Nonequilibrium Green Function Approach Research Articles

Thermomagnetic Anomalies by Magnonic Criticality in Ultracold Atomic Transport.

We investigate thermomagnetic transport in an ultracold atomic system with two ferromagnets linked via a magnetic quantum point contact. Using the nonequilibrium Green's function approach, we show a divergence in spin conductance and a slowing down of spin relaxation that manifest in the weak effective-Zeeman-field limit. These anomalous spin dynamics result from the magnonic critical point at which magnons become gapless due to spontaneous magnetization. Our findings unveil untapped dynamics in ultracold atomic systems, opening new avenues in thermomagnetism.

Susceptible Detection of Organic Molecules Based on C3B/Graphene and C3N/Graphene van der Waals Heterojunction Gas Sensors.

Constructing van der Waals (vdW) heterostructures is a prospective approach that is essential for developing a new generation of functional two-dimensional (2D) materials and designing new conceptual nanodevices. Using density-functional theory combined with a nonequilibrium Green's function approach allows for the theoretical and systematic exploration of the electronic structure, transport properties, and sensitivity of organic small molecules adsorbed on 2D C3B/graphene (Gra) and C3N/Gra vdW heterojunctions. Calculations show the metallic properties of C3B/Gra and C3N/Gra after the formation of heterojunctions. Interestingly, the heterojunctions C3B/Gra (C3N/Gra) for the adsorption of small organic molecules (C2H2, C2H4, CH3OH, CH4, and HCHO) at the C3B (C3N) side are sensitive to the chemisorption of C2H2 and C2H4. Similarly, the Gra/C3B is chemisorbed for both C2H2 and C2H4 when adsorbed on Gra side, while it is only chemisorbed for C2H2 in Gra/C3N. Interestingly, all heterojunctions on different sides are physisorbed for CH3OH, CH4, and HCHO. Furthermore, the calculated I-V curves demonstrate that the devices based on the adsorption of C2H2 and C2H4 at each side of the heterojunction have remarkable anisotropy, in with the current being considerably greater in the zigzag direction than in the armchair direction. More specifically, with C2H2 adsorbed on the Gra side, the sensitivity along the armchair direction is up to 85.0% for Gra/C3B and close to 100% for Gra/C3N. This study reveals that C3B/Gra (C3N/Gra) heterojunctions with high selectivity, high anisotropy, and excellent sensitivity are highly prospective 2D materials for applications, which further contributes new insights into the development of future electronic nanodevices.

The thermal transport properties of Boron-doped C2N nanoribbons

In this paper, we focus on the thermal spin transport properties of hydrogen-saturated graphene-like nanoribbons substituted for doped nonmetallic B by using density functional theory combined with non-equilibrium Green's function approach. We find that AC2NNR-H(B) is a distinctly narrow bandgap semiconductor material and exhibits significant spin Seebeck effect, temperature switching effect, and giant magnetoresistance. Surprisingly, the spin Seebeck coefficients can reach 1.16 mV/K and the magnetoresistance value not only reaches 108% at low temperature, but also 104% near room temperature. These unique transport properties can be explained using spin-dependent transmission spectra combined with energy band structure. The current work further extends the theoretical study of graphene-like derivatives and provides new ideas for the construction of spintronic devices.

Giant tunneling resistance and robust switching behavior in ferroelectric tunnel junctions of WS2/Ga2O3 heterostructures: The influence of metal–semiconductor contacts

The ferroelectric tunneling junctions (FTJs) are widely recognized as one of the non-volatile memories with significant potential. Ferroelectricity usually fades away as materials are thinned down below a critical value, and this problem is particularly acute in the case of shrinking device sizes, thus attracting attention to two-dimensional ferroelectric materials (2DFEMs). In this work, we designed 2D ferroelectric Ga2O3-based FTJs with out-of-plane polarization, and the influence of metal–semiconductor contact in the electrode region on the system is considered. Here, using density functional theory combined with the non-equilibrium Green's function approach to quantum transport calculations, we demonstrate robust ferroelectric polarization-controlled switching behavior between metallic and semiconducting states in Ga2O3/WS2 ferroelectric heterostructures. The potential barrier of the metal–semiconductor contact in the electrode region is lower than that of the intrinsic material, thereby resulting in an increased probability of electron tunneling. Our results reveal the crucial role of 2DFEMs in the construction of FTJs and highlight the significant impact of electrode contact types on performance. This provides a promising approach for developing high-density ferroelectric memories based on 2D ferroelectric semiconductor heterostructures.

Nonequilibrium Green's function approach to low-energy fission dynamics: Fluctuations in fission reactions

Nonequilibrium Green's function approach to low-energy fission dynamics: Fluctuations in fission reactions

Time‐Resolved Optical Response of the Dicke's Model via the Nonequilibrium Green's Function Approach

Due to their conceptual appeal and computational convenience, two‐level systems (TLSs) and generalizations are used to investigate nonlinear behavior in quantum optics and assess the applicability of theoretical methods. Herein, the focus is on second‐harmonic generation (SHG) and, as system of interest, on the Dicke model, which consists of several TLSs inside an optical cavity. The main aspect addressed is the scope of nonequilibrium Green's function (NEGF) to describe the effect of inhomogeneities and electron–electron (e–e) interactions on the SHG signal. For benchmarking purposes, exact diagonalization (ED) results are also presented and discussed. SHG spectra obtained with NEGF and ED are found to be in very good mutual agreement in most situations. Furthermore, inhomogeneity in the TLS and e–e interactions reduce the SHG signal, and the reduction is stronger with inhomogeneity than with interactions. This trend is consistently noted across different (small to large) system sizes. Finally, a modified NEGF approach is proposed to account for cavity leakage, where the quantum photon fields are coupled to a bath of classical oscillators. As expected, within this mixed quantum‐classical scheme, a decrease in the intensity of the fluorescent spectra takes place depending on the entity of leakage.

Molecular heat transport across a time-periodic temperature gradient.

The time-periodic modulation of a temperature gradient can alter the heat transport properties of a physical system. Oscillating thermal gradients give rise to behaviors such as modified thermal conductivity and controllable time-delayed energy storage that are not present in a system with static temperatures. Here, we examine how the heat transport properties of a molecular lattice model are affected by an oscillating temperature gradient. We use analytical analysis and molecular dynamics simulations to investigate the vibrational heat flow in a molecular lattice system consisting of a chain of particles connected to two heat baths at different temperatures, where the temperature difference between baths is oscillating in time. We derive expressions for heat currents in this system using a stochastic energetics framework and a nonequilibrium Green's function approach that is modified to treat the nonstationary average energy fluxes. We find that emergent energy storage, energy release, and thermal conductance mechanisms induced by the temperature oscillations can be controlled by varying the frequency, waveform, and amplitude of the oscillating gradient. The developed theoretical approach provides a general framework to describe how vibrational heat transmission through a molecular lattice is affected by temperature gradient oscillations.

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

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

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

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

Significant effects of magnetic electrodes on rhenium phthalocyanine molecules

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

Nonequilibrium Dynamics of the Hubbard Dimer

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

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

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

Full statistics of nonequilibrium heat and work for many-body quantum Otto engines and universal bounds: A nonequilibrium Green's function approach.

We consider a generic four-stroke quantum Otto engine consisting of two unitary and two thermalization strokes with an arbitrary many-body working medium. Using the Schwinger-Keldysh nonequilibrium Green's function formalism, we provide an analytical expression for the cumulant generating function corresponding to the joint probability distribution of nonequilibrium work and heat. The obtained result is valid up to the second order of the external driving amplitude. We then focus on the linear response limit and obtained Onsager's transport coefficients for the generic Otto cycle and show that the traditional fluctuation-dissipation relation for the total work is violated in the quantum domain, whereas for heat it is preserved. This leads to remarkable consequences in obtaining universal constraints on heat and work fluctuations for engine and refrigerator regimes of the Otto cycle and further allows us to make connections to the thermodynamic uncertainty relations. These findings are illustrated using a paradigmatic model that can be feasibly implemented in experiments.

Rate equations description of the asymmetric double barrier electronic cooler

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

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

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

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

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

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

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

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

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

A Strongly Correlated Quantum Dot Heat Engine with Optimal Performance: A Nonequilibrium Green's Function Approach

Herein, an analytical study of a strongly correlated quantum dot‐based thermoelectric particle‐exchange heat engine for both finite and infinite on‐dot Coulomb interaction is presented. Employing Keldysh's nonequilibrium Green's function formalism for different decoupling schemes in the equation of motion, the thermoelectric properties within the nonlinear transport regime have been analyzed. Initially, Hubbard‐I approximation has been used to study the quantum dot level position (), thermal gradient (), and on‐dot Coulomb interaction (U) dependence of the thermoelectric properties. Furthermore, as a natural extension, a decoupling beyond Hubbard‐I (Lacroix approximation) with infinite‐U limit (strong on‐dot Coulomb repulsion) has been used to provide additional insight into the operation of a more practical quantum dot heat engine. Within this infinite‐U limit, the role of the symmetric dot‐reservoir tunneling (Γ) and external serial load resistance (R) in optimizing the performance of the strongly correlated quantum dot heat engine is examined. The infinite‐U results show a good quantitative agreement with recent experimental data for a quantum dot coupled to two metallic reservoirs.

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

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