Nonequilibrium Green Function Research Articles

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.

Interlayer coupling modulated for thermoelectric transport characterization of black arsenic nanoscale devices.

Modulating interlayer coupling modes can effectively enhance the thermoelectric properties of nano materials or nanoscale devices. By using density functional theory (DFT) combined with non-equilibrium Green's function (NEGF) method, we investigate the thermoelectric properties of zigzag-type black arsenic nanoscale devices with varying interlayer coupling modes. Our results show that altering the interlayer coupling mode significantly modulates the thermoelectric properties of the system. Specifically, we consider four coupling modes with different strengths, by modulating different interlayer overlap patterns. Notably, in the weaker interlayer coupling mode, the system exhibits enhanced thermoelectric properties due to increased interface phonon scattering, reaching a peak value of 2.23 at μ =-0.73eV. Furthermore, we explore the temperature-dependent behavior of each coupling model. The results suggest that the thermoelectric characteristics are more sensitive to temperature variations in the weaker coupling modes. These insights provide valuable guidance for enhancing the thermoelectric performance of nano-scale devices through precise interlayer coupling modulation.

Layer-number parity-dependent oscillatory spin transport in β -Ga2O3 magnetic tunnel junctions

Spintronics devices have been a research hotspot due to their rich theoretical and application value. The widebandgap semiconductor β-Ga2O3 has excellent application potential in spintronics due to the controllability of its electron behavior via ultraviolet light. This paper employs first-principles calculations and the Wenzel–Kramers–Brillouin (WKB) approximation to comprehensively investigate spin transport based on magnetic tunnel junctions (MTJs) comprising β-Ga2O3 nanosheets. The magnetic moment of the ferromagnetic layer in β-Ga2O3 MTJs is found to be positively correlated with tunnel magnetoresistance (TMR). Interestingly, layer-number parity-dependent oscillation of TMR in β-Ga2O3 MTJs is observed, which is explained by the non-equilibrium Green function and the WKB approximation. TMR reaches a maximum of 1077% at five layers, and bias-dependent stability is observed in the monolayer model under biases of 0–20 mV. This study not only expands the application potential of β-Ga2O3 and predicts its superiority in spintronics but also enriches the related condensed matter theory.

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.

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.

Broadband spin-filtered minimalistic magnetic tunnel junction

The tri-layer magnetic tunnel junction (MTJ) has surfaced as a building block for engineering next-generation integrated circuits while combining the attributes of non-volatility and meager energy consumption. Nevertheless, the perceptible switching energy (≈20–50 fJ/bit) and sub-optimal tunnelmagnetoresistance (TMR) (≈200%–300%) have acted as major hindrances, concealing its potential to supersede the capabilities of static and dynamic random access memories. In this work, we introduce a novel device that features a minimalistic non-uniform heterostructure/superlattice instead of the oxide layer in a conventional MTJ and analyze it in the premise of the self-consistent coupling of the Non-Equilibrium-Green’s Function (NEGF) and the Landau-Liftshitz-Gilbert-Slonczewski (LLGS) equation. We ascertain that the coupling of the electrodes to the proposed heterostructure renders a highly spin-selective broadband transmittance, thereby enabling a towering TMR (%) of 3.7 × 104% along with a significant reduction in the spin transfer torque (STT) switching energy (≈1.96 fJ). Furthermore, the sizable slonczewski term (Is‖) originating from the heterostructure facilitates a swift STT-switching within the scale of a few hundred picoseconds (≈400 ps).

Efficient detection of lung cancer biomarkers using functionalized transition metal dichalcogenides (MoS2) Monolayers: DFT study

Selective sensing properties of transition metal dichalcogenides (MoX2, X = S, Se) towards specific volatile organic compounds (VOCs) associated with lung-cancer are investigated using state-of-the-art density-functional theory (DFT) methods. In the present investigation, a combination of DFT and the non-equilibrium Green’s functions (NEGF) formalism are employed to probe the sensing of four VOCs; namely: (i) Isoprene “C5H8”, (ii) Toluene “C7H8”, (iii) Cyclopropanone “C3H4O”, and (iv) Isopropanol “C3H8O”; and four interfering air molecules CO2, H2O, N2 and O2. We find that the doping of single atom of selected transition metals (TMs = Mn, Fe, Ni, Cu) can enhance both the sensitivity and the selectivity of MoX2. Our results show that the selectivity is rather distinct towards the detection of VOCs when TMs doping is targeting the chalcogenide site. Adsorption energies, charge transfers, electronic properties through density of states and band structures, and the sensor responses are obtained in all the cases, particularly for C5H8 and C3H8O, which show superior selectivities. Enhanced selectivity is attributed to the enhancement in the polarity of the substrate after the TMs doping targeting the chalcogenide sites. Our work demonstrates the potential of MoX2 based single atom catalysts as efficient biosensor towards the specific VOCs for the early diagnosis of lung cancer.PACS Numbers: 31.15.E-, 68.43.-h, 68.43.Fg, 82.33.Pt, 87.15.Aa, 87.15.Kg, 87.19.Xx, 87.19.xj.

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.

Analyzing the electronic and conductive characteristics of zigzag graphene nanoribbons upon NOx and N2O Adsorption: An ab-initio study

Achieving a cleaner and more sustainable environment relies on the precise identification and elimination of toxic gases. Addressing this imperative, our current study utilizes a symmetric zigzag graphene nanoribbon (ZGNR)-based sensor device to effectively detect gases from the NOx family at low concentrations. The ZGNR serves as the channel element within a simulated sensing device, with its electrodes connected to a variable direct voltage (d.c) source. The performance of the device is validated by monitoring changes in adsorption energy, charge transfer, dipole moment, recovery time and conductance. The manuscript utilizes non-equilibrium Green’s function (NEGF) theory and density functional theory (DFT). These theories serve as the foundation for electronic structure calculations, supplemented by the application of the Landauer-Büttiker equation for computing electrical characteristics.Three common gasses namely NO, NO2, and N2O have been taken into consideration for ZGNR-based sensor device detection. Notably, the suggested sensor device exhibits a higher sensor response (S= 4.63) and a selectivity of about 21% for NO gas. This is likely due to the high adsorption energy (-1.07 eV), and large dipole moment (4.86×10−30Cm) that substantiate larger charge transfer (-0.14e) as compared to NO2 and N2O.Meanwhile, the ZGNR sensor device shows great gas sensing performances, including excellent selectivity, repeatability (due to small recovery time of 103.00 and 0.01 microseconds), and stability. It can therefore be explored as a sensor device that is comparable as well as superior to various carbon-based chemical entities of both 1D and 2D nanomaterials.

How to repair the performance degradation of WSeTe SBFET caused by channel vacancy defects: A first principles study

The QuantumATK software package based on density functional theory and non-equilibrium Green's function is used to predict the performance of a 5.1 nm in-plane contact double-gate WSeTe Schottky barrier SBFET with 1 T-WTe2 as electrodes and Janus WSeTe as channel. It is found that the performance of 5.1 nm WSeTe SBFET meets the prediction parameters of ITRS(2013 version) about high performanc device for the production year 2028, but Te (Se) vacancy defect at the channel near the source will degrade its performance. How to repair the performance of the WSeTe SBFET with Te (Se) vacancy defect at the channel near the source? The effects of O, Cl and N passivation of Te (Se) vacancy defects on the performance of WSeTe SBFET were discussed. The results indicate that O Passivation can repair and even improve the performance of WSeTe SBFET by repairing the DOS changes of monolayer WSeTe induced by Te (Se) vacancy defects.

Non-equilibrium Green's function analysis of charge plasma-based source-drain electrode P-type MoTe2 MOSFET for high sensitivity hydrogen sensing

—The paper presents a novel P-type molybdenum ditelluride (MoTe2) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) designed for high sensitivity hydrogen sensing applications. Through the utilization of palladium at the source-drain charge plasma electrode, P+ regions at both ends are generated, obviating the necessity for low-dimensional MoTe2 doping. Simultaneously, the inclusion of palladium on both regions act as the sensing area, resulting in notably heightened sensitivity. The study employs Non-Equilibrium Green's Function (NEGF) formalism to investigate the carrier transport properties of the device. The results provide valuable insights into less explored area of p-type MOSFET performance as Hydrogen(H2) gas sensor and its potential.

Combined resonant tunneling and rate equation modeling of terahertz quantum cascade lasers

Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs.

Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers.

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

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.

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.

A computational study of rectification behavior of doped α-graphyne nanotubes

α-graphyne is a carbon nanosheet with sp and sp2 hybridization. This sheet is a hexagonal semi-metal like graphene. In this paper, we roll α-graphyne sheet and build zigzag nanotubes with three different size (n = 5, 6, 7). We used density functional tight binding (DFTB) to compute the band structures of nanotubes. The results show that the gap energies for n = 5, 6, 7 are about 0.648, 0.115, 0.309 eV, respectively. We add impurity atoms such as boron (B) and nitrogen (N) to nanotube. We found that the band gap energy of these doped nanotubes decrease. In addition, the structures that carbon atoms with sp2 hybridization replaced by impurity atoms are more stable. For the first time, we build two doped nanotube diodes at low (M1 model) and high (M2 model) concentration and compute current-voltage (I–V) curves with non-equilibrium Green's functions (NEGF). The I–V curves of doped nanotubes show non-linear asymmetric behavior. Form I–V curves, we compute rectification ratio and negative differential resistance (NDR). The low doped zigzag nanotube with n = 5 (M1 model) shows the highest rectification ratio (about 12.236 × 104) at 0.3 V. In addition, the rectification ratio of this diode at high concentration (M2) is about 3.67 × 103 at 0.6 V. The I–V curve of M2 model illustrations a notably negative differential resistance (NDR) at reverse bias. The ratio of current at peak to current at valley is about 26.83. To describe the I–V curves, we used transmission spectrum, the band structures of electrodes and the density of states (DOS) of central scattering region. In general, transition occurs when the energy levels of three regions (left electrode, scattering and right electrode) overlap.

Ballistic Performance of Quasi-One-Dimensional Hafnium Disulfide Field-Effect Transistors

Hafnium disulfide (HfS2) monolayer is one of the most promising two-dimensional (2D) materials for future nanoscale electronic devices, and patterning it into quasi-one-dimensional HfS2 nanoribbons (HfS2NRs) enables multi-channel architectures for field-effect transistors (FETs). Electronic, transport and ballistic device characteristics are studied for sub-7 nm-wide and ~15 nm-long zigzag HfS2NR FETs using non-equilibrium Green’s functions (NEGF) formalism with density functional theory (DFT) and maximally localized Wannier functions (MLWFs). We provide an in-depth analysis of quantum confinement effects on ON-state performance. We show that bandgap and hole transport mass are immune to downscaling effects, while the ON-state performance is boosted by up to 53% but only in n-type devices. Finally, we demonstrate that HfS2NR FETs can fulfill the industry requirements for future technology nodes, which makes them a promising solution for FET architectures based on multiple nanosheets or nanowires.

Comparison of SiGeC/SiGe/SiC–Si heterojunction based vertical nanowire FET using non equilibrium Green's function

Advancement in FET technology has forced researchers to seek alternate semiconductor devices and materials. TFETs, junctionless FETs, and nanowires are a few examples of modern FETs, whereas Si alloys, graphene, and TMDC are a few materials that have shown promising behaviour to replace Si in the future. In this work, SiGeC/SiGe/SiC–Si based vertical heterostructure nanowires have been investigated. Due to the nanoscale dimensions of the FET and to incorporate the quantum effects along with Schrodinger-Poisson equations, non-equilibrium Green's function has been used as a principle simulator. For simulation, Ge/Carbon mole concentration of 0.25 and 0.01, respectively is used. To thoroughly investigate the effect of variation in doping profile and temperature range, simulated heterojunction nanowire showed that SiGeC-based nanowire result in a better ION/IOFF ratio as compared to SiGe and SiC-based heterojunction nanowire.