Nonequilibrium Green Function Research Articles

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.

Investigation of thermoelectric properties of cadmium selenide CdnSen (n= 7, 11, 13) molecular junctions: a DFT study.

The thermoelectric properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated before and after adding hydrogen atoms. The effects of hydrogen passivation on the transmission and thermopower curves were analyzed. CdSe-diamantane (Cd7Se7) and CdSe-tetramantane (Cd11Se11) junctions exhibited the best thermoelectric performance due to their low surface reconstruction energy, which is attributed to the number of dangling and unsaturated bonds. This study guides the design of new molecular junctions with desired thermoelectric properties. The electrical and thermal properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated using a ballistic quantum transport method based on the non-equilibrium Green's function (NEGF) approach. Thermoelectric properties were calculated for the molecular junctions with different structures before and after hydrogen passivation. Density functional theory (DFT) calculations were performed at the B3LYP level with the 3-21G basis set for the Cd atoms and the 6-31G** basis set for the Se atoms. The SIESTA and GOLLUM codes were used to study the effect of changing the shape and size of each structure on its electrical and thermal characteristics.

Modeling of light absorption in self-assembled truncated conical quantum dot structures

Quantum Dots have shown a significant potential as a top candidate for infrared photodetection at higher temperatures. In the presented work, a theoretical model for estimating the coefficient of optical absorption of self-assembled truncated conical quantum dot is developed. This model considers both bound-to-continuum and bound-to-bound absorption mechanisms that increase the accuracy of the absorption coefficient estimation. The developed model is based on estimating the bound states by diagonalizing the Hamiltonian matrix, where the density of states is computed using the Non-Equilibrium Greens function and the effective mass theory to obtain the unbound states. The kinetic equation of Green’s function is solved numerically by finite difference method. Besides, the effects of quantum dot size, height, aspect ratio, and density on the coefficient of the optical absorption are investigated. The results of the developed model are contrasted with those of other alternative QD structures where the truncated conical QD structure results in a higher absorption coefficient in infrared range than semispherical and conical QD structures.

The interfacial properties of 2D metal-monolayer blue phosphorene heterojunctions and transport properties of their field-effect transistors

Monolayer blue phosphorene (BlueP) has attracted much interest as a potential channel material in electronic devices. Searching for suitable two-dimensional (2D) metal materials to use as electrodes is critical to fabricating high-performance nanoscale channel BlueP-based field effect transistors (FETs). In this paper, we adopted first-principles calculations to explore binding energies, phonon calculations and electronic structures of 2D metal-BlueP heterojunctions, including Ti3C2-, NbTe2-, Ga(110)- and NbS2-BlueP, and thermal stability of Ti3C2-BlueP heterojunction at room temperature. We also used density functional theory coupled with the nonequilibrium Green function method to investigate the transport properties of sub-5 nm BlueP-based FETs with Ti3C2-BlueP electrodes. Our calculated results indicate that Ti3C2-BlueP has excellent thermal stability and may be used as a candidate electrode material for BlueP-based FETs. The double-gate can more effectively improve the device performance compared with the single-gate. The estimated source leakage current of sub-5 nm transistors reaches up to 369 µA µm−1, which is expected to meet the requirements of the international technology roadmap for semiconductors for LP (low-power) devices. Our results imply that 2D Ti3C2 may act as an appropriate electrode material for LP BlueP-based FETs, thus providing guidance for the design of future short-gate-length BlueP-based FETs.

Andreev reflection imaging and longitudinal shift of pseudospin-1 fermions in two-dimensional materials

In this work, the transverse magnetic focusing of pseudospin-1 fermions in two-dimensional materials with a superconducting contact is studied. The transmission coefficient and Andreev reflection coefficient from injector lead to collector lead are calculated based on the Landauer–Büttiker formula combined with the non-equilibrium Green's function method. The local state density diagrams of the electron and hole transmission in the center region are given, which provide a complete image of the ballistic electron–hole cyclotron orbits. Using these images, the positive longitudinal shift in Andreev reflection of the pseudospin-1 fermion can be obviously seen. Furthermore, it is found that the total offset is proportional to the number of Andreev reflection occurrence. This setup provides the possibility of detecting this shift experimentally.

Quantum Transport of Topological and Trivial Edge States in Sn‐Doped SbH Nanoribbons

Although the transport of topological edge states may be insensitive to impurities, impurities may induce the interedge scattering and backscattering in topological nanoribbons due to the finite size effect. It may be interesting to compare the transport of topological edge states with that of trivial edge states in nanoribbons. Here, the quantum transport of edge states in the Sn‐doped SbH nanoribbons using the density functional theory combined with nonequilibrium Green's function method is studied. Both topological and trivial edge states can be found in the zigzag SbH nanoribbons. The impurity generates a higher barrier near the doping position and leads to the scattering of the states. From calculations, the transmission of topological edge states can be smaller than that of trivial edge states due to the antiresonance behavior. Compared with topological edge states, the transport of trivial edge states is more sensitive to the impurity position. Besides, the edge impurity is more likely to induce scattering for both topological and trivial edge states than the central impurity in narrow SbH nanoribbons, since the transport channels are limited to the edges. This study may be helpful for understanding the transport of edge states in nanoribbons.

Enhanced thermoelectric performances in graphene nanoribbons via BN dimers doping: Theoretical study

Improving the thermoelectric performance of designed graphene nanoribbons is a key stage in the production of thermoelectric nanodevices with many applications. The chemical doping allows armchair graphene nanoribbons (AGNRs) to exhibit controllable thermoelectric characteristics. Here, we use density functional theory-based tight-binding (DFTB) coupled with the non-equilibrium Green's function (NEGF) to study the electronic and thermoelectric properties of AGNR with boron nitride (BN) dimers at room temperature. Changing the concentrations (from 4.17 % (BN)1-structure to 12.5 % (BN)3-structure) and geometrical pattern (ortho, meta, and para form) of BN dimers in the graphene nanoribbons may have a significant effect on the thermoelectric (TE) properties. Our results show that the TE properties of AGNR depend not only on the amount of BN dimers but also on the arrangement of the BN dimers in the AGNR. The thermoelectric figure of merit (ZT) of nanoribbons at room temperature has improved from less than 0.7 to more than 2. These results could be used as an indicator to design nanodevices that have good TE applications.

B3C2P3 monolayers based highly sensitive and selective room-temperature gas sensors for reusable NO and NO2 detection

Motivated by the development of high-performance gas sensors using two-dimensional (2D) nanomaterials, herein using the combination methods of density functional theory (DFT) and nonequilibrium Green's function (NEGF), the structural, electronic, transport, and gas sensing properties of the B3C2P3 monolayer adsorbed with various gases (NO2, NO, NH3, SO2, H2S, CO2, CO, H2O, O2 and CH2O) have been systematically studied. The NO2 (or NO) molecules were chemically adsorbed (or strong physisorbed) on the B3C2P3 monolayer, while other molecules were weak-physically adsorbed on the substrate. Furthermore, only the NO2 and NO adsorption results in the remarkable change in electronic properties of B3C2P3. The transport properties such as current-voltage (I–V) characteristics and transmission functions indicate the B3C2P3 monolayer is highly sensitive and selective towards NO2 and NO gases. The recovery times of NO2 and NO were calculated to be 2.27 and 0.0785 ms at 300 K, indicating the reusability of the B3C2P3-based NO2 (NO) sensors. The existence of moisture hardly influences the adsorption of NO and NO2 on the B3C2P3 monolayer. Our results indicated that the B3C2P3 monolayer, as a room-temperature reusable gas sensor, is highly sensitive and selective for NO2 and NO detection regardless of the existence of moisture, O2 and CO2.

Mechanistic Insights into Electronic Current Flow through Quinone Devices.

Molecular switches based on functionalized graphene nanoribbons (GNRs) are of great interest in the development of nanoelectronics. In experiment, it was found that a significant difference in the conductance of an anthraquinone derivative can be achieved by altering the pH value of the environment. Building on this, in this work we investigate the underlying mechanism behind this effect and propose a general design principle for a pH based GNR-based switch. The electronic structure of the investigated systems is calculated using density functional theory and the transport properties at the quasi-stationary limit are described using nonequilibrium Green's function and the Landauer formalism. This approach enables the examination of the local and the global transport through the system. The electrons are shown to flow along the edges of the GNRs. The central carbonyl groups allow for tunable transport through control of the oxidation state via the pH environment. Finally, we also test different types of GNRs (zigzag vs. armchair) to determine which platform provides the best transport switchability.

Josephson current across a Double Quantum Dot Josephson junction in T-Shape Configuration

By implementing the Keldysh non-equilibrium Green's function equation of motion approach, Josephson current has been examined across a T-shaped uncorrelated double quantum dot Josephson junction. The behavior of the Josephson current as a function of the main quantum dot energy level for varied interdot tunneling and different dot-lead coupling strengths is examined. With this configuration, we illustrate that the side-attached quantum dot offers an alternative route for electron transmission, which modifies the Josephson current by varying interdot tunneling. Further, we also investigate how the dot-lead coupling strengths affect the Josephson current.

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

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

Coupling effects in bipyridines linked to carbon nanowires

In this work, we conducted an investigation into the electronic transport properties of four distinct devices formed by bipyridines (BPY) as the central region, linked to carbon nanowires (polyyne-type carbyne) electrodes. These four devices have two symmetrical (2,2′ and 4,4′ BPY) and two asymmetrical (2,4′ and 2,6′ BPY) configurations. We employed Density Functional Theory combined with Non-Equilibrium Green's Function (DFT-NEGF) to calculate the molecular geometry and the electronic transport properties. Our analysis, using Fowler-Nordheim (F-N) and Millikan-Lauritsen (M-L) plots, reveals that electronic transport primarily occurs through direct tunneling without any inflection points, indicating the absence of nonlinear effects such as resonance or negative differential resistance (NDR). This demonstrates the importance of the study including an analysis of Transition Voltage Spectroscopy (TVS) which shows that the presence of nitrogen atoms in the electrode-molecule contacts, specifically in the cases 2,4′ and 4,4′ BPY, significantly alters the structural and electronic characteristics of the devices. This alteration leads to a strong coupling of the junction, improved structural and electronic properties. These findings are supported by the Device Density of States (DDOS) and Frontier Molecular Orbitals (FMOs). Moreover, we observed that the electronic characteristics of all four devices remain consistent even under bias voltage (Ve).

Gate voltage-controlled spin-rectifier diode based on Janus transition metal nitride MXene with spin gapless semiconductor

With the continuous application of spintronics technology, spintronic devices have been expected to become a major direction in the development of next-generation electronic devices. However, for spin rectifier diodes, how to provide a large number of spin-polarized carriers is still a key technical issue. Conventional spin-polarization injection models use ferromagnetic (FM)/semiconductor structures, but because of the conductivity mismatch can limit the injection of their polarization. This makes the search for spin-gapless semiconductor which has the conductivity between metals and semiconductors an effective way to solve this problem. Recently, MXene, a new two-dimensional material with high spin polarization, has been expected to undergo modulation of the electronic structure through external field or proximity effects. Subsequently, we propose a Janus transition metal structure of the MXene which modulates the spin properties by breaking the symmetry of the transition metals. Moreover, we find the band structure of TiCrNO2 can be effectively modulated by an applied electric field, which transforms itself from a magnetic semiconductor to a spin-gapless semiconductor under an electric field strength of 1 V/Å. With this as a basis, we construct a tunable spin rectifier diode and study its spin rectification effect by the first-principle calculations combined with non-equilibrium Green's function. The results show that the rectification ratio of the spin rectification diode can be adjustable by varying the strength of the external electric field, and its rectification rate exceeds the highest value currently available for two-dimensional rectifier devices. Our results provide a reference value for the application of nitride MXene in spintronic devices.

Local resonance mechanism for enhancing the thermoelectric performance of PBCF-graphene nanoribbons

Phonon local resonance mechanism can effectively block phonon transport and thus reduce phonon thermal conductance. Inspired by this, we investigated the thermoelectric performance of Armchair-PBCF-graphene nanoribbons (A-PBCF-GNRs) and its branched structures by using non-equilibrium Green's function method. The results show that the thermoelectric performance of branched A-PBCF-GNRs can be significantly improved. Especially, the room-temperature ZT value of single branch A-PBCF-GNRs (SB-A-PBCF-GNRs) and double branch A-PBCF-GNRs (DB-A-PBCF-GNRs) can reach to 0.56 and 0.80, which is 1.5 and 2 times greater than that of A-PBCF-GNRs. Analysis shows that this is mainly attributed to the significant decrease in thermal conductance of branched A-PBCF-GNRs caused by the localized electronic and phonon states. Moreover, the mode resolved phonon transmission probabilities demonstrated that the local resonance mechanism of branched structure blocks the low-frequency acoustic phonon transmission. This work indicates the broad application prospects of the local resonance mechanism of branched structure in the thermoelectric field.

Ultrafast excitonic radiative recombination in Al-rich semipolar AlGaN quantum wells for far-UVC emitters

Ultrafast excitonic radiative recombination in Al-rich semipolar AlGaN quantum wells was characterized by photoluminescence and analyzed with a non-equilibrium Green's function (NEGF) model. The heterostructures consisted of Al0.69Ga0.31N/Al0.9Ga0.1N multiple quantum wells grown on (202¯1) bulk AlN substrates by MOVPE. The quantum wells were 3 nm thick, separated by 10-nm thick barrier layers, and possessed a low (<100 kV/cm) polarization field. Radiative lifetimes as low as ∼60 ps were recorded at 75 K. Experimental results validated the NEGF model, which incorporated excitons and free carriers and enabled calculation of the radiative lifetimes for different growth planes, temperatures, and carrier densities. Including intersubband scattering in the NEGF model significantly reduced the calculated radiative lifetimes for QW widths larger than the Bohr radius of the excitons in semipolar and nonpolar orientations, for which the polarization field is reduced or eliminated. These wide wells with low polarization fields and high radiative recombination are less susceptible to the deleterious effects of alloy and interface fluctuations, making them an attractive possibility for future devices.

Thermoelectric enhancement of a parallel double quantum dot system connecting to Luttinger liquid leads

In this work, we study the thermoelectric properties of the parallel-coupled double quantum dot connected to Luttinger liquid (LL) leads. By applying nonequilibrium Green's function methods, the expressions of the electron current, heat currents, and transport coefficients are derived. We mainly focus on the effect of intralead Coulomb interaction and the spatial asymmetry of the system. Various causes for the enhancement of the thermoelectric efficiency are investigated. We find that Coulomb interactions in the LL not only enhance quantum interference but also considerably enhance thermoelectric effects. For moderately strong intralead interaction, the figure of merit can be considerably high and reach over 60 in the case of weakly coupled to LL leads and detuning of the energy levels, while in the case of strongly coupled to LL leads and the two aligned levels, the figure of merit approaches 8. The quantum interference induced by the Coulomb interaction yields a very high thermoelectric efficiency when the intralead interaction is very strong. These results may provide a design rule for the development of high-efficiency thermoelectric devices and facilitate optimization of device performance.

Ultrahigh spin filter efficiency and large spin Seebeck polarization of binuclear manganese phthalocyanine molecular junctions on nickel electrodes

In this paper, we investigated the spin transport properties of binuclear manganese phthalocyanine (Mn2Pc2) spintronic devices sandwiched between two nickel electrodes using the non-equilibrium Green's function method in combination with density functional theory. Based on the calculation results, the Mn2Pc2 device exhibited excellent spin-filtering capabilities, demonstrating an exceptionally high spin filter efficiency (SFE). Irrespective of the parallel or antiparallel orientation of magnetization in the electrodes, we observed that when both manganese atoms were in a spin-up state, the SFE of spin-resolved currents under finite bias and the thermoelectric currents induced by temperature gradients at fixed temperatures were both close to 100%. The large spin Seebeck polarization of the Mn2Pc2 device was also obtained at low reference temperatures. This study explores the potential for developing multifunctional spintronic single-molecule devices using Ni−Mn2Pc2.

Tuning the thermoelectric properties of graphene nanoribbons by vacancy defect with Ge-doping

In this work, the influence of vacancy defects and germanium (Ge)-doping on structural stability, electronic and thermoelectric (TE) characteristics of armchair graphene nanoribbons (AGNRs) have been studied by density functional theory (DFT) based tight-binding coupled with nonequilibrium green function (NEGF) calculations. Three concentrations of Ge impurities, single, two, and three, are identified at different sites with vacancy defects. As a result, the DFT calculations suggest that the defected AGNRs with three Ge impurities are structurally favorable configurations due to the lowest required cohesive energy. At low doping ratio, small bandgap for defected AGNRs is predicted leading to high Seebeck coefficient and figure of merit. In contrast, at high doping ratio, low Seebeck coefficient and figure of merit are noticed. The results show that the TE properties of AGNRs do not only depend on the Ge concentrations with vacancy defect but also depend on the geometrical pattern of Ge impurities. As a result, exploiting the electronic and thermoelectric properties of AGNRs to create nanostructure, which can be used in many important applications for nanoelectronics and spintronics.

Electron transport in a stressed moiré bigraphene structure

Within the framework of density functional theory in combination with the method of nonequilibrium Green's functions, the transport characteristics (band structure, transmission spectrum, current–voltage characteristic, and differential conductivity) of unstressed, stressed, and strongly stressed moiré bigraphene nanodevices with interlayer distances of 3.4 Å, 2.85 Å, and 1.7 Å, respectively, and consisting of two layers of graphene, where one of the layers is connected to electrodes (active graphene) and the other layer is electrically neutral (passive graphene) have been determined. It is established that, when passive graphene is twisted to certain angles before the moiré pattern is obtained (∼4° and ∼ 12°), the energy gap is opened, the value of range 1.66–1.82 eV and 3.78–4.69 eV in the unstressed state (in the stressed state 2.27–2.67 eV and 4.28–4.93 eV), respectively. It is shown that the transmission spectrum of bilayer graphene has features in the form of peak structures; with decreasing intergraphene distance these structures begin to blur and their quasiperiodic behavior is partially broken. The current–voltage characteristic of such nanostructures has areas with negative differential conductivity. Such nonlinear behavior of transport characteristics of bigraphene moiré structures can be used for the construction of electronic components of nanoelectronics on their basis.

Quantum nonlinear ac transport theory at low frequency

Based on the nonequilibrium Green’s function (NEGF), we develop a quantum nonlinear theory to study time-dependent ac transport properties in the low frequency and nonlinear bias voltage regimes. By expanding NEGF in terms of time to the linear order in Wigner representation, we can explicitly include the time-dependent self-consistent Coulomb interaction induced by external ac bias. Hence this theory automatically satisfies two basic requirements, i.e. current conservation and gauge invariance. Within this theory, the nonlinear ac current can be evaluated at arbitrarily large bias voltages under the low frequency limit. In addition, we obtain the expression of time-dependent current under the wide band limit and derive the relation between the nonlinear electrochemical capacitance and the bias voltage, which are very useful in predicting the dynamical properties of nanoelectronic devices. This quantum theory can be directly combined with density functional theory to investigate time-dependent ac transport from first-principles calculation.