Nonequilibrium Green Research Articles

Nernst and seebeck effects in α−T3 lattice

By using the tight-binding Hamiltonian and non-equilibrium Green’s function methods, the Seebeck and Nernst effects of α−T3 lattice are investigated, in which the lattice interpolates between graphene and the dice lattice via the parameter α. For α = 0 (graphene), flat bands are always present in the band structure. The Seebeck and Nernst coefficients are consistent with those in graphene. When α is non-zero at zero magnetic field, the Seebeck coefficient is an odd function of the Fermi energy. It produces a very large and wide first peak within the band gap for the zigzag boundary. Under the influence of magnetic fields, the first peak of the Seebeck coefficient in the gap region increases with α increasing. The Nernst effect occurs under the influence of a magnetic field. The height of the zeroth peak of the Nernst coefficient increases with α increasing. When α reaches a certain value, the zeroth peak splits. The post-split peak decreases with α increasing for the zigzag boundary, but continues to become wider and higher for the armchair boundary.

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.

Photon-assisted destructive interference through a parallel-coupled double quantum dots

In this paper, we establish a model to realize the photon-assisted destructive interference phenomena, the model consisting of parallel-coupled double quantum dots and Luttinger liquid leads. By applying the canonical transformation and the nonequilibrium Green’s function technique, the general time-dependent current formula is developed. The joint effects of the intralead Coulomb interaction and an external ac field on the quantum interference are investigated. For weak intralead interactions, besides the usual photon-assisted tunneling resonance, there are two types of photon-assisted tunneling destructive interference: one is related to interdot coupling and the other to intralead interactions. For moderately strong intralead interaction, the photon-assisted sidebands are suppressed and the differential conductance scales as a power law in bias voltage. In particular, we show the multiphoton absorption and emission up to four photons in an electron tunneling process. We believe that the photo-assisted destructive interference results obtained in this paper are instructive for future experiments.

First-Principles Simulation and Materials Screening for Spin-Orbit Torque in 2D van der Waals Heterostructures.

Recent advancements in spin-orbit torque (SOT) technology in two-dimensional van der Waals (2D vdW) materials have not only pushed spintronic devices to their atomic limits but have also unveiled unconventional torques and novel spin-switching mechanisms. The vast diversity of SOT observed in numerous 2D vdW materials necessitates a screening strategy to identify optimal materials for torque device performance. However, such a strategy has yet to be established. To address this critical issue, a combination of density functional theory and non-equilibrium Green's function is employed to calculate the SOT in various 2D vdW bilayer heterostructures. This leads to the discovery of three high SOT systems: WTe2/CrSe2, MoTe2/VS2, and NbSe2/CrSe2. Furthermore, a figure of merit that allows for rapid and efficient estimation of SOT is proposed, enabling high-throughput screening of optimal materials and devices for SOT applications in the future.

Study of Carbon monoxide adsorption on B-N doped hydrogenated Sn nanoribbons towards nanosensor applications

This study utilized Van der Waals corrected density functional theory (DFT) and nonequilibrium Green's function (NEGF) to consider the potential of hydrogenated Armchair stanene nanoribbons of width 8 (HASnNR(8)) for CO gas detection. The study analyzed the most stable adsorption configuration, charge transfer, electronic transport, adsorption energy, sensitivity and electronic properties of CO gas molecules on HASnNR(8), both before and after B-N doping. The results showed that B-N doping led to a significant increase in adsorption energy and a gap opening at the Gamma point, resulting in semiconductor behavior. Analysis of charge transfer indicated that CO molecules acted as donors to HASnNR(8). Moreover, there was a notable change in the transmission functions of CO adsorbed on B-N doped HASnNR(8) compared to pure HASnNR(8). Based on the I-V characteristics, the study concluded that CO molecules could be easily detected after B-N doping. Finally, the study performed a recovery time analysis and sensitivity CO adsorbed on bare and B-N doped HASnNR(8) surfaces, with the results indicating that B-N doped HASnNR is a promising material for CO molecule sensing applications.

Biaxila strain modulated high anisotropic gas-sensing performance of C5N-based two-dimensional devices: A first-principles study

Two-dimensional carbon nitride materials have been widely used in many applications such as device fabrication, gas adsorption and separation due to their abundant element resources, high physicochemical stability and excellent electronic properties. In this paper, density functional theory and non-equilibrium Green's function method are used to study the electronic structure, transport characteristics and gas sensitivity of C5N-based structures. The results show that the electron transport exhibits obvious anisotropy, in which the electron transport in the armchair direction is more conductive than that in the zigzag direction. It is worth noting that the negative differential resistance effect exists in both directions. Furthermore, the transport and sensing characteristics of inorganic molecules (NO, CO, NO2, SO2, and NH3) adsorbed on the C5N monolayer are studied. The results show that CO, SO2 and NH3 exist on the surface of C5N in the form of physical adsorption, while NO and NO2 adhere to the surface of C5N in the form of chemical adsorption. The designed C5N gas sensor exhibits high sensitivity to NO and NO2 molecules, reaching 81 % sensitivity to NO2 at a 0.1 V bias voltage. Finally, the effect of strain on the adsorption properties of gas sensing devices is studied. Studies have shown that the application of -4 % strain in the armchair direction significantly increases the current of NO and NO2, significantly improving the performance of the gas sensor. Whether strain is applied to the device or gas adsorption, C5N materials always maintain significant anisotropy. This study shows that C5N is a highly anisotropic and sensitive two-dimensional material, which has broad application potential in the field of electronic properties and gas sensing.

Transition Metal (Co, V, W, Zr) Single-Atom Decorated Biphenylene for Enhancing the Sensing Performance of SF6 Decomposition Molecules.

The highly sensitive gas sensors used to monitor the decomposition of toxic gases in the dielectric materials of electrical equipment are vital in preventing safety problems arising from corrosion of the equipment. Recently, biphenylene (BPN) has been prepared through surface interpolymer hydrofluorination (HF zipper) reaction, whereas potential gas-sensitive devices based on the BPN monolayer have lacked in-depth investigation. The stable geometries, adsorption energies, interlayer distances, and charge transfers of small molecules of toxic gases (H2S, SO2, SOF2, SO2F2) produced by SF6 chalcogenide molecules of decomposition adsorbed on the original BPN monolayer are systematically researched by using nonequilibrium Green's function methods and density functional theory. The results indicated that all small molecules adsorbed on the BPN monolayer are physisorbed, while the type of adsorption turned from physisorption to chemisorption when BPN carried out adsorption with adsorbing a transition metal atom (TMA). In addition, the characteristics of current-voltage (I-V) curves of H2S and SO2 based on the TMA-BPN gas sensors revealed that the currents in BPN-based gas sensors displayed an obvious anisotropy, and the currents in the zigzag direction are larger than that in the armchair orientation regardless of the molecular adsorption cases. Moreover, the difference of currents for TMA-decorated BPN sensors changed more remarkably before and after the adsorption of H2S and SO2 in the zigzag direction. This work offers insights into the design of gas-sensitive devices through the adsorption of small molecules on the TMA-decorated BPN monolayer.

Accelerating Nonequilibrium Green Functions Simulations: The G1–G2 Scheme and Beyond

The theory of nonequilibrium Green functions (NEGF) has seen a rapid development over the recent three decades. Applications include diverse correlated many‐body systems in and out of equilibrium. Very good agreement with experiments and available exact theoretical results could be demonstrated if the proper selfenergy approximations were used. However, full two‐time NEGF simulations are computationally costly, as they suffer from a cubic scaling of the computation time with the simulation duration. Recently the G1–G2 scheme that exactly reformulates the generalized Kadanoff–Baym ansatz with Hartree–Fock propagators (HF‐GKBA) into time‐local equations is introduced, which achieves time‐linear scaling and allows for a dramatic speedup and extension of the simulations (Schluenzen et al. Phys. Rev. Lett. 2020, 124, 076601). Remarkably, this scaling is achieved quickly, and also for high‐level selfenergies, including the nonequilibrium GW and T‐matrix approximations (Joost et al. Phys. Rev. B 2020, 101, 245101). Even the dynamically screened ladder approximation is now feasible (Joost et al. Phys. Rev. B 2022, 105, 165155), and also applications to electron‐boson systems are demonstrated. Herein, an overview on recent results that are achieved with the G1–G2 scheme is presented. Problems and open questions are discussed and further ideas of how to overcome the current limitations of the scheme and present are presented. The G1–G2 scheme is illustrated by presenting applying it to the excitation dynamics of Hubbard clusters, to optical excitation of graphene, and to charge transfer during stopping of ions by correlated materials.

Edge-dependent electronic transport and diode effect in C3N nanoribbons

Quantum transport calculations are conducted for C3N nanoribbons with different edge terminations within the nonequilibrium Green’s function formalism in combination with density functional theory. The electronic transport in the system strongly depends on the properties of the edge states. For example, the current in metallic carbon-terminated nanoribbons is several orders of magnitude higher than the current in nitrogen-terminated nanoribbons with a semiconducting nature. In addition, C-terminated nanoribbons show very pronounced negative differential resistance. A diode structure with a rectification ratio of 10 is proposed, consisting of a heterojunction of C- and N-terminated nanoribbons. These findings can be of practical importance in creating functional device structures from this 2D material.

Charge Transport Enhancement in Some Acene-Based Molecular Junctions

The field of single molecule electronics research has been spurred by the need to add functionality to next-generation electronic circuits  and the desire to miniaturize devices. We focused on some acenebased molecular junctions, in which molecules are connected to Au  electrodes via anchor groups. The frontier molecular orbitals (FMOs) and charge transport nature of the junction were examined by employing the density functional theory (DFT) in conjunction with the non-equilibrium Green's functional (NEGF) formalism. Particular  attention was paid to the anchoring groups (NH2, S, CN, and SH), chemical impurity doping (B, N, and NB), and side groups (-CH3, -NH2,  and –NO2). The findings demonstrate that the FMOs can change depending on the dopants or substituents employed. Additionally, it is  noted that depending on the materials employed, charge transfer may be p-type or ntype. It is discovered that the molecular junctions  have the highest conductivity when cyanides are used as anchor groups. It is therefore the ideal anchor material to utilize with tetracene  and pentacene molecules. Understanding and creating n-type or p-typed high conductivity single molecule electronic components can be  greatly aided by these discoveries. 

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.

Exploring Intertwined Quantum and Cryogenic Behaviour in Ultra-Scaled 10nm MOSFET: A NEGF Quantum Ballistic Simulation

Abstract Silicon-based spin qubits have emerged as promising candidates for scalable quantum information processing. This study first time investigates the behaviour of ultra-scaled 10nm gate length and 3nm channel thickness nanoscale double gate metal-oxide semiconductor field-effect transistors (MOSFETs) over a broad temperature range, from deep cryogenic (4 K) to room temperature (300 K). Employing the Non-Equilibrium Green's Function (NEGF) method, the research explores the intertwined quantum and cryogenic behaviours of the various quantum phenomena, including eigen energies, eigen-functions, electron concentration, current characteristics, and more. This comprehensive analysis sheds light on the intricate interplay of quantum effects in nanoscale transistors under deep cryogenic conditions, offering valuable insights into the development of cryo-CMOS circuits for quantum computing.

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.

Exploring stable half-metallicity and magnetism of Cr-based double half-Heusler alloys and its high magnetoresistance ratio in magnetic tunnel junction

A new series of double half-Heusler (DHH) alloy Cr2FeCoZ2 (Z = As, Ge, S, P) are investigated by using the non-equilibrium Green's function in combination with the first-principles calculations. The calculations on magnetism reveal that these Cr-based DHH alloys are ferrimagnets due to the antiparallel magnetic coupling between Cr and Fe (Co). The electronic structure calculations of the Cr-based DHH alloys reveal half-metallicity, and Cr2FeCoAs2 possesses the largest spin-down gap of 1.181 eV. Within the c/a ranges from 1.60 to 2.52, Cr2FeCoAs2 maintains its 100 % spin-polarization, and the changes in total and spin-resovled atomic magnetic moment are negligible, showing a stable half metallicity and magnetism against the tetragonal distortion. Futhermore, the transport properties are investigated by constructing the Cr2FeCoZ2/GaAs/Cr2FeCoZ2 magnetic tunnel junction (MTJ) model. By changing the magnetic arrangement of the left and right electrodes into antiparallel configuration, the capability of transportation in both spin channels is significantly limited. Conversely, by modulating the magnetic arrangement of the left and right electrodes into parallel configuration, the transportation of electrons in spin-up channel is exceptionally strong while that of the spin-down channel is severely impeded. The Cr2FeCoAs2/GaAs/Cr2FeCoAs2 MTJ owns the highest tunneling magnetoresistance (TMR) ratio of ∼7.42 × 108 %, indicating that Cr2FeCoAs2 is the most promising candidate for high performance spintronic devices.

Topological Phonons and Thermoelectric Conversion in Crystalline Materials

AbstractTopological phononics, a fascinating frontier in condensed matter physics, holds great promise for advancing energy‐related applications. Topologically nontrivial phonons typically possess gapless edge or surface states. These exotic states of lattice vibrations, characterized by their nontrivial topology, offer unique opportunities for manipulating and harnessing energy transport. The exploration of topological phonons opens new avenues in understanding and controlling thermal transport properties, with potential applications in fields such as thermoelectric materials, phononic devices, and waste heat recovery. Here, an overview of concepts such as Berry curvature and topological invariants, along with the applications of phonon tight‐binding method and nonequilibrium Green's function method in the field of topological phononics is provided. This review encompasses the latest research progress of various topological phonon states within crystalline materials, including topological optical phonons, topological acoustical phonons, and higher‐order topological phonons. Furthermore, the study delves into the prospective applications of topological phonons in the realm of thermoelectric conversion, focusing on aspects like size effects and symmetry engineering.

First principles study of electronic structure and transport in graphene grain boundaries

Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions. We generated 150 different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB. We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.

Realization of IR photodetector based on AGNRs operating in the atmospheric windows

Nowadays, the importance of Atmospheric windows in optoelectronic industries is not hidden from anyone. On the other hand, using graphene nanoribbons widely has been applied in optical devices. So, armchair graphene nanoribbons (AGNRs) from three different families as IR photodetectors are proposed and modeled, in this work. Our results show that the energy gap of AGNRs has been in the 1–3 μm and 8–14 μm atmospheric windows in the pure state and with nitrogen (N)/boron (B) substitutional doping. Also, responsivity and specific detectivity (D*) have been calculated, by introducing a simple structure as an IR photodetector. The energy gap and dark current (Jdark) calculations have been done by density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism, respectively. The specific detectivity peaks on the order of 109 cmHz1/2/W for 9-AGNR and 10-AGNR at 1–3 μm atmospheric window have been concluded. Furthermore, our results show that 11-AGNR with B dopants can be used to design dual-band detectors in the 1–3 μm and 8–14 μm atmospheric windows.

High rectifying performance of armchair C[formula omitted]N heterojunction based on the interface between pristine and H-passivated nanoribbons

Two-dimensional polyaniline with a C3N structure is a newly fabricated layered material that is expected to have fascinating electronic, thermal, mechanical, and chemical properties. By performing first-principles calculations based on density functional theory and the nonequilibrium Green’s function, we first perform a stoichiometric study of the energy bands of armchair C3N nanoribbons (AC3NNRs) without and with H-passivation. The results show that the pristine AC3NNRs are metals, whereas the H-passivated nanoribbons are either direct or indirect band gap semiconductors with different edge configurations. Interestingly, additional transport calculations demonstrate that the AC3NNR-based heterojunction shows good rectification behavior. The average rectification ratio (RR) can reach up to 103 under voltage bias within the range from 0.2 to 0.4 V. In particular, extending the length of the scattering region in the heterojunction, which leads to a reduction in the current passing through the junction, allows the RR to be enlarged obviously. The average value of RR increases to a magnitude of the order of 104 under bias voltages in the range from 0.25 to 0.4 V, with a boosted maximum of up to 105 at 0.35 V. The findings of this work may be helpful in the design of functional nanodevices based on AC3NNRs in the future.

Modulating the electronic transport properties of zigzag phosphorene nanoribbons through the coupling with the chromium triiodide molecules

Based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) method, we have investigated spin-dependent electronic transport properties of zigzag phosphorene nanoribbons (ZPNRs) coupled with chromium triiodide (CrI3) molecules. Three different adsorption models have been considered: CrI3 molecule is placed on ZPNR surface (C-type), and CrI3 molecule is lateral adsorbed on the edge of ZPNR with its molecule panel vertical (V-type) or parallel (P-type). We discovered that ZPNR's electrical transport clearly displays spin polarization, owing to the coupling between ZPNR and CrI3 molecules. And spin-polarization is tightly dependent on the absorbing manners. Moreover, the negative differential resistance (NDR) characteristics can be observed from current-voltage curves and modulated by absorbing molecules. Furthermore, we also discovered that CrI3 is vertically adsorbed on the surface of the ZPNRs has a better spin-polarization efficiency than the other two cases.

The spin transport properties of Ni(111)/SnSe/Ni(111) lateral tunnel junction

Spin-resolved transport properties of the armchair-edged SnSe lateral contact with two magnetic Ni(111) leads are investigated by density functional theory combined with nonequilibrium Green's function method. The spin-polarized currents, magnetoresistances and k-resolved transmission spectrums are simulated in spin parallel and antiparallel configurations. It is found that the heterostructure exhibits a stable spin injection efficiency which can reach up to 80% at low bias range in spin parallel configuration, suggesting the large spin-diffusion length of SnSe. The magnetoresistance can be maintained stably at about 40% at a wide bias range. These results make SnSe material possible to be used in spintronic devices.