Nonequilibrium Green Function Research Articles

Exploring Intertwined quantum and cryogenic behaviour in ultra-scaled 10 nm MOSFET: a NEGF quantum ballistic simulation

Silicon-based spin qubits have emerged as promising candidates for scalable quantum information processing. This study first time investigates the behaviour of ultra-scaled 10 nm gate length and 3 nm 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.

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.

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.

Super-Suppression of Long-Wavelength Phonons in Constricted Nanoporous Geometries.

In a typical semiconductor material, the majority of the heat is carried by long-wavelength, long-mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean free paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessen's rule. In general, however, long-wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale molecular-dynamics simulations, non-equilibrium Green's function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries that create narrow constrictions in the passage of phonons lead to anticorrelated heat currents in the phonon spectrum. This effect results in super-suppression of long-wavelength phonons due to heat trapping and reductions in the thermal conductivity to values well below those predicted by Matthiessen's rule.

Transport properties of Mn-doped/adsorbed zigzag boron nitride nanoribbon based nanodevices

We are conducting research into the spin transport characteristics of models that are built from zigzag Boron Nitride Nanoribbons (ZBNNRs) that have been doped and adsorbed with Mn. These investigations are made possible by using the non-equilibrium Green’s function (NEGF) formalism in conjunction with Density Functional Theory (DFT). We have scrutinized the electronic structures of six different models, and have chosen two for a deeper exploration of its’ transport characteristics. Our computational findings highlight an unusual occurrence where a Mn atom substitutes a B atom in ZBNNRs, resulting in ZB(Mn)NNRs that exhibit significant Giant Magnetoresistance (GMR) and Rectification ratio (RR) - peaking at 106 and 105 respectively, along with the presence of negative differential conductance (NDC) effects. The detected GMR, NDC, and rectification characteristics in ZB(Mn)NNRs offer an optimistic outlook for the conceptualization of future nanodevices.

Exploring the Odd–Even Effect, Current Stabilization, and Negative Differential Resistance in Carbon-Chain-Based Molecular Devices

The transport properties of molecular devices based on carbon chains are systematically investigated using a combination of non-equilibrium Green’s function (NEGF) and density functional theory (DFT) first-principle methods. In single-carbon-chain molecular devices, a distinct even–odd behavior of the current emerges, primarily influenced by the density of states (DOS) within the chain channel. Additionally, linear, monotonic currents exhibit Ohmic contact characteristics. In ladder-shaped carbon-chain molecular devices, a notable current stabilization behavior is observed, suggesting their potential utility as current stabilizers within circuits. We provide a comprehensive analysis of the transport properties of molecular devices featuring ladder-shaped carbon chains connecting benzene-ring molecules. The occurrence of negative differential resistance (NDR) in the low-bias voltage region is noted, with the possibility of manipulation by adjusting the position of the benzene-ring molecule. These findings offer a novel perspective on the potential applications of atom chains.

Surface-dominated conductance scaling in Weyl semimetal NbAs

Protected surface states arising from non-trivial bandstructure topology in semimetals can potentially enable advanced device functionalities in compute, memory, interconnect, sensing, and communication. This necessitates a fundamental understanding of surface-state transport in nanoscale topological semimetals. Here, we investigate quantum transport in a prototypical topological semimetal NbAs to evaluate the potential of this class of materials for beyond-Cu interconnects in highly-scaled integrated circuits. Using density functional theory (DFT) coupled with non-equilibrium Green’s function (NEGF) calculations, we show that the resistance-area RA product in NbAs films decreases with decreasing thickness at the nanometer scale, in contrast to a nearly constant RA product in ideal Cu films. This anomalous scaling originates from the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70% in NbAs thin films. We also show that this favorable RA scaling persists even in the presence of surface defects, in contrast to RA sharply increasing with reducing thickness for films of conventional metals, such as Cu, in the presence of surface defects. These results underscore the potential of topological semimetals as future back-end-of-line (BEOL) interconnect metals.

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.

Spin filtering, magnetoresistance and seebeck effects in phthalocyanine based molecular junctions between phosphorus nanoribbon electrodes

This research utilized the non-equilibrium Green’s function (NEGF) method combined with density functional theory (DFT) to explore spin transport in binuclear transition metal phthalocyanine (TMPc) molecules between black phosphorus nanoribbon electrodes. The device achieves near 100% SFE with ferromagnetic coupling of Manganese phthalocyanine (MnPc) and Iron phthalocyanine (FePc). When the MnPc and FePc molecules exhibited anti-ferromagnetic coupling, with the transports in the two spin channels significantly smaller than in the ferromagnetic coupling scenario, demonstrating giant magnetoresistance effects. MnPc shows high Seebeck polarization at 149 K, while FePc maintains it at a low temperature. This research opens new avenues for designing molecular spintronic devices with two-dimensional electrodes.

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.

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.

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.

Effect of lattice defects on electronic structure and thermoelectric properties of 2D monolayer MoS2

Molybdenum disulfide (MoS2) is considered as a promising economical and non-toxic thermoelectric material due to its low thermal conductivity and high Seebeck coefficient. However, its power factor is not ideal, which limits its energy conversion efficiency in thermoelectric applications. In this paper, the electronic structure of pure and lattice-defect MoS2 2D monolayer is studied by using the first principles method and non-equilibrium Green's function (DFT-NEGF). The effects of atom substitution and vacancy defect on the thermoelectric properties of MoS2 2D monolayer are also investigated. The result shows that the S-vacancy changes the bandgap by introducing donor and acceptor levels, while the substitution of Se and Te atoms slightly increases the bandgap value. In the case of n-type doping, the ZT value of the S atom substituted MoS2 reaches 1.19 at 300 K, while the Te atom substitution MoS2 has a higher ZT value of almost 2.26 at 800 K. The analysis shows that the thermal conductivity of MoS2 decreases due to the presence of S-vacancy, while the atomic substitution of Se and Te increases the power factor of MoS2. This work may provide a new method and theoretical guidance for achieving high performance of thermoelectric devices in the energy industry.

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.

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.

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.