Nonequilibrium Green Function Method Research Articles

Increasing the output power of a heavily doped terahertz quantum cascade laser by avoiding the subband misalignment

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions.

Quantized charge-pumping in higher-order topological insulators

We study the quantized charge pumping of higher-order topological insulators (HOTIs) with edge-corner correspondences based on the combination of the rotation of in-plane magnetic field and the quantum spin Hall effect. A picture of a specific charge pumping process is uncovered with the help of the non-equilibrium Green's function method. Significantly, we demonstrate that the quantized charge pumping current is achieved without the participation of bulk states, and the charges move along the boundary of the sample. Furthermore, the effects of external parameters on the pumping current is also studied. We find that the magnitude and direction of the pumping current can be manipulated by adjusting the coupling strength between the leads and sample. Our work deepens the understanding of the charge pumping in HOTIs and extends the study of their transport properties.

Destructive interference and Fano antiresonance in T-shaped double quantum dots coupled to Luttinger liquid leads

We investigate the interplay of quantum interference effects and the electronic correlation on the transport through T-shaped spinless double quantum dots coupled to Luttinger liquid leads by using the nonequilibrium Green's function method. We find that the weak intralead Coulomb interaction leads to a zero-bias antiresonance dip in the differential conductance for small interdot tunneling coupling, which is distinct destructive interference characteristic. In particular, an intriguing Fano antiresonance profile for asymmetrically coupling to the Luttinger liquid leads is exhibited, consistent with the experiment findings but different physics screening origin. The strong intralead Coulomb interaction or large interdot coupling tunneling quenches the quantum interference effects, resulting in the disappearance of Fano antiresonance. The scaling behavior of differential conductance on temperature is presented for different intralead interactions. The relationship between destructive interference and two-channel Kondo effect is given. Our results provide a way of adjusting in and out of antiresonance to achieve quantum interference transistors and of switching by tuning the gate voltage controlling the interdot tunneling. • The interplay of interference and intralead Coulomb interaction on transport of T-shaped quantum dots is studied. • Weak intralead interaction leads to zero-bias dip in differential conductance for small interdot tunneling. • New physical mechanism is presented to explain for the antiresonance dip and Fano effect. • Scaling behavior of differential conductance on temperature is presented for different intralead interactions.

Ab Initio Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect

Magnetic tunnel junctions (MTJs) have attracted strong research interest within the last decades due to their potential use as nonvolatile memory such as magnetoresistive random access memory as well as for magnetic logic applications. Half-metallic magnets (HMMs) have been suggested as ideal electrode materials for MTJs to achieve an extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode effect, which was achieved in a magnetic tunnel junction concept [E. \ifmmode \mbox{\c{S}}\else \c{S}\fi{}a\ifmmode \mbox{\c{s}}\else \c{s}\fi{}\ifmmode \imath \else \i \fi{}o\ifmmode \breve{g}\else \u{g}\fi{}lu et al., ACS Appl. Electron. Mater. 1, 1552--9 (2019)] based on HMMs and type-II spin-gapless semiconductors (SGSs). The proposed concept has recently been experimentally demonstrated using Heusler compounds. In the present work, we investigate from first-principles MTJs based on type-II SGS and HMM quaternary Heusler compounds $\mathrm{Fe}\mathrm{V}\mathrm{Ta}\mathrm{Al}$, $\mathrm{Fe}\mathrm{V}\mathrm{Ti}\mathrm{Si}$, $\mathrm{Mn}\mathrm{V}\mathrm{Ti}\mathrm{Al}$, and $\mathrm{Co}\mathrm{V}\mathrm{Ti}\mathrm{Sb}$. Our ab initio quantum transport calculations based on a nonequilibrium Green's function method have demonstrated that the MTJs under consideration exhibit current rectification with relatively high on:off ratios. We show that, in contrast to conventional semiconductor diodes, the rectification bias voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that the diode effect can be configured dynamically, i.e., depending on the relative orientation of the magnetization of the electrodes, the MTJ allows the electrical current to pass either in one or the other direction, which leads to an inverse TMR effect. The combination of nonvolatility, reconfigurable diode functionality, tunable rectification voltage window, and high Curie temperature of the electrode materials makes the proposed MTJs very promising for room-temperature spintronic applications and opens ways to magnetic memory and logic concepts as well as logic-in-memory computing.

TPV radical-based multifunctional molecular spintronic device: A first-principles study

We design a multifunctional molecular spintronic device consisting of two 1,3,5-triphenylverdazyl (TPV) radicals with carbon nanotube bridge and electrodes , and investigate its spin-polarized transport properties using the first-principles density functional theory and nonequilibrium Green's function method. The results show that the spin-polarized transport properties depend strongly on the external magnetic field modulation and high-efficiency spin-filtering, giant magnetoresistance , spin-rectifying and low-bias voltage negative differential resistance effects can be realized in this designed device. The mechanisms are proposed for these interesting physical effects based on the bias-dependent spin-resolved transmission function, projected density of states and corresponding molecular projected self-consistent Hamiltonian orbitals analysis. These results hold great potential for the development of high-performance multifunctional molecular spintronic devices. High-efficiency spin-filtering, giant magnetoresistance, spin-rectifying and low-bias voltage negative differential resistance effects are realized in a TPV radical-based molecular spintronic device with carbon nanotube bridge and electrode. • Spin-polarized transport properties of a TPV radical-based molecular spintronic device are investigated. • Spin-polarized transport properties depend strongly on the external magnetic field modulation. • High-efficiency spin-filtering, GMR, spin-rectifying and low-bias voltage NDR effects are realized.

Self-energy corrected DFT-NEGF for conductance in molecular junctions: an accurate and efficient implementation for TRANSIESTA package applied to Au electrodes

In view of the development and the importance that the studies of conductance through molecular junctions is acquiring, robust, reliable and easy-to-use theoretical tools are the most required. Here, we present an efficient implementation of the self-energy correction to density functional theory non-equilibrium Green functions method for TRANSIESTA package. We have assessed the validity of our implementation using as benchmark systems a family of acene complexes with increasing number of aromatic rings and several anchoring groups. Our theoretical results show an excellent agreement with experimentally available measurements assuring the robustness and accuracy of our implementation.

σ-σ Stacked supramolecular junctions.

Intermolecular charge transport plays an essential role in organic electronic materials and biological systems. To date, experimental investigations of intermolecular charge transport in molecular materials and electronic devices have been restricted to conjugated systems in which π-π stacking interactions are involved. Herein we demonstrate that the σ-σ stacking interactions between neighbouring non-conjugated molecules offer an efficient pathway for charge transport through supramolecular junctions. The conductance of σ-σ stacked molecular junctions formed between two non-conjugated cyclohexanethiol or single-anchored adamantane molecules is comparable to that of π-π stacked molecular junctions formed between π-conjugated benzene rings. The current-voltage characteristics and flicker noise analysis demonstrate the existence of stacked molecular junctions formed between the electrode pairs and exhibit the characteristics of through-space charge transport. Density functional theory calculations combined with the non-equilibrium Green's function method reveal that efficient charge transport occurs between two molecules configured with σ-σ stacking interactions.

Influence of contact interface on electric transport in in-plane graphene/MoSSe heterojunction

Janus transition-metal dichalcogenides (JTMDS) have attracted much attention due to their obvious potential for electronic and optical applications. The electronic transport properties of in-plane graphene/MoSSe heterojunctions are studied using density functional theory and nonequilibrium Green's function method. It is found that different contact patterns lead to different accumulation and depletion on the contact interface and exhibit a P-type Schottky barrier. Moreover, the Schottky barrier can be modulated by doping. The ohmic contact is formed when the doping concentration is 3 × 1013 cm−2, which is proved by the approximately linear characteristics of the current-voltage curve. These results would provide an insight into designing a high-performance device.

Applying bias voltage to improve rectification performance of molecular junction through side gate-group

Molecular diode is one of the most significant molecular devices for it being promised as electronic component in the molecular logic circuit. Controlling electronic transport to improve the rectification ratio of molecular diode is a crucial target for researchers. Here, based on the density functional theory and non-equilibrium Green's function method, the rectification performances of diblock co-oligomer molecules are investigated systematically. The numerical results show that, side gate-group can slightly enhance rectification ratios of diblock-molecule diodes, but it produces evident hindering effects on the electronic transports of the molecular junctions. Using electrode protrusion to contact with the side gate-group, the bias voltage is introduced to modulate the electronic transports of molecular junctions. By this strategy, the voltage at which the lowest unoccupied molecular orbital (LUMO) enters bias window is lowered evidently in the positive bias regime, which further significantly enhances the rectification ratios of the molecular diodes. The calculations additionally show that the LUMO is less delocalized under positive bias than it under negative bias, which is disadvantageous for enhancing the rectification performances of diblock co-oligomer molecular diode, thus very large rectification ratios are difficult to be achieved when one fabricates diblock co-oligomer molecular diodes in experimental fabrications.

Thermoelectric performance of porphyrin molecular junction with graphene nanoribbon electrodes

Thermoelectric performance of porphyrin molecular junction is investigated by using first-principles calculation combined with nonequilibrium Green function (NEGF) method. It is found that porphyrin molecule rotation (90[Formula: see text] rotated) has no obvious effect on its thermoelectric conversion efficiency due to the conjugate structure of central molecule. Through the functions of energy and thermal conductivity, it is found that phonon thermal conductivity accounts for the main part, which provides a potential for further optimizing thermoelectric performance. Moreover, it can be seen from phonon transmission spectrum that porphyrin molecule has a certain filtering effect on high-frequency phonon mode. For power factor and figure of merit (ZT), wave functions for two cases highly coincide. The maximum ZT values of por1 and por2 are 0.38 and 0.34 near Fermi level (FL), respectively. Porphyrin molecular junction could have a good application prospect in nanoscale thermoelectric devices.

Over One Watt Output Power Terahertz Quantum Cascade Lasers by Using High Doping Concentration and Variable Barrier‐Well Height

A GaAs/AlGaAs‐based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. To realize a high output power, an effective way is to increase the electron density at the active region by using a high doping level. This work aims to optimize a design enabling to use a heavy doping level, by realigning the resonant tunneling which can be seriously degraded due to a band bending from this heavy local doping. With variable quantum well and barrier height, the design owns an improved parameter tuning freedoms, and a saturation of peak gain can be achieved at a relatively high doping concentration of 1.0 × 1017 cm−3, which is twice higher than the recently reported device. Nonequilibrium Green's function method including full carriers scattering effect is used to carry out the design optimization, mainly including the realignment of resonant tunneling and suppression of leakages via high‐energy confined levels. In the experiment, a 14 μm thick active layer for this design is grown and also large mesa size is developed. To ensure a quasi‐continuous wave (quasi‐CW) operation, the heat dissipation is also managed.

Reverse strain-induced snake states in graphene nanoribbons

Strain can tailor the band structures and properties of graphene nanoribbons (GNRs) with the well-known emergent pseudo-magnetic fields and the corresponding pseudo-Landau levels (pLLs). We design one type of the zigzag GNR (ZGNR) with reverse strains, producing pseudo-magnetic fields with opposite signs in the lower and upper half planes. Therefore, electrons propagate along the interface as "snake states", experiencing opposite Lorentz forces as they cross the zero field border line. By using the Landauer-Buttiker formalism combined with the nonequilibrium Green's function method, the existence and robustness of the reverse strain-induced snake states are further studied. Furthermore, the realization of long-thought pure valley currents in monolayer graphene systems is also proposed in our device.

Microscopic theory of photon-induced energy, momentum, and angular momentum transport in the nonequilibrium regime

We set up a general microscopic theory for the transfer of energy, momentum, and angular momentum mediated by photons. Using the nonequilibrium Green's function method, we propose a unified Meir-Wingreen formalism for the energy emitted, force experienced, and torque experienced by the objects due to the fluctuating electromagnetic field. Our theory does not require the local thermal equilibrium that is the central assumption of the conventional theory of fluctuational electrodynamics (FE). The obtained formulas are valid for arbitrary objects as well as the environment without the requirement of reciprocity. To show the capability of our microscopic theory, we apply the general formulas to transport problems of graphene edges in both equilibrium and nonequilibrium situations. We show the local equilibrium energy radiation of graphene obeys the well-known $T^4$ law with a converged theoretical emissivity of 2.058$\%$. In the ballistic nonequilibrium situation driven by chemical potential biases, we observe nonzero results for force and torque from the graphene edges, which go beyond the predictive ability of the FE theory. Our method is general and efficient for large systems, which paves the way for studying more complex transport phenomena in the nonequilibrium regime.

Insights into Electron Transport in a Ferroelectric Tunnel Junction.

The success of a ferroelectric tunnel junction (FTJ) depends on the asymmetry of electron tunneling as given by the tunneling electroresistance (TER) effect. This characteristic is mainly assessed considering three transport mechanisms: direct tunneling, thermionic emission, and Fowler-Nordheim tunneling. Here, by analyzing the effect of temperature on TER, we show that taking into account only these mechanisms may not be enough in order to fully characterize the performance of FTJ devices. We approach the electron tunneling in FTJ with the non-equilibrium Green function (NEGF) method, which is able to overcome the limitations affecting the three mechanisms mentioned above. We bring evidence that the performance of FTJs is also affected by temperature–in a non-trivial way–via resonance (Gamow-Siegert) states, which are present in the electron transmission probability and are usually situated above the barrier. Although the NEGF technique does not provide direct access to the wavefunctions, we show that, for single-band transport, one can find the wavefunction at any given energy and in particular at resonant energies in the system.

Influence of phonon-assisted tunneling on photovoltaic properties of BaSi2 and BaGe2 p–n homojunction solar cell devices

The solar cell properties of crystalline BaSi2 and BaGe2 p–n homojunctions are explored using density functional theory combined with a nonequilibrium Green function method. In particular, the quantitative estimates of solar cell parameters such as photocurrent, open-circuit voltage (Voc), short-circuit current (|Jsc|), and efficiency (η) are obtained for LDA and GGA-1/2 functionals. The effect of temperature on solar cell parameters is included through electron–phonon coupling (EPC) using the special thermal displacements method. The magnitudes of Jsc, Voc, and η for BaSi2 (BaGe2) at 300 K are found to be 27.35 mA/cm2 (26.1 mA/cm2), 0.84 V (0.78 V), and 18.0% (16.6%), respectively. Our study strongly suggests that the phonon-assisted photon absorption and thereby EPC significantly affect the photocurrent, and its inclusion is necessary for a proper description of various solar cell parameters. The computed solar cell parameters for BaSi2 (BaGe2) p–n homojunctions can be used as benchmark ab-initio quantum mechanical results and can be used in simulations based on continuum models.

Site-dependent spin-polarized tunneling via hybrid interface states on molecule/ferromagnet surface

Based on density functional theory and nonequilibrium Green's function method, we investigate the spin polarization and transport ability of hybrid interface states at benzene/Ni surface by introducing the top probe electrode. The results demonstrate that when the top electrode connects the molecule with different sites, the spin polarization of the projected density of states and the interfacial spin density distribution are disturbed in different ways. A contact site induced inversion of the spin polarization is observed at Fermi energy. The transport calculation indicates that the center contact is better to achieve a large tunneling magnetoresistance at low bias, but the edge and top contacts is useful to get a little larger spin polarization of the current. This work points out that the spin polarization and transport property of the hybrid interface states in a molecular junction depends on the way to introduce the second probe electrode, which can be suitable designed to achieve an excellent performance.

Time-linear scaling nonequilibrium Green's function method for real-time simulations of interacting electrons and bosons. II. Dynamics of polarons and doublons

Nonequilibrium dynamics of the open chain Holstein-Hubbard model is studied using the linear time-scaling GKBA+ODE scheme developed in the preceeding paper. We focus on the set of parameters relevant for photovoltaic materials, i.\,e., a pair of electrons interacting with phonons at the cross-over between the adiabatic and anti-adiabatic regimes and at moderately large electron-electron interaction. By comparing with exact solutions for two corner cases, we demonstrate the accuracy of the $T$-matrix (in the $pp$ channel) and the second-order Fan ($GD$) approximations for the treatment of electronic ($e$-$e$) and electron-phonon ($e$-ph) correlations, respectively. The feedback of electron on phonons is consistently included and is shown to be mandatory for the total energy conservation. When two interactions are simultaneously present, our simulations offer a glimpse into the dynamics of doublons and polarons unveiling the formation, propagation and decay of these quasiparticles, energy redistribution between them and self-trapping of electrons.

Time-linear scaling nonequilibrium Green's function methods for real-time simulations of interacting electrons and bosons. I. Formalism

Simulations of interacting electrons and bosons out of equilibrium, starting from first principles and aiming at realistic multiscale scenarios, is a grand theoretical challenge. Here, using the formalism of nonequilibrium Green's functions and relying in a crucial way on the recently discovered time-linear formulation of the Kadanoff-Baym equations, we present a versatile toolbox for the simulation of correlated electron-boson dynamics. A large class of methods are available, from the Ehrenfest to the dressed $GD$ for the treatment of electron-boson interactions in combination with perturbative, i.e., Hartree-Fock and second-Born, or nonperturbative, i.e., $GW$ and $T$-matrices either without or with exchange effects, for the treatment of the Coulomb interaction. In all cases the numerical scaling is linear in time and the equations of motion satisfy all fundamental conservation laws.

Optimization of gain region in mid-IR ( ≈ 5 μm) QCL.

Non-equilibrium Green's function (NEGF) formalism is used to optimize the gain region of a quantum cascade laser (QCL) tailored to emit radiation at ∼5 µm wavelength, originally designed by Evans et al. [Appl. Phys. Lett., 88,051105(2006)10.1063/1.2171476]. The optimization strategy uses electron-photon selfenergies to find characteristics of devices under the "operating conditions," i.e., interacting with the laser field. These conditions can be quite different from the one when the device is in no-lasing state and the unsaturated gain is being optimized. The saturation caused by the optical field can push the structure from strong to weak coupling conditions, what changes laser parameters in a non-linear manner. Moreover, the NEGF method does not require any phenomenological parameters (such as, e.g., the phase relaxation times), so the quantities dependent on these parameters are determined solely on physical grounds. The use of the above procedure for the structure under investigation shows that the increase of the quantum efficiency by 24% and the output power by 83% in comparison to the original design can be achieved when the widths of injection and extraction barriers are changed to their optimal values.

Electronic phase transition, spin filtering effect, and spin Seebeck effect in 2D high-spin-polarized VSi2X4 (X = N, P, As)

Spin gapless semiconductors (SGSs) and bipolar magnetic semiconductors (BMSs) are highly desirable for advanced spintronic applications due to their unique electronic and magnetic properties. Being inspired by the recent synthesis of the 2D MA2X4 family with various electronic properties, including nontrivial topological properties, 2D ferromagnetism, Ising superconductivity, and robust valley polarization, we systematically studied the electron structure, magnetism, and spin transport properties for VSi2X4 (X = N, P, As) monolayers by the density functional theory combined with the nonequilibrium Green's function method. The results show that VSi2X4 monolayers are ferromagnetic type-II SGS and BMS with high Curie temperatures 230–250 K. The interesting electronic phase transitions of type-II-SGS-to-metal-to-type-I-SGS for VSi2P4 and BMS-to-type-I-SGS-to-type-II-SGS for VSi2As4 can be achieved by applying the biaxial strain. Furthermore, the VSi2P4 monolayer exhibits the bias-dependent spin filtering effect and the temperature-driven spin Seebeck effect. The high Curie temperature, the versatile strain-tuned electronic phase transitions, and the excellent spin transport characteristics make 2D VSi2X4 the promising candidate for spintronic applications and will stimulate intensive studies on this class of high-spin-polarized 2D systems.