Green Function Formalism Research Articles

Design and simulation of a binary full adder based on quantum rings

In this paper, a binary full adder circuit is designed with eight quantum rings where each ring is threaded by a magnetic flux which is an odd multiple of half of the magnetic flux quantum. The quantum rings are divided into two categories with four rings connected to each other in parallel. They are attached symmetrically to two semi-infinite one-dimensional metallic electrodes , and three gate voltages are applied as three inputs of the full adder. The Hamiltonian of the system is approximated by the tight-binding method, and the calculations are performed by using Green's function formalism. The current-voltage characteristics are obtained for different values of applied gate voltages in the strong and weak-coupling strengths between the quantum rings and source and drain electrodes. The results are in accordance with the truth table of a binary full adder and show this quantum circuit behaves as a binary full adder. • This work proposes a binary full adder consisting of eight quantum rings where a constant magnetic flux is passing through each of them. • The designed full adder with eight quantum rings is divided into two separate parts (Sum and Carry-out) which are connected symmetrically to the source and drain electrodes. • The values of the applied gate voltages are considered as the full adder inputs and the electric current is the output of the system. • The output current of the proposed circuit based on the system input voltages is in accordance with the truth table of a binary full adder.

Bound states in the continuum in a two-channel Fano-Anderson model

In this article, we study the formation of the bound states in the continuum (BICs) in a two-channel Fano-Anderson model. We employ the Green's function formalism, together with the equation of motion method, to analyze the relevant observables, such as the transmission coefficient and the density of states. Most importantly, our results show that the system hosts true BICs for the case of a symmetric configuration with degenerate impurity levels, and a complete transmission channel is then suppressed. Finally, we argue that the proposed mechanism could be relevant for the realization of BICs in electronic and photonic systems.

Critical dynamics of the superfluid phase transition: Multiloop calculation of the microscopic model.

Results and method of a three-loop renormalization-group calculation in the model of a Bose gas with a local density-density interaction in the formalism of time-dependent Green functions at finite temperature are presented. The results provide support to the recent conjecture [J. Honkonen, M.V. Komarova, Y.G. Molotkov, and M.Y. Nalimov, Nucl. Phys. B 939, 105 (2019)0550-321310.1016/j.nuclphysb.2018.12.015; Y.A. Zhavoronkov, M.V. Komarova, Y.G. Molotkov, M.Y. Nalimov, and J. Honkonen, Theor. Math. Phys. 200, 1237 (2019)0040-577910.1134/S0040577919080142] that the dynamics of the superfluid phase transition is described by a model which belongs to the same universality class as the stochastic model A.

Modeling of a vertical tunneling transistor based on Gr-hBN- χ 3 borophene heterostructure

We present a computational study on the electrical behavior of the field-effect transistor based on vertical graphene-hBN-χ3 borophene heterostructure and vertical graphene nanoribbon-hBN-χ3 borophene nanoribbon heterostructure. We use nonequilibrium the Green function formalism along with an atomistic tight-binding (TB) model. The TB parameters are calculated by fitting tight-binding band structure and first-principle results. Also, electrical characteristics of the device, such as ION/IOFF ratio, subthreshold swing, and intrinsic gate-delay time, are investigated. We show that the increase of the hBN layer number decreases subthreshold swing and degrades the intrinsic gate-delay time. The device allows current modulation 177 at room temperature for a 1.2 V gate-source bias voltage.

Proposal for All-Electrical Spin Manipulation and Detection for a Single Molecule on Boron-Substituted Graphene.

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S=1 and S=3/2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fe-d_{z^{2}} and B-p_{z} orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (>10% for typical coverage). Our work paves the way to realize all-electrical spintronics devices using molecular spinterfaces.

Bilayer armchair graphene nanoribbon photodetector with Stone-Wales defect: A computational study

Bilayer armchair graphene nanoribbons (BAGNRs), due to their low sensitivity to low frequency noise and high controllability of their bandgap have drawn a lot of attention as building blocks of photodetectors in recent years. In this work, using tight binding (TB) method and non-equilibrium Green's function formalism (NEGF) a computational study has been done on the BAGNR photodetector with perfect and Stone-Wales (SW) defected channel and the effect of defects' number and position on the device performance has been investigated. In order to validate the TB parameters used in this work, the bandstructures of both perfect and SW defected BAGNRs are compared with the results of DFT method and a good matching is obtained. Simulation results show that perfect BAGNR photodetector has quantum efficiency (QE) of 19.77% at 0.12 eV photon energy. Among defected photodetectors, highest QE of 21.77% belongs to the structure with single SW defect near the right contact (higher bias voltage) at 0.14 eV. This structure also possesses the highest Iph/Idark of 64.45 which is 1.8 times larger than Iph/Idark of perfect BAGNR photodetector. Moreover, the structure with two SW defects located at left and right sides of the channel exhibits a drastically low Idark which results in a high Iph/Idark. In addition, this structure yields the high QE (19.72%) at blue-shifted photon energy of 0.2 eV which makes it a suitable choice for mid-wavelength infrared applications.

Electronic transport properties of compressed and stretched helicene-graphene nanostructures, a theoretical study

We present our ab initio simulations of strained helicene molecular junction. First, we present ab-initio models of [n]helicenes up to [100]helicene done by two different recent reactive force field molecular geometry optimization methods (ReaxFF CHOCsKNaClIFLi, 2019 and CHONSMgPNaCuCl, 2018). We used the [4]helicene and [10]helicene models for Density Functional Theory (DFT) electronic transport calculations of various graphene-helicene-graphene molecular junction configurations on Local Density Approximation (LDA) or Generalized Gradient Approach (GGA) level of exchange and correlation energy with Perdew-Zinger (PZ) or Perdew–Burke–Ernzerhof (PBE) functionals and Grimme DFT-D3 van der Waals correction. The electronic transport properties are studied by using of the Green's functions formalism. The electron transmissions and densities of states of [4]helicene (and [10]helicene) attached as a molecular junction in three different configurations to zigzag graphene nanoribbon (ZGNR) electrodes of three different widths were obtained. The structures are analyzed while relaxed, compressed or stretched. The current-voltage and current-strain characteristics are presented and results are compared and discussed. The work is supported by a short review summarizing some of the recent studies related to graphene, helicene, and molecular junctions under strain.

A statistical quasi-particles thermofield theory with Gaussian environments: System-bath entanglement theorem for nonequilibrium correlation functions.

For open quantum systems, the Gaussian environmental dissipative effect can be represented by statistical quasi-particles, namely, dissipatons. We exploit this fact to establish the dissipaton thermofield theory. The resulting generalized Langevin dynamics of absorptive and emissive thermofield operators are effectively noise-resolved. The system-bath entanglement theorem is then readily followed between an important class of nonequilibrium steady-state correlation functions. All these relations are validated numerically. A simple corollary is the transport current expression, which exactly recovers the result obtained from the nonequilibrium Green's function formalism.

Nonlinear Quantum Optics With Structured Light: Tightly Trapped Atoms in the 3D Focus of Vectorial Waves

Atomic gases tightly trapped near the focus of an electromagnetic wave interact with photons that exhibit a complex structure, displaying strong gradients of field amplitude and local polarization that can lead to topological phase singularities. We illustrate the consequences of this structure on a paradigmatic nonlinear optical process: three-wave mixing. The process begins by proper selection of the pump field, whose spatial structure is tailored to present huge gradients of the EM field that enhance atomic excitations through forbidden transitions. Atoms can then be depopulated via two electric dipole decays in a cascade configuration, thus providing the three necessary waves. The properties of the down-converted photons are conditioned to those of the pump field through phase matching conditions. It is emphasized that the expression of the photons must incorporate both the structure of the vectorial EM modes and the spatial configuration of the atomic trap. Due to the three-dimensional focusing, the slowly varying envelope approximation becomes inadequate when describing the scattered EM field. We discuss an alternative using a Green function formalism valid for any configuration of the field that also allows to identify the phase matching conditions. Spherical vectorial waves exemplify most concepts here discussed, including the possibility of observing nonlinear quantum phenomena at the single photon level.

Green's Function Formulation of Quantum Defect Embedding Theory.

We present a Green's function formulation of the quantum defect embedding theory (QDET) where a double counting scheme is rigorously derived within the G0W0 approximation. We then show the robustness of our methodology by applying the theory with the newly derived scheme to several defects in diamond. Additionally, we discuss a strategy to obtain converged results as a function of the size and composition of the active space. Our results show that QDET is a promising approach to investigate strongly correlated states of defects in solids.

Study of the transport properties of cobalt atomic contact under mechanical strain in a nitrogen atmosphere

Achieving an efficient spin-filter effect and large magneto-resistance in molecular junctions is very important in spintronics. By employing the non-equilibrium Green's function formalism combined with the density functional theory, this study investigated the atomic structures and spin-resolved transport properties of molecular junctions consisting of nitrogen (N2) sandwiched between two cobalt (Co) electrodes with different crystal lattices as a function of stretching distance. In the case of face-centred cubic Co electrodes, our calculations showed that the transport properties depend strongly on the stretching process, which changes the contact between the N2 and Co electrodes. Specifically, the maximum spin polarization is due to the configuration of N2 molecules sitting in parallel with the protruding Co atoms. Furthermore, the Co dxz,yz orbitals interact with the N px,y orbitals, which contribute to the high transmission coefficient at the Fermi level for spin-down electrons. Meanwhile, the Co s-dz2 hybridization orbitals, which interact weakly with the N p orbitals, contribute to the small transmission coefficient at the Fermi level. This can also be verified by the molecular junction model adopting simple monatomic Co chain electrodes, which also show a perfect spin-filter effect and achieve an ideal magnetoresistance. In contrast, in the case of hexagonal Co electrodes, the spin-filter effect disappears. However, the effect appears again by adding one more Co atom to the central point contact. Our findings show that the spin-resolved electronic transport of molecular junctions depends on the stretching process and the nature of the electrodes, which can help in the future design of molecular devices based on the spin degree of freedom.

Plasmonic Waveguides: Enhancing quantum electrodynamic phenomena at nanoscale

The emerging field of plasmonics can lead to enhanced light matter interactions at extremely nanoscale regions. Plasmonic (metallic) devices promise to efficiently control both classical and quantum properties of light. Plasmonic waveguides are usually used to excite confined electromagnetic modes at the nanoscale that can strongly interact with matter. The analysis of these nanowaveguides exhibits similarities with their low frequency microwave counterparts. In this article, we review ways to study plasmonic nanostructures coupled to quantum optical emitters from a classical electromagnetic perspective. These quantum emitters are mainly used to generate single photon quantum light that can be employed as a quantum bit or qubit in the envisioned quantum information technologies. We demonstrate different ways to enhance a diverse range of quantum electrodynamic phenomena based on plasmonic configurations by using the classical dyadic tensor Green function formalism. More specifically, spontaneous emission and superradiance are analyzed by using the Green function based field quantization. The exciting new field of quantum plasmonics will lead to a plethora of novel optical devices for communications and computing applications operating in the quantum realm, such as efficient single-photon sources, quantum sensors, and compact on-chip nanophotonic circuits.

Controllable sensitivity mechanism in an energetic compound of [FeII(Rtrz)6] as a molecular switch

Transition metal iron complexes can be designed for spin crossover compounds with low spin and high spin bistable (LS and HS) states, spin transition can be triggered by external excitation, which regulates stability (sensitivity) and magnetism of complexes. [FeII(Rtrz)6] systems are bulit with the nitrogen-rich group triazole as the ligand, potentially making them high-energy compounds that can be applied in pyrotechnics and especially good candidates as green initiator for weapon systems. In this paper, two kinds of mononuclear complexes [FeIIN6-1,2,4-triazole] (1) and [FeIIN6-1,2,3-triazole] (2) are constructed, and LS ⇌ HS transition occurs by altering the bond lengths of metal ligand at room temperature; therefor, complex 1 and 2 may be spin crossover energetic materials. The ground state of both complexes is the LS state (low sensitivity), and the stable trend is 1 > 2. The spin transport mechanism is calculated by the non-equilibrium Green's function formalism combined with density functional theory. The theoretical results show that both 1 and 2 can be used as molecular switches in a small bias range, but 2 cannot be used when the bias voltage is high. In short, complex 1 is an ideal molecular switch from HS state (high-sensitivity) to LS state (low-sensitivity), acting as a reversible on ⇌ off switch. The spin state transition of complex 1 can be induced with an electric field; therefore, it is expected to control the safety of primary explosives precisely in the long term.

Calculated characterisation of a sensitive gas sensor based on PEDOT:PSS

The interactions between poly (3,4-ethylene dioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) and small gas molecules are studied using non-equilibrium Green's function formalism based on the density functional theory. The proposed method is implemented in the Tran SIESTA code to benefit from the potential application of PEDOT:PSS as a gas sensor. The results show that doping with nanoparticles can drastically improve the sensitivity of polymer-based chemical gas sensors. Moreover, among various PEDOT:PSS doping materials, silver nanoparticles have an appropriate response to ammonia, while platinum shows the best response to carbon dioxide. The numerical results can be useful to design PEDOT:PSS-based gas sensors.

Accelerated causal Green's function molecular dynamics

A Green's function formalism has been applied to solve the equations of motion in classical molecular dynamics simulations. This formalism enables larger time scales to be probed for vibration processes in carbon nanomaterials. In causal Green's function molecular dynamics (CGFMD), the total interaction potential is expanded up to the quadratic terms, which enables an exact solution of the equations of motion to be obtained for problems within the harmonic approximation, reasonable energy conservation, and fast temporal convergence. Differently from conventional integration algorithms in molecular dynamics, CGFMD performs matrix multiplications and diagonalizations within its main loop, which make its computational cost high and, therefore, has limited its use. In this work, we propose a method to accelerate CGFMD simulations by treating the full system of N atoms as a collection of N smaller systems of size n. Diagonalization is performed for smaller nd×nd dynamical matrices rather than the full Nd×Nd matrix (d=1,2, or 3). The eigenvalues and eigenvectors are then used in the CGFMD equations to update the atomic positions and velocities. We applied the method for one-dimensional lattices of oscillators and have found that the method rapidly converges to the exact solution as n increases. The computational time of the proposed method scales linearly with N, providing a considerable gain with respect to the O(N3) full diagonalization. The method also exhibits better accuracy and energy conservation than the velocity-Verlet algorithm. An OpenMP parallel version has been implemented and tests indicate a speedup of 14× for N= 50000 in affordable computers. Our findings indicate that CGFMD can be an alternative, competitive integration technique for molecular dynamics simulations.

Electron Transport Through Experimentally Controllable Parabolic Bubbles on Graphene Nanoribbons

We present a theoretical study of electron transport properties through experimentally controllable graphene nanobubbles [P. Jia et al., Nat. Commun. 10 (2019) 1] employing a tight-binding model and the non-equilibrium Green's function formalism. Sharp conductance peaks are observed at low energy region which signifies the emergence of quasi-bound states caused by pseudomagnetic field in the strained nanobubbles. Analysis based on local density of states reveals the nature of electron transmission at peak energies. Our results also show that the emergence of quasi-bound states and its role in electron transport depend on both strain strength and bubble size: when the strain or size of the bubble increases, more quasi-bound states emerge and resonant tunnelling assisted by these quasi-bound states becomes dominant.

Nonlinear Hall effect induced by internal Coulomb interaction and phase relaxation process in a four-terminal system with time-reversal symmetry

We numerically investigate the second-order nonlinear Hall transport properties of a four-terminal system with time-reversal symmetry and broken inversion symmetry. Within the nonequilibrium Green's function formalism, the second-order nonlinear conductances are derived, where the internal Coulomb potential in response to external voltages is explicitly included to guarantee the gauge invariance. For the system with single mirror symmetry ${\mathcal{M}}_{x}$, nonlinear Hall properties are only observable in the $y$ direction and contributed solely from the second-order nonlinear effect. From the symmetry point of view, the observed nonlinear Hall transport phenomena have one-to-one correspondence with the Berry curvature dipole induced nonlinear Hall effect semiclassically obtained for the same Hamiltonian. In addition to the nonlinear Hall effect originated from symmetries of the system, it is found that the internal Coulomb potential has the same symmetry of the four-terminal system, which gives rise to an extra nonlinear Hall response. Moveover, the phase relaxation mechanism modeled by virtual probes leads to the dephasing-induced nonlinear Hall effect.

High‐Performance Monolayer SiMe‐Graphene n‐Type Field‐Effect Transistors with Low Supply Voltage and High On‐State Current in Sub‐5 nm Gate Length

AbstractWith the development of device miniaturization, traditional Si‐based field effect transistors (FETs) are approaching their scaling limits. It is necessary to find more alternative materials to improve the performance of FETs. 2D semiconductors attract great attention for channel material due to their atomic‐scale smoothness and dangling‐bond‐free surface, and the gate electrostatic properties and charge carrier mobility are better than traditional 3D Si‐based semiconductor. In this paper, using density functional theory combined with nonequilibrium Green's function formalism, the electronic properties and device performance of sub‐5 nm monolayer SiMe‐graphene n‐type field‐effect transistors are evaluated. Remarkably, the on‐state current value of the FET can reach 3070 µA µm−1, which greatly surpasses most of reported FETs based on other 2D semiconductors. In addition, the on‐state current, power dissipation, and delay time of the SiMe‐graphene FETs satisfy the high‐performance requirements of the 2020 International Roadmap for Device and Systems under different supply voltages, which indicates that the FETs can work at different operating frequencies and power consumption. Moreover, via controlling electrode doping concentration, the channel length can be reduced to 4 nm. In total, the SiMe‐graphene is a favorable channel material for future FETs devices.

First-principles mobility prediction for amorphous semiconductors

Carrier mobility in amorphous semiconductors remained unpredictable due to random electronic states in the absence of the long-range order in a lattice structure, although amorphous semiconductors have been investigated over several decades and widely used in diverse electronic devices. In this work, we develop a method to predict mobility of disordered systems by virtue of the first-principles calculation without using any empirical parameters. Quantum transport modeling based on the nonequilibrium Green's function formalism enables us to establish a formula to connect first-principles results with amorphous-phase mobility. Finally, the developed approach is quantitatively validated by comparing the theoretical predictions with previously measured mobilities of amorphous metal oxides $({\mathrm{SnO}}_{2},{\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ZnO) and amorphous silicon. Localization analysis provides further physical insight into a distinct feature between the amorphous metal oxides and amorphous silicon.

Drifting Electrons: Nonreciprocal Plasmonics and Thermal Photonics

Light propagates symmetrically in opposite directions in most materials and structures. This fact -- a consequence of the Lorentz reciprocity principle -- has tremendous implications for science and technology across the electromagnetic spectrum. Here, we investigate an emerging approach to break reciprocity that does not rely on magneto-optical effects or spacetime modulations, but is instead based on biasing a plasmonic material with a direct electric current. Using a 3D Green function formalism and microscopic considerations, we elucidate the propagation properties of surface plasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this type, revealing some previously overlooked, anomalous, wave-propagation effects. We show that SPPs can propagate in the form of steerable, slow-light, unidirectional beams associated with inflexion points in the modal dispersion. We also clarify the impact of dissipation (due to collisions and Landau damping) on nonreciprocal effects and shed light on the connections between inflexion points, exceptional points at band edges, and modal transitions in leaky-wave structures. We then apply these concepts to the important area of thermal photonics, and provide the first theoretical demonstration of drift-induced nonreciprocal radiative heat transfer between two planar bodies. Our findings may open new opportunities toward the development of nonreciprocal magnet-free devices that combine the benefits of plasmonics and nonreciprocal photonics for wave-guiding and energy applications.