Nonequilibrium Green Function Approach Research Articles

Effect of oxide barrier height in spin dependent tunneling in MTJ of FeO-MgO multilayer structure

We study the spin dependent tunneling current properties through oxide multilayers in a magnetic tunnel junction (MTJ). For this purpose, nonequilibrium Green's function approach along-with the density-functional theory have been applied. We employed three structural models of FeO-MgO-FeO multi-layer with three different width of FeO and MgO layer. An atomistic model is considered to describe the effect of oxide multilayers of different heights. Spin dependent study for tunneling reveals that the parallel spin shows higher tunneling current whereas anti-parallel spin conducts very less. Further, the lowest tunneling current is obtained for the case where the FeO and MgO each has 3 atomic layers of height whereas the tunneling current is highest in 4 atomic layers of FeO/1 atomic layers of MgO/4 atomic layers of MgO multilayer structure. Importantly, when the MgO or FeO layers are increased or decreased from this level, the tunneling current decreases significantly. The study reveals that the layer height in the tunneling domain can be important factor for tuning and adjusting tunneling current in the nanoscale regime of oxide layer thickness.

Tunable photonic cavity coupled to a voltage-biased double quantum dot system: Diagrammatic nonequilibrium Green's function approach

We investigate gain in microwave photonic cavities coupled to voltage-biased double quantum dot systems with an arbitrary strong dot-lead coupling and with a Holstein-like light-matter interaction, by adapting the diagrammatic Keldysh nonequilibrium Green's function approach. We compute out-of-equilibrium properties of the cavity : its transmission, phase response, mean photon number, power spectrum, and spectral function. We show that by the careful engineering of these hybrid light-matter systems, one can achieve a significant amplification of the optical signal with the voltage-biased electronic system serving as a gain medium. We also study the steady state current across the device, identifying elastic and inelastic tunnelling processes which involve the cavity mode. Our results show how recent advances in quantum electronics can be exploited to build hybrid light-matter systems that behave as single-atom amplifiers and photon source devices. The diagrammatic Keldysh approach is primarily discussed for a cavity-coupled double quantum dot architecture, but it is generalizable to other hybrid light-matter systems.

Tuning the conductance of benzene-based single-molecule junctions

Single-molecule junctions are elementary building blocks in novel electronics devices, and therefore, it is essential to understand the charge transport mechanisms at the single-molecular level. According to recent studies, the linker atoms connecting the organic molecule with the electrodes play a crucial role in optimizing the transport properties as well as thermodynamical stability of a molecular junction. We address this issue by considering N, O, S, and Se atoms as prospective linkers anchoring the benzene molecule to the Au(1 0 0) electrodes. Calculations based on non-equilibrium Green's function approach are performed within the framework of the density functional theory. Electron transport is studied in detail by analyzing the transmission spectra, density-of-states distributions, and current–voltage characteristics. Results show that the choice of linkers strongly affects the conductance of the junctions under study: at low bias regime, the current through N-linked molecules is remarkably higher as compared to the case of S and Se linkers, whereas the thermodynamical stability is similar. This offers an additional means of modifying the current–voltage characteristics of benzene-based molecular junctions by an appropriate selection of linkers.

Antidamping spin-orbit torque driven by spin-flip reflection mechanism on the surface of a topological insulator: A time-dependent nonequilibrium Green function approach

Motivated by recent experiments observing spin-orbit torque (SOT) acting on the magnetization $\stackrel{P\vec}{m}$ of a ferromagnetic (F) overlayer on the surface of a three-dimensional topological insulator (TI), we investigate the origin of the SOT and the magnetization dynamics in such systems. We predict that lateral F/TI bilayers of finite length, sandwiched between two normal metal leads, will generate a large anti-damping-like SOT per very low charge current injected parallel to the interface. The large values of anti-damping-like SOT are spatially localized around the transverse edges of the F overlayer. Our analysis is based on adiabatic expansion (to first order in $\ensuremath{\partial}\stackrel{P\vec}{m}/\ensuremath{\partial}t$) of time-dependent nonequilibrium Green functions (NEGFs), describing electrons pushed out of equilibrium both by the applied bias voltage and by the slow variation of a classical degree of freedom [such as $\stackrel{P\vec}{m}(t)$]. From it we extract formulas for spin torque and charge pumping, which show that they are reciprocal effects to each other, as well as Gilbert damping in the presence of SO coupling. The NEGF-based formula for SOT naturally splits into four components, determined by their behavior (even or odd) under the time and bias voltage reversal. Their complex angular dependence is delineated and employed within Landau-Lifshitz-Gilbert simulations of magnetization dynamics in order to demonstrate capability of the predicted SOT to efficiently switch $\stackrel{P\vec}{m}$ of a perpendicularly magnetized F overlayer.

Time-resolved photoabsorption in finite systems: A first-principles NEGF approach

We describe a first-principles NonEquilibrium Green's Function (NEGF) approach to time-resolved photoabsortion spectroscopy in atomic and nanoscale systems. The method is used to highlight a recently discovered dynamical correlation effect in the spectrum of a Krypton gas subject to a strong ionizing pump pulse. We propose a minimal model that captures the effect, and study the performance of time-local approximations versus time-nonlocal ones. In particular we implement the time-local Hartree-Fock and Markovian second Born (2B) approximation as well as the exact adiabatic approximation within the Time-Dependent Density Functional Theory framework. For the time-nonlocal approximation we instead use the 2B one. We provide enough convincing evidence for the fact that a proper description of the spectrum of an evolving admixture of ionizing atoms requires the simultaneous occurrence of correlation and memory effects.

Single and double photoemission and generalizations

A unified diagrammatic treatment of single and double electron photoemission currents is presented. The irreducible lesser density-density response function is the starting point of these derivations. Diagrams for higher order processes in which several electrons are observed in coincidence can likewise be obtained. For physically relevant situations, in which the photoemission cross-section can be written as the Fermi Golden rule, the diagrams from the nonequilibrium Green's function approach can be put in direct correspondence with the diagrams of the scattering theory.

Simple electron-electron scattering in non-equilibrium Green's function simulations

In this work we include electron-electron interaction beyond Hartree-Fock level in our non-equilibrium Green's function approach by a crude form of GW through the Single Plasmon Pole Approximation. This is achieved by treating all conduction band electrons as a single effective band screening the Coulomb potential. We describe the corresponding self-energies in this scheme for a multi-subband system. In order to apply the formalism to heterostructures we discuss the screening and plasmon dispersion in both 2D and 3D systems. Results are shown for a four well quantum cascade laser with different doping concentration where comparisons to experimental findings can be made.

Contact resistances in trigate and FinFET devices in a non-equilibrium Green's functions approach

We compute the contact resistances Rc in trigate and FinFET devices with widths and heights in the 4–24 nm range using a Non-Equilibrium Green's Functions approach. Electron-phonon, surface roughness, and Coulomb scattering are taken into account. We show that Rc represents a significant part of the total resistance of devices with sub-30 nm gate lengths. The analysis of the quasi-Fermi level profile reveals that the spacers between the heavily doped source/drain and the gate are major contributors to the contact resistance. The conductance is indeed limited by the poor electrostatic control over the carrier density under the spacers. We then disentangle the ballistic and diffusive components of Rc and analyze the impact of different design parameters (cross section and doping profile in the contacts) on the electrical performances of the devices. The contact resistance and variability rapidly increase when the cross sectional area of the channel goes below ≃50 nm2. We also highlight the role of the charges trapped at the interface between silicon and the spacer material.

The effect of domain walls on leakage current in PbTiO3 thin films

Combining nonequilibrium Green function's approach with density functional theory, the effects of the 180° domain walls on the leakage current of PbTiO3 (PTO) thin films have been investigated. It is found that the leakage current of the ultrathin PTO film with 180° domain walls is greatly increased compared with that of perfect PTO film. Moreover, we found that the effect of domain walls on the leakage current in TiO2-terminated PTO films is larger than that in PbO-terminated PTO films. The enhancement of the leakage current is mainly attributed to the extra transport channels introduced by the domain walls according to the calculated transmission coefficient and the band structures. These results suggest that the leakage current of ferroelectric thin films can be decreased by controlling the occurrence of domain walls and their orientation in the design of nanoscale electronic devices.

Nonequilibrium Green Functions Approach to Strongly Correlated Fermions in Lattice Systems

AbstractQuantum dynamics in strongly correlated systems are of high current interest in many fields including dense plasmas, nuclear matter and condensed matter and ultracold atoms. An important model case are fermions in lattice systems that is well suited to analyze, in detail, a variety of electronic and magnetic properties of strongly correlated solids. Such systems have recently been reproduced with fermionic atoms in optical lattices which allow for a very accurate experimental analysis of the dynamics and of transport processes such as diffusion. The theoretical analysis of such systems far from equilibrium is very challenging since quantum and spin effects as well as correlations have to be treated non‐perturbatively. The only accurate method that has been successful so far are density matrix renormalization group (DMRG) simulations. However, these simulations are presently limited to one‐dimensional (1D) systems and short times. Extension of quantum dynamics simulations to two and three dimensions is commonly viewed as one of the major challenges in this field. Recently we have reported a breakthrough in this area [N. Schlünzen et al., Phys. Rev. B (2016)] where we were able to simulate the expansion dynamics of strongly correlated fermions in a Hubbard lattice following a quench of the confinement potential in 1D, 2D and 3D. The results not only exhibited excellent agreement with the experimental data but, in addition, revealed new features of the short‐time dynamics where correlations and entanglement are being build up. The method used in this work are nonequilibrium Green functions (NEGF) which are found to be very powerful in the treatment of fermionic lattice systems filling the gap presently left open by DMRG in 2D and 3D.In this paper we present a detailed introduction in the NEGF approach and its application to inhomogeneous Hubbard clusters. In detail we discuss the proper strong coupling approximation which is given by T ‐matrix selfenergies that sum up two‐particle scattering processes to infinite order. The efficient numerical implemen‐tation of the method is discussed in detail as it has allowed us to achieve dramatic performance gains. This has been the basis for the treatment of more than 100 particles over large time intervals. The numerical results presented in this paper concentrate on the diffusion in 1D to 3D lattices. We find that the expansion dynamics consist of three different phases that are linked with the build‐up of correlations. In the long time limit, a universal scaling with the particle number is revealed. By extrapolating the expansion velocities to the macroscopic limit, the obtained results show excellent agreement with recent experiments on ultracold fermions in optical lattices. Moreover we present results for the site‐resolved behavior of correlations and entanglement that can be directly compared with experiments using the recently developed atomic microscope technique. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-controlled spin filter and spin valve based on Fe-doped monolayer MoS2.

The thermal transport properties of an iron-doped molybdenum disulfide system were explored theoretically using the density functional theory calculations combined with the Keldysh non-equilibrium Green's function approach. The results indicate that a perfect spin filtering effect and spin Seebeck effect are induced thermally. Excellently, there exists thermal colossal magnetoresistances, which exhibit a transition between positive and negative that can be tuned using temperature. These features were elucidated using the band structures of the electrodes and the transmission function together with current spectra. Our findings may be helpful in the design of highly efficient spin caloritronic devices.

Tunable ballistic thermal conductance of electrons in strained graphene nanoribbons

The thermal transport properties of electrons in graphene nanoribbon (GNR) subjected to strain fields are studied by non-equilibrium Green's function approach. The results show that the stretching of the carbon–carbon bond in strained GNR can enhance the ballistic thermal conductance of electrons. The enhancement originates from that the change of C–C bond length can modulate the nearest-neighbour π-orbital overlap and reduce the threshold energies of electron transport modes. A brief analysis of these results is given.

Full counting statistics of vibrationally assisted electronic conduction: Transport and fluctuations of thermoelectric efficiency

We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double quantum dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic- a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second-order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the non-equilibrium Green's function approach (harmonic mode) and a kinetic quantum master equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property.

First-principles nonequilibrium Green's-function approach to transient photoabsorption: Application to atoms

We put forward a first-principle NonEquilibrium Green's Function (NEGF) approach to calculate the transient photoabsorption spectrum of optically thin samples. The method can deal with pump fields of arbitrary strength, frequency and duration as well as for overlapping and nonoverlapping pump and probe pulses. The electron-electron repulsion is accounted for by the correlation self-energy, and the resulting numerical scheme deals with matrices that scale quadratically with the system size. Two recent experiments, the first on helium and the second on krypton, are addressed. For the first experiment we explain the bending of the Autler-Townes absorption peaks with increasing the pump-probe delay $\t$, and relate the bending to the thickness and density of the gas. For the second experiment we find that sizable spectral structures of the pump-generated admixture of Kr ions are fingerprints of {\em dynamical correlation} effects, and hence they cannot be reproduced by time-local self-energy approximations. Remarkably, the NEGF approach also captures the retardation of the absorption onset of Kr$^{2+}$ with respect to Kr$^{1+}$ as a function of $\t$.

Carrier transport in Bi2Se3 topological insulator slab

Electron transport in Bi2Se3 topological insulator slabs is investigated in the thermal activation regime (>50K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi2Se3 slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments.

Varistor characteristics of a nano-device containing graphene and oxidised graphene: verification by DFT + NEGF

Electron transport and quantum conductance through an armchair graphene and its oxidised graphene-containing form were investigated by the density functional theory method and the implementation of the non-equilibrium Green function approach. The computed I − Vb(current as a function of bias voltage) characteristic of the studied systems showed the tunnelling phenomenon in bias and gate voltages considered. Along with the transport properties, electronic properties including density of states were calculated in the studied systems. A close examination of the results showed that the I − Vb curve for graphene behaved like at some bias voltages, while for the oxidised graphene-containing form, its trend was the same as that of a voltage dependent resistor (VDR-VARiable resISTOR), I∝Vβb, at the whole range of the applied bias.

Effect of nonadiabatic electronic-vibrational interactions on the transport properties of single-molecule junctions

The interaction between electronic and vibrational degrees of freedom in single-molecule junctions may result from the dependence of the electronic energies or the electronic states of the molecular bridge on the nuclear displacement. The latter mechanism leads to a direct coupling between different electronic states and is referred to as nonadiabatic electronic-vibrational coupling. Employing a perturbative nonequilibrium Green's function approach, we study the influence of nonadiabatic electronic-vibrational coupling in model molecular junctions. Thereby, we distinguish between systems with well-separated and quasidegenerate electronic levels. The results show that the nonadiabatic electronic-vibrational interaction can have a significant influence on the transport properties. The underlying mechanisms, in particular the difference between nonadiabatic and adiabatic electronic-vibrational couplings, are analyzed in some detail.

Electron-Transport Properties of Few-Layer Black Phosphorus.

We perform the first-principles computational study of the effect of number of stacking layers and stacking style of the few-layer black phosphorus (BPs) on the electronic properties, including transport gap, current-voltage (i-v) relation, and differential conductance. Our computation is based on the nonequilibrium Green's function approach combined with density functional theory calculations. Specifically, we compute electron-transport properties of monolayer BP, bilayer BP, and trilayer BP as well as bilayer BPs with AB-, AA-, or AC-stacking. We find that the stacking number has greater influence on the transport gap than the stacking type. Conversely, the stacking type has greater influence on i-v curve and differential conductance than on the transport gap. This study offers useful guidance for determining the number of stacking layers and the stacking style of few-layer BP sheets in future experimental measurements and for potential applications in nanoelectronic devices.

Charge transport and ac response under light illumination in gate-modulated DNA molecular junctions

Using a two-strand tight-binding model and within nonequilibrium Green's function approach, we study charge transport through DNA sequences and sandwiched between two Pt electrodes. We show that at low temperature DNA sequence exhibits coherent charge carrier transport at very small bias, since the highest occupied molecular orbital in the GC base pair can be aligned with the Fermi energy of the metallic electrodes by a gate voltage. A weak distance dependent conductance is found in DNA sequence with large NTA. Different from the mechanism of thermally induced hopping of charges proposed by the previous experiments, we find that this phenomenon is dominated by quantum tunnelling through discrete quantum well states in the TA base pairs. In addition, ac response of this DNA junction under light illumination is also investigated. The suppression of ac conductances of the left and right lead of DNA sequences at some particular frequencies is attributed to the excitation of electrons in the DNA to the lead Fermi surface by ac potential, or the excitation of electrons in deep DNA energy levels to partially occupied energy levels in the transport window. Therefore, measuring ac response of DNA junctions can reveal a wealth of information about the intrinsic dynamics of DNA molecules.

A new possible transition from two- to one-channel Kondo physics in mesoscopic transport

We study the joint effects of the Coulombic dot–lead interaction and intralead electron interaction on nonequilibrium transport through a quantum dot coupled to Luttinger liquid leads in the Kondo regime. By applying the nonperturbative canonical transformation technique and extended equation of motion method of nonequilibrium Green function approach, we find two types of zero-bias anomalies with an enhanced conductance for the renormalized exponent Y<1 and a suppressed conductance for Y>1. The transition between the two behaviors is found at Y≈1. For Y>1 and Y<1, tunneling between the dot and Luttinger liquid leads is suppressed and enhanced, respectively, and they reflect two different types of exciton-assist tunneling. These physics phenomena provide opportunities to easily study the transition of two-channel Kondo physics in the future experiments.