Nonequilibrium Green Function Approach Research Articles

Detecting Hachimoji DNA: An Eight-Building-Block Genetic System with MoS2 and Janus MoSSe Monolayers.

In the pursuit of personalized medicine, the development of efficient, cost-effective, and reliable DNA sequencing technology is crucial. Nanotechnology, particularly the exploration of two-dimensional materials, has opened different avenues for DNA nucleobase detection, owing to their impressive surface-to-volume ratio. This study employs density functional theory with van der Waals corrections to methodically scrutinize the adsorption behavior and electronic band structure properties of a DNA system composed of eight hachimoji nucleotide letters adsorbed on both MoS2 and MoSSe monolayers. Through a comprehensive conformational search, we pinpoint the most favorable adsorption sites, quantifying their adsorption energies and charge transfer properties. The analysis of electronic band structure unveils the emergence of flat bands in close proximity to the Fermi level post-adsorption, a departure from the pristine MoS2 and MoSSe monolayers. Furthermore, leveraging the nonequilibrium Green's function approach, we compute the current-voltage characteristics, providing valuable insights into the electronic transport properties of the system. All hachimoji bases exhibit physisorption with a horizontal orientation on both monolayers. Notably, base G demonstrates high sensitivity on both substrates. The obtained current-voltage (I-V) characteristics, both without and with base adsorption on MoS2 and the Se side of MoSSe, affirm excellent sensing performance. This research significantly advances our understanding of potential DNA sensing platforms and their electronic characteristics, thereby propelling the endeavor for personalized medicine through enhanced DNA sequencing technologies.

High sensitivity of s-triazine based g-C3N4 surface to pollutant gases

Rapid detection of harmful airborne substances caused by daily activities and industrial activity is vital. In the current study, the applicability of s-triazine based g-C3N4 nanosheet as a potential sensor for environmentally dangerous gases, NO, N2O, NO2, SO, SO2, and SO3, was investigated using the density functional theory and non-equilibrium Green's function (NEGF) method. Sulfur-containing gases exhibited stronger interactions with the surface. The obtained adsorption energies are in the range of −0.21 to −1.13 eV, which is associated with charge transfer in the range of −0.09 to −0.29 e. The molecules' overall structures are preserved during the adsorption process. The sensitivity of the surface to the adsorbed molecules was confirmed by analyzing the work function changes and calculating the transmission coefficients using the NEGF approach. All adsorbate species, except SO3, have short recovery times of up to 3.9 s at room temperature, proving the efficiency of this surface as a viable sensor material. Furthermore, by analyzing the humidity impact, the g-C3N4 ability as a sensor in more realistic conditions was also validated. The results indicate that the g-C3N4 surface can be an excellent candidate for toxic gas detection with high selectivity and sensitivity.

Significant effects of magnetic electrodes on rhenium phthalocyanine molecules

The investigation focused on examining the spin polarization and thermal spin polarization transport characteristics of dual rhenium phthalocyanine (Re2PC2) molecular junctions. This analysis was conducted using first-principles density-functional theory and the non-equilibrium Green's function approach. Calculations were performed on molecular junctions in two configurations: one at a non-magnetic electrode (Au) and the other at a magnetic electrode (Ni). A comparison of the spin transport properties of the two electrodes shows significant differences. The magnetic electrode has a notable effect on the spin-polarization and thermal spin-polarization transport properties of the devices. In contrast, the nickel electrode bis-phthalocyanine rhenium molecular device exhibits perfect spin/thermal spin-filtering efficiencies in a parallel structure. The properties of the dual phthalocyanine rhenium molecular devices were significantly affected by the nature of the electrodes. This makes dual rhenium phthalocyanine molecular junctions have great potential for the development of high-performance multifunctional spintronic and spin caloritronic devices.

Nonequilibrium Dynamics of the Hubbard Dimer

Electron dynamics in a two‐sites Hubbard model is studied using the nonequilibrium Green's function approach. The study is motivated by the empirical observation that a full solution of the integro‐differential Kadanoff–Baym equation (KBE) is more stable and often accompanied by artificial damping [Marc Puig von Friesen, C. Verdozzi, and C.‐O. Almbladh (2009)] than its time‐linear reformulations relying on the generalized Kadanoff–Baym ansatz (GKBA). Additionally, for conserving theories, numerical simulations suggest that KBE produces natural occupations bounded by one and zero in agreement with the Pauli exclusion principle, whereas, in some regimes, GKBA‐based theories violate this principle. As the first step for understanding these issues, the electron dynamics arising in the adiabatic switching scenario is studied. Many‐body approximations are classified according to the channel of the Bethe–Salpeter equation in which electronic correlations are explicitly treated. They give rise to the so‐called second Born, T‐matrix, and GW approximations. In each of these cases, the model is reduced to a system of ordinary differential equations, which resemble equations of motion for a driven harmonic oscillator with time‐dependent frequencies. A more complete treatment of electronic correlations is achieved by combining different correlation channels, with parquet theory serving as a starting point.

A twist for tunable electronic and thermal transport properties of nanodevices.

Twisted graphene-layered materials with nonzero interlayer twist angles (θ) have recently become appealing, as they exhibit a range of attractive physical properties, which include a Mott insulating phase and superconductivity. In this study, we consider nanodevices constructed from zigzag graphene nanoribbons with a top rectangular benzenoid [6,3]-flake. Using density functional theory and a non-equilibrium Green's function approach, we explore how the electronic and thermal transport properties in such nanodevices can be tuned through a twist of the top flake by an angle 0° ≤ θ ≤ 8.8° for different stacking configurations. We found a strong dependency of the electronic structure on the stacking type, as well as on the twisting regime, specifically in AA-stacking devices. Electron and hole van Hove singularities (vHSs), which originate, respectively, from the flatness of the top of the valence band for the minor-spin component and the bottom of the conduction band for the major-spin component, are found very close to the Fermi level in the density of states and electronic transmission spectra of AA-stacking devices with a twist angle of 1.1°. We establish that these vHSs in AA-1.1° devices are stable at higher temperatures and, with the increased number of available states, lead to larger values of electron thermal conductivity and finally total thermal conductivity in AA-1.1°. Our work highlights the essential role of twisting and stacking for the fabrication of nanoscale charge and heat switches and spurs future studies of twisted layered structures.

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.

Rate equations description of the asymmetric double barrier electronic cooler

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments.

SnSe Monolayer-Based heavy metal sensors with high Sensitivity, Selectivity, and Reusability: Insights from first principle calculation

Heavy metal pollution has a negative impact on both human health and the environment. The monitoring of heavy metal is therefore very important from a practical standpoint. In this study, the adsorption energy, charge transfer, band, densities of states, and sensitivity of SnSe monolayers toward heavy metals (As, Cd, Cr, Hg, Ni, Pb) were investigated using density functional theory combined with the non-equilibrium Green's function approach. The calculations show that SnSe monolayers have excellent sensitivities exceeding the limit of quantitation (LOQ) towards As, Cd, Hg, and Pb, which are as high as 384467%, 1462%, 1791%, and 26160%, respectively. Moreover, analyzing response peaks at various bias voltages can identify the composition of heavy metals from diverse sources, providing valuable insights for selective heavy metal monitoring. SnSe monolayers exhibit wide bias voltage windows that enhance sensor sensitivity by reaching the detection threshold, while also simplifying sensor encapsulation, resulting in higher reliability. The SnSe monolayer exhibits rapid recovery times for As, Hg, and Cd at room temperature, while for Pb, heating to 498 K is required for rapid recovery. These findings show that SnSe monolayers have a strong potential for constructing extremely sensitive and selective heavy metal sensors that are also reusable, implying that SnSe monolayers might serve as a possible online sensor for environmental monitoring.

Fully relativistic first-principles quantum transport simulation of noncollinear spin transfer and spin Hall current

In this work, we report the fully relativistic (FR) first-principles quantum transport simulation of noncollinear spin transfer and spin Hall current in the device structure. In this method, the noncollinear FR exact muffin-tin orbital method is combined with Keldysh's nonequilibrium Green's function approach and mean-field theory to account for the multiple disorder scattering. We adopt the Bargmann-Wigner polarization operator to define the appropriate FR spin current so that the current-induced spin transfer, in the noncollinear magnetic device or due to the spin Hall effect, can be studied from first principles. As applications, we calculate the spin transfer torque in noncollinear spin valves Co/Cu/FM/Cu (FM = Co, ${\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}$) and spin Hall angles in various ${\mathrm{Pt}}_{1\ensuremath{-}x}{Y}_{x}$ [$Y$ = vacancy (Va), Au, Ag, Pd] alloys. We find that our FR results agree well with previous theoretical simulations and experimental measurements. Moreover, it is found that the applied finite bias can significantly enhance the spin Hall angle in ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Va}}_{x}$, and PtAg alloy presents a much higher spin Hall angle than that of PtAu and PtPd alloys. Our implementation of the FR method provides an important first-principles tool for studying various nonequilibrium spin phenomena and the associated relativistic effects in realistic device structures with atomic disorders, including both current-induced spin transfer and spin-orbit torques.

The influence of single carbon atom impurity on the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes

In this study, the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes ((6, 3) TSC-SWBNNTs) located between two electrodes of (5, 5) conductive carbon nanotubes is investigated.Introducing carbon impurities instead of nitrogen and boron atoms in different locations of two side-closed (6, 3) SWBNNTs would change the transmission spectrum and reduce the gap. As the bias voltage increases, the peaksof the transmission spectrum become sharper and narrower. Substituting carbon impurities instead of nitrogenatoms leads to larger currents than substituting carbon impurities instead of boron. For the carbon impurity insteadof N atoms, the current in the center is observed to be larger than currents on the left and right sides. In addition,negative resistance can be seen in current-voltage diagrams, which are used in the construction of high-speedelectronic switches in electrical circuits. Due to the larger number of atoms, the study of structures by the common approach is time-consuming and sometimes impossible. Hence, in this research, the Quantum ATK simulation software is used to investigate the characteristics of electronic transport. For this purpose, the Slater-Koster method and the tight-closure approximation are used. In this method, the non-equilibrium Green function (NEGF) approach is employed.

Correlated Mott insulators in a strong electric field: The effects of phonon renormalization

We characterize the response of a Mott insulating system to a static electric field in terms of its conducting and spectral properties. Dissipation is included by a coupling to fermionic baths and to either optical or acoustic phonons. This paper extends and completes the analysis made in a previous work by the authors [Mazzocchi et al., Phys. Rev. B 106, 125123 (2022)]. In the present work, phonons are included diagrammatically within the Migdal approximation by also including self-consistency from the electronic feedback. The nonequilibrium steady state is addressed by means of the dynamical mean-field theory based on the nonequilibrium Green's function approach, while the so-called auxiliary master equation approach is employed as impurity solver. With optical phonons the self-consistency suppresses the steady-state current for field strengths comparable to the band gap with respect to the non-self-consistent case. This is due to the interaction of phonons with the hot electrons of the lattice which increases their temperature, thus providing a less effective relaxation channel for the current-induced Joule heat. In addition, in the case of optical phonons, the results are essentially independent of the temperature of the fermionic baths, as the latter is sensibly smaller than their characteristic frequency. On the other hand, with acoustic phonons the steady-state current is slightly suppressed by the self-consistent treatment only at field strengths close to half of the gap and especially at very small phonon frequency. Also, in this case, the results seem to slightly depend on the temperature of the fermionic baths.

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.

Chiral-Induced Spin Selectivity in Photon-Coupled Achiral Matters.

Chiral-induced spin selectivity is a phenomenon in which electron spins are polarized as they are transported through chiral molecules, and the spin polarization depends on the handedness of the chiral molecule. In this study, we show that spin selectivity can be realized in achiral materials by strongly coupling electrons to a circularly polarized mode of an optical cavity or waveguide. Through the investigation of spin-dependent electron transport in a two-terminal setup using the nonequilibrium Green's function approach, it is found that a large spin polarization can be obtained if the rate of dephasing is sufficiently small and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the optical mode. To obtain a wider range of energies for a large spin polarization, chiral molecules can be combined with light-matter interactions. To demonstrate this, the spin polarization of electrons transported through a helical molecule strongly coupled to a circularly polarized optical mode is evaluated.

Electron Transport through Nanoconfined Ferrocene Solution: Density Functional Theory─Nonequilibrium Green Function Approach

We investigate the transport of electrons in a ferrocene aqueous solution nanoconfined between two Pt electrodes using density functional theory and a nonequilibrium Green function method. The system consists of three characteristic phases: metal electrodes, the electrode–solution interface, and the nanoconfined solution phase. By performing the geometry optimization of such systems, it is found that the molecular configuration of water molecules at the Pt surface is adjusted due to the Pt–water interaction, and the charges at the Pt surface are redistributed. Next, by applying the external bias potential via the effective screening medium method during ab initio molecular dynamics simulations, it is revealed that water molecules are packed at the Pt–water interfacial region, and the electrostatic potential profile in the water phase is significantly changed due to aligned water layers. From the analysis of the electron transport characteristics, it is discovered that the ferrocene molecule generates a strong transmission function near the Fermi level and high electron density of states spatially connecting both electrodes through the water phase. These features explain the drastic enhancement in electrical current–voltage characteristics. Therefore, it is concluded that ferrocene enhances electron transport through nanoconfined solutions, indicating that it can be used as a signaling molecule in nanoscale systems for electrochemical sensor applications.

The spin caloritronic transport properties of newly designed devices consisting of a sawtooth graphene nanoribbon and its derived five-member ring structure.

Achieving high spin polarization transport and a pure spin current is particularly desired in spintronics. We use a sawtooth graphene nanoribbon (STGNR) and its derived five-member ring structure (5-STGNR) to design new spin caloritronic devices, since they have been successfully prepared experimentally and have an interface with no lattice distortion. By using first-principle calculations combined with the non-equilibrium Green's function approach, we have studied the spin caloritronic transport properties of several STGNR-based devices, including the structures with symmetrical and asymmetrical edges, and found some excellent spin caloritronic properties, such as spin polarization, magnetoresistance and the spin Seebeck effect. By introducing a temperature difference, giant magnetoresistance and spin Seebeck effects are achieved in a heterojunction with a symmetrical edge, whereas spin polarization is more effective in a heterojunction with an asymmetrical edge. Meanwhile, the metal-semiconductor-metal junction, which is composed of STGNRs with a symmetrical edge, exhibits approximately 100% spin polarization and produces a perfect thermally induced pure spin current at room temperature. Our results indicate that the devices consisting of a sawtooth graphene nanoribbon and its derived five-member ring structure are promising novel spin caloritronic devices.

Charge filling induced quasi-two dimensional charge/spin density wave channel in a ballistic transport device

Charge filling controlled mean field metal–insulator phase transition is examined in the context of two dimensional Fermi surface nesting and van Hove singularity induced charge density wave (CDW), spin density wave (SDW) condensates. In the framework of a coherent ballistic transport model utilizing the Non-Equilibrium Green Function approach (NEGF), a three terminal device with metallic gate, source, drain and CDW/SDW channel is simulated and studied. Within the validity of mean field approximation, we exposit the commensurability and boundary effects. The efficacy of the Hubbard model for (quasi) two dimensional Charge and Spin Density Wave materials is discussed. A two orbital generalization of the effective Hamiltonian is proposed for transport calculations in rare earth Tellurides RTe 3.

Giant tunnel magnetoresistance in two-dimensional van der Waals magnetic tunnel junctions: Ag/CrI3/MoSi2N4/CrI3/Ag

With intrinsic magnetism, two-dimensional (2D) $\mathrm{Cr}{\mathrm{I}}_{3}$ has attracted tremendous interest because of the potential application in magnetic devices of smaller size. We propose to use 2D $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ material, which has remarkable stability, excellent electronic properties, and high mobility, as tunnel barrier in vertical magnetic tunnel junction (MTJ), and demonstrate that it is able to generate a giant tunnel magnetoresistance (TMR) over ${10}^{5}%$ when integrated with $\mathrm{Cr}{\mathrm{I}}_{3}$ ferromagnetic layers and Ag electrodes. Combining with density-functional theory and nonequilibrium Green's function approach, we systematically investigate the electron transmission, band structures, and projected local density of states and elaborate the transmission mechanism. The TMR and spin injection efficiency maintain high values below 0.25 V. These results indicate that $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ is a promising barrier material in future 2D vertical MTJs and provide important guidance for designing devices.

Multiscale Pseudoatomistic Quantum Transport Modeling for van der Waals Heterostructures

Several electronic and optoelectronic devices have been proposed in recent years based on vertical heterostructures of two-dimensional (2D) materials. The large number of combinations of available 2D materials and the even larger number of possible heterostructures require effective and predictive device-simulation methods, to inform and accelerate experimental research and to support the interpretation of experiments. Here, we propose a computationally effective and physically sound method to model electron transport in 2D van der Waals heterostructures, based on a multiscale approach and quasiatomistic Hamiltonians. The method uses ab initio simulations to extract the parameters of a simplified tight-binding Hamiltonian based on a uniform three-dimensional lattice geometry that enables device simulations using the nonequilibrium Green's function approach in a computationally effective way. We describe the application and limitations of the method and discuss the examples of two use cases of practical electronic devices based on 2D materials, such as a field-effect transistor and a floating-gate memory, composed of molybdenum disulphide, hexagonal boron nitride and graphene.

Combined density functional theory calculation and non-equilibrium Green's function approach to predict the sensitivity of nitrogen-containing gases over PtTenS2-n monolayers (n = 0–2)

Two-dimensional (2D) transition-metal dichalcogenide (TMD) monolayers are potential toxic gas sensors for next-generation industrial applications. In this study, we adopted density functional theory (DFT) calculations to investigate the sensing properties of harmful nitrogen-containing gases (NCGs) on Pt-based TMDs (PtTenS2-n monolayers, where n = 0–2). Our results showed that the electronic interactions of ammonia (NH3) on all TMDs and nitrogen oxides (NOx) on PtS2 were weak. However, the electronic interactions were strong between the NOx molecules and PtTe2 or Janus PtTeS monolayers. Moreover, the bandgap was reduced after NOx adsorption on the PtTe2 and Janus PtTeS monolayers; however, the reduced amplitude of NOx on the Janus PtTeS monolayer was more significant than that on the PtTe2 monolayer. By combining Non-Equilibrium Green’s Function (NEGF) calculations, the sensitivities were all found to be poor for the NCGs on PtS2. Nitrogen dioxide (NO2) exhibited higher sensitivity than those exhibited by NH3 and nitric oxide (NO) at 1.0 V on the PtTe2 monolayer; therefore, the selectivity of NO2 was high under this condition. In addition, NO and NO2 possessed high sensitivities at 0.6 and 0.4 V on the Janus PtTeS monolayer, respectively. Consequently, the sensitivity of NOx molecules on the Janus PtTeS monolayer was higher than that on the PtTe2 and PtS2 monolayers at relatively low applied biases. The results of this study indicate that the Janus PtTeS monolayer is a promising NOx gas sensor with both high selectivity and sensitivity for NCG-sensing applications.

Classical and quantum harmonic mean-field models coupled intensively and extensively with external baths

We study the nonequilibrium steady-state of a fully-coupled network of N quantum harmonic oscillators, interacting with two thermal reservoirs. Given the long-range nature of the couplings, we consider two setups: one in which the number of particles coupled to the baths is fixed (intensive coupling) and one in which it is proportional to the size N (extensive coupling). In both cases, we compute analytically the heat fluxes and the kinetic temperature distributions using the nonequilibrium Green's function approach, both in the classical and quantum regimes. In the large N limit, we derive the asymptotic expressions of both quantities as a function of N and the temperature difference between the baths. We discuss a peculiar feature of the model, namely that the bulk temperature vanishes in the thermodynamic limit, due to a decoupling of the dynamics of the inner part of the system from the baths. At variance with the usual case, this implies that the steady-state depends on the initial state of the bulk particles. We also show that quantum effects are relevant only below a characteristic temperature that vanishes as 1/N. In the quantum low-temperature regime the energy flux is proportional to the universal quantum of thermal conductance.