of fabricating such a molecular spintronics device in the observation of switchable Kondo resonance was demonstrated recently in the dinuclear [Co2(L)(hfac)4] (1) complex (Wagner et al., Nat. Nanotechnol. 2013, 8, 575-579). In this work, employing an array of theoretical tools such as density functional theory (DFT), the ab initio CASSCF/NEVPT2 method, and DFT combined with nonequilibrium Green Function (NEGF) formalism, we studied in detail the role of magnetic coupling, ligand field, and magnetic anisotropy in the transport characteristics of complex 1. Particularly, our calculations not only reproduce the current-voltage (I-V) characteristics observed in experiments but also unequivocally establish that these arise from an exchange-coupled singlet state that arises due to antiferromagnetic coupling between two high-spin Co(II) centers. Further, the estimated spin Hamiltonian parameters such as J, g values, and D and E/D values are only marginally altered for the molecule at the interface. Further, the exchange-coupled state was found to have very similar transport responses, despite possessing significantly different geometries. Our transport calculations unveil a new feature of the negative differential resistance (NDR) effect on 1 at the bias voltage of 0.9 V, which agrees with the experimental I-V characteristics reported. The spin-filtering efficiency (SFE) computed for the spin-coupled states was found to be only marginal (∼25%); however, if the ligand field is fine-tuned to obtain a low-spin Co(II) center, a substantial SFE of 44% was noted. This spin-coupled state also yields a very strong NDR with a peak-to-valley ratio (PVR) of ∼56 - a record number that has not been witnessed so far in this class of compounds. Additionally, we have established further magnetostructural-transport correlations, providing valuable insights into how microscopic spin Hamiltonian parameters can be associated with SFE. Several design clues to improve the spin-transport characteristics, SFE and NDR in this class of molecule, are offered.

Full statistics of nonequilibrium heat and work for many-body quantum Otto engines and universal bounds: A nonequilibrium Green's function approach.

We consider a generic four-stroke quantum Otto engine consisting of two unitary and two thermalization strokes with an arbitrary many-body working medium. Using the Schwinger-Keldysh nonequilibrium Green's function formalism, we provide an analytical expression for the cumulant generating function corresponding to the joint probability distribution of nonequilibrium work and heat. The obtained result is valid up to the second order of the external driving amplitude. We then focus on the linear response limit and obtained Onsager's transport coefficients for the generic Otto cycle and show that the traditional fluctuation-dissipation relation for the total work is violated in the quantum domain, whereas for heat it is preserved. This leads to remarkable consequences in obtaining universal constraints on heat and work fluctuations for engine and refrigerator regimes of the Otto cycle and further allows us to make connections to the thermodynamic uncertainty relations. These findings are illustrated using a paradigmatic model that can be feasibly implemented in experiments.

Automatic differentiable nonequilibrium Green's function formalism: An end-to-end differentiable quantum transport simulator

Automatic differentiable nonequilibrium Green's function formalism: An end-to-end differentiable quantum transport simulator

Tunnel-magneto resistance of double-barrier tunnel junctions in the presence of spin-orbit coupling within spin-filter barriers

The tunnel magneto-resistance ratio is investigated for double-barrier tunnel junctions with spin-filter (magnetic insulators) barriers. The transmission probability is calculated in the tight-binding model by the non-equilibrium Green's function formalism considering spin-orbit coupling within barriers. The formation of resonant states inside a non-magnetic spacer gives rise to an enhancement in tunnel magneto-resistance ratio. Double spin-filter tunnel junctions have also shown a higher tunnel magneto-resistance ratio compared to that in double-barrier magnetic tunnel junctions. These findings will be valuable to improve the performance of spintronics devices.

Influence of a static electric field on magnetic materials monitored by X-ray magnetic circular dichroism

The magnetic circular X-ray dichroism (XMCD) in X-ray absorption spectroscopy (XAS) is extensively used to monitor the response of a magnetic system to an external perturbation. Application of a static external electric field to control its magnetic properties may just lead to a charge rearrangement or to a steady electric current dependent on the geometry. As it is demonstrated, the first situation can be handled with minor modifications of the standard well established schemes to calculate XAS and XMCD spectra. For the second case, however, more advanced schemes have to be used that account for the steady-state out-of-equilibrium situation. It is shown, that this can indeed be achieved by making use of the Keldysh non-equilibrium Green function formalism that allows in particular to express the X-ray absorption coefficient in terms of the corresponding greater Green function G>(E). As an alternative approach, a bi-linear response formalism is presented that can be used to calculate the modification of the XMCD spectra due to the electric field directly.

Performance analysis of fluorinated silicene based magnetic tunnel junction

Two-dimensional material based Spintronic devices are considered to be the promising next generation nano-electronic devices due to their low power, high speed and enhanced data storage capabilities. In this work, firstly we have investigated the effect of fluorination on the electronic/structural properties of zigzag/armchair silicene. Next, we have modelled a magnetic tunnel junction (MTJ) device consisting of CrO2 electrodes and fluorine passivated armchair silicene nanoribbon as scattering region. The spin dependent transport properties are calculated via first principles calculations combined with non-equilibrium Green's function formalism. The modeled MTJ device shows very large magnetoresistance as compared to the state of art reported devices and high spin injection efficiency. The spin-dependent transmission spectrum and density of states were calculated to elucidate the obtained transport characteristics.

Power efficient ReLU design for neuromorphic computing using spin Hall effect

We demonstrate that a magnetic tunnel junction injected with a spin Hall current can exhibit linear rotation of the magnetization of the free-ferromagnet using only the spin current. Using the linear resistance change of the magnetic tunnel junction (MTJ), we devise a circuit for the rectified linear activation (ReLU) function of the artificial neuron. We explore the role of different spin Hall effect (SHE) heavy metal (HM) layers on the power consumption of the ReLU circuit. We benchmark the power consumption of the ReLU circuit with different SHE layers by defining a new parameter called the spin Hall power factor. It combines the spin Hall angle, resistivity, and thickness of the HM layer, which translates to the power consumption of the different SHE layers during spin-orbit switching/rotation of the free FM. We employ a hybrid spintronics-CMOS simulation framework that couples Keldysh non-equilibrium Green’s function formalism with Landau–Lifshitz–Gilbert–Slonzewski equations and the HSPICE circuit simulator to account for the diverse physics of spin-transport and the CMOS elements in our proposed ReLU design. We also demonstrate the robustness of the proposed ReLU circuit against thermal noise and a non-trivial power-error trade-off that enables the use of an unstable free-ferromagnet for energy-efficient design. Using the proposed circuit, we evaluate the performance of the convolutional neural network for MNIST datasets and demonstrate comparable classification accuracies to the ideal ReLU with an energy consumption of 75 pJ per sample.

Controllable gas adsorption via inter-coupled ferroelectricity in In2Se3 monolayer

Recently, two-dimensional ferroelectrics with spontaneous electric polarization have attracted growing interest for application in gas sensors and other electronic devices. This study proposes an in-plane polarization-induced sensor based on the α-In2Se3 ferroelectric material with reversible gas capture or release. A comprehensive quantum mechanical analysis on the characteristics of the device is presented by employing density functional theory calculations, ab initio molecular dynamic simulations, and the non-equilibrium Green's function formalism. The ferroelectricity of α-In2Se3 results in different surface polarizations, leading to different adsorption behaviors on surfaces with opposite polarizations. Based on the adsorption energy, charge transfer, and recovery time, the negatively polarized surface is better suited for gas capture than the positively polarized surface. Consequently, by applying a vertical electric field, which changes the out-of-plane polarization, reversible capture or release of NH3 and NO2 molecules can be achieved. However, applying vertical electric fields in transistor-based gas sensors is rather challenging. Meanwhile, the unique interlocking feature of electric dipoles in α-In2Se3 monolayer can be used to control gas adsorption or desorption by an in-plane bias voltage (electric field). The sensitivities of the positive and negative surfaces to the NH3 molecule are 107.6% and 11.3%, respectively, when an appropriate in-plane bias voltage is applied. The proposed sensor that allows manipulating the out-of-plane polarization by an in-plane bias is of special importance and opens up a new pathway for sensing applications.

Josephson diode based on conventional superconductors and a chiral quantum dot

We propose theoretically a Josephson diode consisting of the conventional superconductors with the plain $s$-wave pairing and a chiral quantum dot. When an external magnetic field is exerted on the quantum dot, the critical current of the Josephson structure is different for the opposite directions of current flow. The strong nonreciprocity can be obtained in a large area of the parameter space. The inversion and the on/off state of the nonreciprocity can be conveniently regulated by adjusting the direction of the external field. The dependencies of the nonreciprocal behaviors on the hopping amplitude, the magnitude of the magnetic field, and the energy level of the quantum dot are investigated in details using the Keldysh nonequilibrium Green's function formalism under the self-consistent procedure. The symmetric and the antisymmetric properties of the nonreciprocity are analyzed from symmetries satisfied by the superconductors. The proposed diode effect also applies to other chiral conductors as interlayer. Its formation has no restriction on the type of the current-phase difference relations. The generality and flexibility of our proposed diode effect provides more possibilities for the design of the dissipationless diode devices.

Negative differential resistance effect and current rectification in WS2 nanotubes: A density functional theory study

Electronic properties of (n,0) zigzag tungsten disulfide WS2 nanotubes and transport properties of (7,0) zigzag nanotube heterojunction are investigated by utilizing non-equilibrium Green's function formalism (NEGF). The results reveal that inclusion of the spin-orbit (SO) coupling significantly reduces the value of the band gap about 15.52\\%. Additionally, strong negative differential resistances take place in voltage regions between −0.2 and −0.4 V as well as between −0.5 and −0.6 V. Moreover, temperature dependent transport properties are elaborately investigated in this work. The results show that, as the temperature increases to 600 K the stronger negative differential resistance occurs in both positive and negative bias voltages. Finally, a reasonably large degree of rectification ratio can be established. Our analysis can provide a comprehensive perspective on the NDR effect and current rectification in WS2 nanotubes and can be used as a beneficial guide for future designing of novel logical and nanoelectronic and spintronic devices.

A Strongly Correlated Quantum Dot Heat Engine with Optimal Performance: A Nonequilibrium Green's Function Approach

Herein, an analytical study of a strongly correlated quantum dot‐based thermoelectric particle‐exchange heat engine for both finite and infinite on‐dot Coulomb interaction is presented. Employing Keldysh's nonequilibrium Green's function formalism for different decoupling schemes in the equation of motion, the thermoelectric properties within the nonlinear transport regime have been analyzed. Initially, Hubbard‐I approximation has been used to study the quantum dot level position (), thermal gradient (), and on‐dot Coulomb interaction (U) dependence of the thermoelectric properties. Furthermore, as a natural extension, a decoupling beyond Hubbard‐I (Lacroix approximation) with infinite‐U limit (strong on‐dot Coulomb repulsion) has been used to provide additional insight into the operation of a more practical quantum dot heat engine. Within this infinite‐U limit, the role of the symmetric dot‐reservoir tunneling (Γ) and external serial load resistance (R) in optimizing the performance of the strongly correlated quantum dot heat engine is examined. The infinite‐U results show a good quantitative agreement with recent experimental data for a quantum dot coupled to two metallic reservoirs.

Dual‐Gate GaAs‐Nanowire FET for Room Temperature Charge‐Qubit Operation: A NEGF Approach

AbstractThe performance of dual‐gate GaAs‐nanowire field‐effect‐transistor (FET) is investigated as a charge‐qubit device operating at room temperature. In compatibility with the state‐of‐the‐art classical bit technology, it is shown that the single gate of a nanowire FET can be replaced by two localized gates to achieve such charge‐qubit operation. On application of relevant biases to the localized gates, two voltage tunable quantum dots are created within the nanowire channel with electrostatically controlled single‐state‐occupancy and interdot coupling leading to charge‐qubit operation at room temperature. The associated electron transport is theoretically modeled on the basis of nonequilibrium Green's function formalism. The “initialization” and “manipulation” for qubit operation are performed by applying suitable gate voltages, whereas the “measurement” is executed by applying a small drain bias to obtain a pulse current of ≈pA order. A ≈25 MHz frequency of coherent oscillation is observed for the qubit and a characteristic decay time of ≈ 70 ns is achieved. The results suggest that such dual gate nanowire FET is a promising architecture for charge‐qubit operation at room temperature.

Phonon-limited mobility and quantum transport in fluorinated diamane MOSFETs from the first-principles calculations

Two-dimensional diamane with outstanding properties is promising for advanced nanodevice applications, whereas a comprehensive understanding of phonon-limited mobility as well as the prediction of device performance limit is still lacking. Here we report on phonon-limited mobility simulation in fluorinated diamane monolayer using first-principles calculations, with consideration of both elastic and inelastic phonon scattering processes based on Boltzmann transport equation. We construct sub-7 nm fluorinated diamane metal-oxide-semiconductor field-effect transistors (MOSFET) to investigate their quantum transport properties by first-principles calculations based on density functional theory coupling with the non-equilibrium Green's function formalism. Our findings show that fluorinated diamane mobility is concentration-dependent, with the electron and hole mobility reaching as high as 4390 and 10100 cm2V−1s−1, respectively, at the 1014 cm−2 carrier concentration. Our simulations reveal that the key figures of merits (FOMs) of fluorinated diamane MOSFETs are benchmarked against the International Technology Roadmap for Semiconductors (ITRS) standards for high-performance (HP) and low-power (LP) applications, showing superior potential compared to the most reported 2D materials. The simulated results demonstrate that the on-current, delay time, and power-delay product meet the ITRS requirements for HP and LP applications, including devices constructed with nano-scale channel length (≥3 and 5 nm) respectively. Finally, we show that the performance of a 32-bit ALU based on fluorinated diamane MOSFETs is comparable with emerging beyond-CMOS devices. Thus, our results shed light on the electronic properties of fluorinated diamane, making it superior to serve as a channel material in the post-silicon era.

Thermoelectricity in a Quasiperiodic Lattice Beyond Nearest‐Neighbor Electron Hopping

AbstractA new route to obtain a large figure of merit (>1) is reported, for the first time, by considering a one‐dimensional quasiperiodic lattice where an electron can hop beyond usual nearest‐neighbor sites. A finite positional correlation is imposed among the constituent lattice sites, following the well‐known Aubry‐André‐Harper (AAH) form. In the presence of second‐neighbor hopping, the AAH lattice exhibits energy dependent mobility edge which plays the central role for efficient energy conversion. Employing a tight‐binding framework, all the thermoelectric quantities are computed based on the standard nonequilibrium Green's function formalism. The analysis can be utilized to investigate thermoelectric phenomena in similar kinds of other aperiodic systems possessing higher order electron hopping.

Half-Metallic Heusler Alloy/MoS2 Based Magnetic Tunnel Junction.

Integration of half-metallic materials and 2D spacers into vertical magnetoresistive spin valves may pave the way for effective low-power consumption storage and memory technologies. Driven by the recent successful growth of graphene/half-metallic Co2Fe(Ge1/2Ga1/2) (CFGG) heterostructure, here we report a theoretical investigation of magnetic tunnel junction (MTJ) based on the ferromagnetic CFGG Heusler alloy and the MoS2 spacer of different thicknesses. Using ab initio approach, we demonstrate that the inherent ferromagnetism of CFGG is preserved at the interface, while its half-metallicity is recovering within few atomic layers. Ballistic transport in CFGG/MoS2/CFGG MTJ is studied within the nonequilibrium Green's function formalism, and a large magnetoresistance value up to ∼105% is observed. These findings support the idea of effective spintronics devices based on half-metallic Heusler alloys and highly diversified transition metal dichalcogenide family.