Green Function Formalism Research Articles

Nonequilibrium green function technique for analyzing electron transport through single and two levels of interacting quantum dot

The electron transport through nanoelectronic device, fabricated by coupling a QD into two metallic electrodes, is analyzed theoretically using Non-equilibrium Green function formalism. The considered QD has one quantum level with one-site coulomb interaction (two-channel for transportation). The spectra function of this QD is obtained by using Feynman diagram technique for single-level model and the equation of motion method for two-level model. Analytical approximations of electric current through this device are presented at low temperature and at finite temperature. Depending on the analytical approaches, we will analyze the dependency of nano-device current on the QD size, level width, and temperature. Finally, the dependency of current on the applied voltages is analyzed with respect to coulomb-blocked diamonds. The current-applied voltages characteristics help us to understand the behavior of nano-devices that has two channels of transport. We will show where and why the two-channel nano-device shows two and one current steps depending on the chosen value of gate voltage.

Strain-induced switching in field effect transistor based on zigzag graphene nanoribbons

In this paper, we investigate the effects of uniaxial and oblique strain on Non-linear transport of zigzag graphene nanoribbons using non-equilibrium Green's function formalism. The current-voltage characteristics and especially negative differential resistance (NDR) are affected by the type, strength, and direction of the applied strain. For the tensile strain along the ribbon axis, the on-off current ratio increases, while for the compressive strain, NDR gradually disappears. In the oblique strain, the on-off current ratio drastically decreases compared to the un-strained case. In context, we demonstrate how strain can affect band structure and transition rules as well as transmission coefficients. Also in our calculations, we consider Coulomb interaction as long-range electrostatic potential at mean-field approximation. Coulomb interaction decreases off current and leads to the increased on-off current ratio in the oblique strain. Here, we observe under the influence of the oblique strain, the geometrical symmetry is broken, so the symmetry of charge profile and local currents are broken across ZGNR. We show an imbalanced accumulated charge located on the nanoribbon edges that as a consequence, appear some transverse local currents between the upper and lower edges. These transverse local currents are increased by the strain strength. The minimum (maximum) imbalanced charge accumulated on the ribbon edges occurs exactly at the on (off)-state of longitudinal bias where the transverse current is decreasing (increasing). Finally, we strongly suggest the strain can operate as a nanoelectromechanical switch.

Transport and spectral properties of magic-angle twisted bilayer graphene junctions based on local orbital models

The electronic properties of junctions defined electrostatically on twisted bilayer graphene can be addressed theoretically using lattice models. Recent works have introduced minimal local orbital models to describe twisted bilayer graphene at the magic angle (MATBLG) with different degrees of approximation and accounting for fragile topology. In the present work we use a Green's function formalism to obtain the spectral and transport properties for MATBLG junctions based on these models. We introduce different symmetry breaking perturbations to simulate the effect of interactions and characterize the topology of the bulk bands by analyzing the corresponding Wilson loops. We then analyze the spectral properties for different types of edges and in the case of a domain wall in the sublattice symmetry breaking parameter. We further consider a three region junction where one could control independently the central and lateral regions doping level. In the limit where the central region is fixed at the charge neutrality point and the lateral ones are heavily doped, the spectral and the two terminal transport properties can be understood in terms of the hybridization of chiral states along the junctions. These properties are found to be extremely sensitive to the orientation of the junctions along the moir\'e lattice.

Magneto-Conductance of Ferromagnetic Metal/Transition Metal Dichalcogenide Bilayers in the Presence of Spin–Orbit Proximity

Magneto-conductance in magnetic bilayers of the form ferromagnetic metal (FM)/monolayer transition metal dichalcogenide (TMD) is investigated in the linear response regime. It is assumed that spin-orbit proximity (SOP) effect in FM monolayers is created by the adjacent monolayer TMD. The transmission function is calculated from the Landauer formula in terms of non-equilibrium Green's function formalism. It is presented here how the magneto-conductance (magneto-resistance) can be tuned in the presence of SOP for FM/TMD bilayers.

Real-space Green's function approach for x-ray spectra at high temperature

There has been considerable interest in properties of condensed matter at finite temperature, including non-equilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time resolved x-ray absorption spectroscopy (XAS) in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of finite-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. The method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

Carrier-Envelope-Phase Modulated Currents in Scanning Tunneling Microscopy.

Carrier-envelope-phase (CEP) stable optical pulses combined with state-of-the-art scanning tunneling microscopy (STM) can track and control ultrafast electronic tunneling currents. On the basis of nonequilibrium Green's function formalism, we present a time and frequency domain theoretical study of CEP-stable pulse-induced tunneling currents between an STM tip and a metal substrate. It is revealed that the experimentally observed phase shift between the maximum tunneling current and maximum electric field is caused by the third-order response to the electric field. The shift is also found to be sensitive to the duration of pulses. The tunneling process can thus be precisely manipulated by varying the phase and duration of these pulses.

Effect of a ferromagnetic STM cobalt tip on a single Co-phthalocyanine molecule adsorbed on a ferromagnetic substrate

The effect of a ferromagnetic scanning tunneling microscope (STM) cobalt tip on the electronic, magnetic and electronic transport properties of a Co-phthalocyanine (CoPc)/Co(111) junction has been investigated in the framework of density functional theory in conjunction with Landauer transport and the non-equilibrium Green function formalism. It is shown that the spin magnetic moment of the CoPc molecule can be flipped by varying the distance between the STM tip and the CoPc molecule when passing from the tunneling regime to the contact regime. Our calculations show that such spin flip of the CoPc molecule leads to a change of the sign of the Tunneling Magneto-Resistance ratio (TMR). The change from the tunneling to contact regime also leads to large changes in the total and spin-polarized I–V characteristics.

From nonequilibrium Green's functions to quantum master equations for the density matrix and out-of-time-order correlators: Steady-state and adiabatic dynamics

We consider a finite quantum system under slow driving and weakly coupled to thermal reservoirs at different temperatures. We present a systematic derivation of the quantum master equation for the density matrix and the out-of-time-order correlators. We start from the microscopic Hamiltonian and we formulate the equations ruling the dynamics of these quantities by recourse to the Schwinger-Keldysh non-equilibrium Green's function formalism, performing a perturbative expansion in the coupling between the system and the reservoirs. We focus on the adiabatic dynamics, which corresponds to considering the linear response in the ratio between the relaxation time due to the system-reservoir coupling and the time scale associated to the driving. We calculate the particle and energy fluxes. We illustrate the formalism in the case of a qutrit coupled to bosonic reservoirs and of a pair of interacting quantum dots attached to fermionic reservoirs, also discussing the relevance of coherent effects.

Long-range correlations in pinned athermal networks

We derive exact results for displacement fields that develop as a response to external pinning forces in two-dimensional athermal networks. For a triangular lattice arrangement of particles interacting through soft potentials, we develop a Green's function formalism which we use to derive exact results for displacement fields produced by localized external forces. We show that in the continuum limit the displacement fields decay as 1/r at large distances r away from a force dipole. Finally, we extend our formulation to study correlations in the displacement fields produced by the external pinning forces. We show that uncorrelated pinned forces at each vertex give rise to long-range correlations in displacements in athermal systems, with a nontrivial system size dependence. We verify our predictions with numerical simulations of athermal networks in two dimensions.

Electronic characteristics of BRCA1 mutations in DNA.

The efficient and low-cost way for gene mutation detection and identification are conducive for the detection of disease. Here, we report the electronic characteristics of the gene of breast cancer 1 in four common mutation types: duplication, single nucleotide variant, deletion, and indel. The electronic characteristics are investigated by the combination of density functional theory and non-equilibrium Green's function formulation with decoherence. The magnitude of conductance of these DNA molecules and mutational changes are found to be detectable experimentally. In this study, we also find the significant mutation type dependent on the change of conductance. Hence these mutations are expected to be identifiable. We find deletion type mutation shows the largest change in relative conductance (~97%), whereas the indel mutation shows the smallest change in relative conductance (~27%). Therefore, this work presents a possibility of electronic detection and identification of mutations in DNA, which could be an efficient method as compared to the conventional methods.

Surface states and related quantum interference in ab initio electron transport

Shockley surface states (SS) have attracted much attention due to their role in various physical phenomena occurring at surfaces. It is also clear from experiments that they can play an important role in electron transport. However, accurate incorporation of surface states in $\textit{ab initio}$ quantum transport simulations remains still an unresolved problem. Here we go beyond the state-of-the-art non-equilibrium Green's function formalism through the evaluation of the self-energy in real-space, enabling electron transport without using artificial periodic in-plane conditions. We demonstrate the method on three representative examples based on Au(111): a clean surface, a metallic nanocontact, and a single-molecule junction. We show that SS can contribute more than 30\% of the electron transport near the Fermi energy. A significant and robust transmission drop is observed at the SS band edge due to quantum interference in both metallic and molecular junctions, in good agreement with experimental measurements. The origin of this interference phenomenon is attributed to the coupling between bulk and SS transport channels and it is reproduced and understood by tight-binding model. Furthermore, our method predicts much better quantized conductance for metallic nanocontacts.

Influence of pseudo-gap and interlayer coupling on isotope effect in bilayer cuprate superconductors

The present investigations deal with the study of the isotope effect in bilayer cuprate superconductors. We have considered the BCS-effective Hamiltonian that includes the intra and inter-layer kinetic energies and in plane attractive interaction responsible for pairing in these systems. In order to look for pseudo-gap having momentum dependence of the form of d-wave superconducting order parameter, a pseudo-gap order parameter is also introduced in the in-plane quasi-particle energy. Such a model calculation can be applicable for the high Tc cuprates like Bi2Sr2CaCu2O8+x having two CuO2 planes per unit cell and also for several other cuprate systems with a possibility to extend for the multi-layer cuprates. Employing Green's function formalism, we obtain the expressions for the superconducting order parameter and hence the isotope effect coefficient as a function of interlayer coupling, transition temperature and pseudo-gap order parameter. On the basis of numerical computation, we have pointed out that isotope effect increases due to coupling between the planes and decreases with the transition temperature. The presence of phenomenological pseudo-gap parameter in the quasi-particle spectrum enhances the isotope effect. These results are viewed in terms of recent experimental works on isotope effect in layered cuprate superconductors.

Anharmonic phonon-phonon scattering at the interface between two solids by nonequilibrium Green's function formalism

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via non-equilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and non-local anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.

Investigation of transport in edge passivated armchair silicene nanoribbon field effect transistor by ab-initio based Wannierised tight binding

In this work, comparative analysis of hydrogen(H), hydroxyl (OH), fluorine (F) and chlorine (Cl) edge passivated silicene nanoribbon (SiNR) field effect transistors (FETs) having nanoribbon width of 7 dimers along the armchair direction has been carried out. The ab-initio tight binding simulations of SiNR with different edge passivation uses the multi-scale approach which consists of density functional theory , Wannier function based tight binding and the non-equilibrium Green's Function formalism. It has been found that, the band gap of hydrogen passivated SiNR is comparatively larger than the other edge passivated SiNRs. To choose the optimum number of Wannier functions for tight binding approximations, the contribution of orbitals has also been analyzed. It has been found that, in transport characteristics of X-edge passivated SiNR FETs (X is H, F, Cl and OH), the H-edge passivated SiNR FET shows improved transfer characteristics in comparison to OH, Cl and F–edge passivated SiNR FETs. • Performance analysis of various edge passivated Silicene armchair nanoribbon. • Bandgap of H-passivated SiNR is higher than OH, Cl and F–edge passivated SiNR. • Use of projected density of states to choose the optimum number of Wannier function. • Higher I ON / I OFF in case of H-passivated SiNR FET in comparison other–edge passivated SiNR FET. • Performance of H-passivated SiNR FET is superior among other–edge passivated SiNR FET.

Role of carbon nanotubes as an acceptor to enhance the photovoltaic performances of organic solar cells based on π ‐conjugated thiophene as a donor materials

Filled semiconducting single-walled carbon nanotubes (SWNTs), with π-conjugated polymer as a light harvester and charge transporter in the active layer, could play a key role in the development of more qualified organic solar cells (OSCs) in the sense of high energy conversion efficiency and long-term stability. In this paper,we used three computational approaches to investigate a series of oligothiophene (nT) (n = 2, 4 or 6) encapsulated inside SWNTs. The first approach is based on a combination of the density functional theory (DFT), molecular mechanics, bond polarizability model and the spectral moment's method (SMM) to compute the nonresonant Raman spectra of the encapsulated systems. We reported the optimal tube diameter allowing the nT encapsulation. The influence of the encapsulation on the radial breathing mode and G-band modes of the selected semiconducting SWNTs zigzag (11,0), labeled (NT11), is testified to the presence of the charge transfer (CT) in the nT@NT11 hybrid systems without specifying their direction. The second approach is based on the electronic and optical properties with DFT at the generalized gradient approximation. The CT and its direction in nT@NT11 hybrid systems is identified by electronic and optical calculations. The third approach is based on electronic transport properties with DFT in combination to nonequilibrium Green's function formalism. We have investigated the transmission spectra and current-voltage characteristics of nT@NT11 hybrid. It was observed that a strong correlation between transmission spectra and DOS near the Fermi level due to delocalization of electronic states, and the I-V characteristic exhibits interesting electronic transport properties. The results illustrate that the filled semiconducting SWNTs exhibiting type II heterojunctions are expected to be a good candidate as a light harvester and charge transporter in the active layer, which can contribute to developing highly efficient filled SWNT-based OSCs.

Thermal rectification through a nonlinear quantum resonator

We present a comprehensive and systematic study of thermal rectification in a prototypical low-dimensional quantum system -- a non-linear resonator: we identify necessary conditions to observe thermal rectification and we discuss strategies to maximize it. We focus, in particular, on the case where anharmonicity is very strong and the system reduces to a qubit. In the latter case, we derive general upper bounds on rectification which hold in the weak system-bath coupling regime, and we show how the Lamb shift can be exploited to enhance rectification. We then go beyond the weak-coupling regime by employing different methods: i) including co-tunneling processes, ii) using the non-equilibrium Green's function formalism and iii) using the Feynman-Vernon path integral approach. We find that the strong coupling regime allows us to violate the bounds derived in the weak-coupling regime, providing us with clear signatures of high order coherent processes visible in the thermal rectification. In the general case, where many levels participate to the system dynamics, we compare the heat rectification calculated with the equation of motion method and with a mean-field approximation. We find that the former method predicts, for a small or intermediate anharmonicity, a larger rectification coefficient.

Non-Hermitian Skin Effects in Hermitian Correlated or Disordered Systems: Quantities Sensitive or Insensitive to Boundary Effects and Pseudo-Quantum-Number

There is a common belief in the condensed matter community that bulk quantities become insensitive to the boundary condition in the infinite-volume limit. Here we reconsider this statement in terms of recent arguments of non-Hermitian skin effects-strong dependence of spectra on boundary conditions for the non-Hermitian Hamiltonians-in the traditional Green's function formalism. We find the criterion for quantities to be sensitive or insensitive against the boundary condition in Hermitian correlated or disordered systems, which is characterized by the residue theorem. We also discuss the uncertainty of the quasiparticle energy under the skin effects in terms of non-normal pseudospectra, which can be tested via the sharp optical absorption from the bulk-surface coupling. Our result indicates that pseudo quantum number emerges as a consequence of large nonnormality.

Nanoscale functionalized superconducting transport channels as photon detectors

Single-photon detectors have typically consisted of macroscopic materials where both the photon absorption and transduction to an electrical signal happen. Newly proposed designs suggest that large arrays of nanoscale detectors could provide improved performance in addition to decoupling the absorption and transduction processes. Here we study the properties of such a detector consisting of a nanoscale superconducting (SC) transport channel functionalized by a photon absorber. We explore two detection mechanisms based on photo-induced electrostatic gating and magnetic effects. To this end we model the narrow channel as a one-dimensional atomic chain and use a self-consistent Keldysh-Nambu Green's function formalism to describe non-equilibrium effects and SC phenomena. We consider cases where the photon creates electrostatic and magnetic changes in the absorber, as well as devices with strong and weak coupling to the metal leads. Our results indicate that the most promising case is when the SC channel is weakly coupled to the leads and in the presence of a background magnetic field, where photo-excitation of a magnetic molecule can trigger a SC-to-normal transition in the channel that leads to a change in the device current several times larger than in the case of a normal-phase channel device.

Photoinduced phase transition and associated timescales in the excitonic insulator Ta2NiSe5

We investigate the non-equilibrium electronic structure and characteristic time scales in a candidate excitonic insulator, Ta$_2$NiSe$_5$, using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a time scale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity above the Fermi energy decreases with increasing excitation strength, while the relaxation time of the electron population towards equilibrium shows an opposite behaviour. We argue that these experimental observations can be consistently explained by an excitonic origin of the band gap in the material. The excitonic picture is supported by microscopic calculations based on the non-equilibrium Green's function formalism for an interacting two-band system. We interpret the speedup of the rise time with fluence in terms of an enhanced scattering probability between photo-excited electrons and excitons, leading to an initially faster decay of the order parameter. We show that the inclusion of electron-phonon coupling at a semi-classical level changes only the quantitative aspects of the proposed dynamics, while the qualitative features remain the same. The experimental observations and microscopic calculations allow us to develop a simple and intuitive phenomenological model that captures the main dynamics after photoexcitation in Ta$_2$NiSe$_5$.

Spinor Boltzmann equation approach to domain-wall motion driven by spin-polarized current

Based on the nonequilibrium Green's function formalism, the spinor Boltzmann equation beyond gradient approximation is derived in a ferromagnetic metal with a single domain wall (DW). We further obtain the charge continuity equation and the spin diffusion equation by integrating over the momentum. By using the spin diffusion equation, we get a generalized spin transfer torque (STT), in which the usual STT is extended to the case beyond the gradient approximation and with inhomogeneous current. We also calculate numerically the physical observables such as charge density $n(x)$, spin accumulation $\mathbit{m}(x)$, current density $j(x)$, spin current density ${\mathbit{j}}_{s}(x)$, etc., by the use of the spinor distribution function. Along with the Landau-Lifshitz-Gilbert-Slonczewski equation that contains the above generalized STT, we can study the motion domain wall, and the critical electric field at an initial velocity of the DW is obtained in terms of the linear stability analysis method.