First-principles modelling of scanning tunneling microscopy using non-equilibrium Green’s functions

The investigation of electron transport processes in nano-scale architectures plays a crucial role in the development of surface chemistry and nano-technology. Experimentally, an important driving force within this research area has been the concurrent refinements of scanning tunneling microscopy (STM) techniques. The theoretical treatment of the STM operation has traditionally been based on the Bardeen and Tersoff–Hamann methods which take as input the single-particle wave functions and eigenvalues obtained from finite cluster or slabs models of the surface-tip interface. Here, we present a novel STM simulation scheme based on non-equilibrium Green’s functions (NEGF) and Wannier functions which is both accurate and very efficient. The main novelty of the scheme compared to the Bardeen and Tersoff–Hamann approaches is that the coupling to the infinite (macroscopic) electrodes is taken into account. As an illustrating example we apply the NEGF-STM method to the Si(001)-(2×1):H surface with sub-surface P doping and discuss the results in comparison to the Bardeen and Tersoff–Hamann methods.     

Design and simulation of double-lightly doped MOSCNT using non-equilibrium Green’s function

In this paper, we propose a new ohmic-structure of ballistic carbon nanotube field-effect transistors (CNTFETs) in which the source and drain regions are doped stepwise and the device acts as MOSFET like CNTFET (MOSCNT). The number of lightly doped regions and their doping concentrations are optimized to obtain the lowest OFF current. To study the device characteristics, the Poisson–Schrödinger equations are solved self-consistently using the Nonequilibrium Green’s Function (NEGF) formalism in the mode space approach. To find the Hamiltonian matrix, the tight-binding approximation with only p _z orbital is used. The obtained results show that the stepwise regions lead to barrier widening due to the reduction in potential gradient. Therefore, the band-to-band tunneling (BTBT) and the ambipolar behavior of the device decrease due to band engineering. This causes to the superior reduction of OFF current and dissipative power. In addition, the device performance shows lower subthreshold swing (SS), smaller drain induced barrier lowering (DIBL), and larger current ratio than that of the previous structures.     

Studies of imaging experiment for photon scanning tunneling microscopy

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Mode-detailed analysis of transmission based directly on Green’s functions

Fisher-Lee relation\hbox{$\bm{t}= {i}\bm{\Gamma}_L^{1/2}\bm{G}\bm{\Gamma}^{1/2}_R$}t=iΓL1/2GΓR1/2is a well-established tool to decode the modeinformation from Green’s function and coupling parameters. Using the Bloch eigen-modes ofthe leads, we show that the\hbox{$\bm{\Gamma}^{1/2}_{L/R}$}ΓL/R1/2term can be expressed by the Bloch eigen-mode vectorsand the wave velocities which give unambiguous algorithm of\hbox{$\bm{\Gamma}^{1/2}_{L/R}$}ΓL/R1/2in the Fish-Lee relation. Using this approach, wepresent an accurate and convenient technique to analyze all transport modes and also thedominant channels of an electronic transport system in tight-binding model. We studygraphene nanoribbon structures to demonstrate the typical application of our technique.     

Green’s functions and strength functions for stationary quantum systems

By using the analytic properties of the retarded Green’s function for a stationary quantum system, the strength function that coincides with the energy distribution of an unperturbed state of the system over its exact states in a perturbing field is constructed. It is shown that, in general, this strength function has the form of a Breit-Wigner distribution with energy-dependent parameters and that its moments are determined by the expectation values of various powers of the exact Hamiltonian for the wave function of the unperturbed state. The strength function averaged over a certain energy interval is calculated, and its properties are investigated for a global regime of averaging. The resulting strength functions are used to determine the mean field and the optical potential for nucleons in nuclei and to investigate conditions under which quantum chaos emerges in various systems.     

Coherently controllable non-equilibrium charge accumulation inside a magnetic quantum dot: a non-equilibrium Green’s function approach

We have performed a non-equilibrium quantum transport calculations for a two-terminal mesoscopic system including a magnetic quantum dot. Using the non-equilibrium Green’s function technique, we have obtained electric current and charge distribution in the temperature range from 1 to 10 K as a function of magnetic field. Results indicate that the density of carriers essentially can be controlled by temperature and bias voltage.     

Gauge invariant quark Green’s functions with polygonal Wilson lines

Properties of gauge invariant two-point quark Green’s functions, defined with polygonal Wilson lines, are studied. The Green’s functions can be classified according to the number of straight line segments their polygonal lines contain. Functional relations are established between the Green’s functions with different numbers of segments on the polygonal lines. An integrodifferential equation is obtained for the Green’s function with one straight line segment, in which the kernels are represented by a series of Wilson loop vacuum averages along polygonal contours with an increasing number of segments and functional derivatives on them. The equation is exactly solved in the case of two-dimensional QCD in the large-N _c limit. The spectral properties of the Green’s function are displayed.     

Atomic resolution scanning tunneling microscopy imaging of Pt electrodes interfaced with β″-Al2O3

Solid electrolytes can be used as active catalyst supports to induce significant and reversible catalytic activity and selectivity enhancement via the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion which has been recently reported for over fourty catalytic reactions. Atomically resolved Scanning Tunneling Microscopy was used to image the reversible electrochemically controlled dosing (backspillover) of sodium on Pt(111) interfaced to β″-Al₂O₃ at atmospheric pressure, which has been proposed as the cause of the NEMCA effect in the case of Na⁺ conductors. It was found that electrical current application between the Pt(111) monocrystal and a counter electrode also in contact with the β″-Al₂O₃ Na⁺-conducting solid electrolyte causes reversible migration (backspillover and spillover) of sodium which forms a (12×12) hexagonal structure on the Pt(111) surface. In addition to explaining the phenomenon of Electrochemical Promotion in Heterogeneous Catalysis when using Na-β″-Al₂O₃ solid electrolyte these observations provide the first STM confirmation that:

1. spillover-backspillover phenomena can take place over enormous (~mm) atomic distances, and
2. promoters can form ordered structures on catalyst surfaces under ambient conditions relevant to industrial practice.

     

Green’s function calculations of light nuclei

The influence of short-range correlations in nuclei was investigated with realistic nuclear force. The nucleon-nucleon interaction was renormalized with V_lowk technique and applied to the Green’s function calculations. The Dyson equation was reformulated with algebraic diagrammatic constructions. We also analyzed the binding energy of ⁴He, calculated with chiral potential and CD-Bonn potential. The properties of Green’s function with realistic nuclear forces are also discussed.     

Stable and efficient evaluation of periodized Green’s functions for the Helmholtz equation at high frequencies

A difficulty that arises in the context of infinite d-periodic rough-surface scattering relates to the effective numerical evaluation of the corresponding “quasi-periodic Green function” G_qp. Due to its relevance in a variety of applications, this problem has generated significant interest over the last 40 years, and a variety of numerical methods have been devised for this purpose. None of these methods to evaluate G_qp however, were designed for high-frequency calculations. As a result, in this regime, these methods become prohibitively expensive and/or unstable. Here we present a novel scheme that can be shown to outperform every alternative numerical evaluation procedure and is especially effective for high-frequency calculations. Our new algorithm is based on the use of some exact integrals that arise on judicious manipulation of the integral representation of G_qp and which reduce the overall problem to that of evaluation of a sequence of simpler integrals that can be effectively handled by standard quadrature formulas. We include a variety of numerical results that confirm that, indeed, our algorithm compares favorably with alternative methods.     

Chemical microanalysis of rare-earth–transition metal–boron alloys and magnets using scanning electron microscopy

This paper reports the results of investigations of chemical microanalyses of various alloys and sintered magnets with well-known compositions. Analyses of neodymium, praseodymium, iron, cobalt, niobium and copper have been carried out using energy-dispersive X-ray spectrometry. Quantitative analyses of boron have been carried out using wavelength-dispersive X-ray spectrometry. The resulting mean values were compared with data reported in the literature for the various phases present in these magnetic materials.     

Side-branch resonators modelling with Green׳s function methods

This paper deals with strategies for computing efficiently the propagation of sound waves in ducts containing passive components. In many cases of practical interest, these components are acoustic cavities which are connected to the duct. Though standard Finite Element software could be used for the numerical prediction of sound transmission through such a system, the method is known to be extremely demanding, both in terms of data preparation and computation, especially in the mid-frequency range. To alleviate this, a numerical technique that exploits the benefit of the FEM and the BEM approach has been devised. First, a set of eigenmodes is computed in the cavity to produce a numerical impedance matrix connecting the pressure and the acoustic velocity on the duct wall interface. Then an integral representation for the acoustic pressure in the main duct is used. By choosing an appropriate Green?s function for the duct, the integration procedure is limited to the duct–cavity interface only. This allows an accurate computation of the scattering matrix of such an acoustic system with a numerical complexity that grows very mildly with the frequency. Typical applications involving Helmholtz and Herschel–Quincke resonators are presented.     

Dyadic Green’s function of a PEMC cylinder

The dyadic Green’s function of a PEMC cylinder is derived with the aid of the principle of scattering superposition and Ohm–Rayleigh method. The PEMC boundary conditions are presented in dyadic form and it shows that how the impedance parameter of PEMC and cross-polarized fields appear in the Green’s function. The asymptotic expansions of the dyadic function is calculated in order to attain a closed form for the electrical field.     

A new model for deformed carbon nanotubes using Green’s function

A new method for modeling and analysis of deformed carbon nanotubes (CNTs) using Green’s function, is presented in this paper, for the first time. Using the proposed method, a new circuit model is obtained for the deformation region of a deformed single-walled CNT (SWCNT), which the values of its elements depend on the type of deformation and also the deformation parameters such as the coupling matrices and the energy variations of deformation region. The comparison between the obtained results from the analysis of proposed model and the literature gives a good match which approves the accuracy and correctness of the proposed model.     

A scanning tunneling microscopy investigation of the 1 × 2 ⇌ 1 × 1 structural transformation of the pt(110) surface

Lifting of the reconstruction of the clean Pt(110) surface under the influence of adsorbed CO proceeds at 300 K through homogeneous nucleation of small holes (with about 10 × 10 Ų size). At 350 K more correlated displacements of [110] strings take place, a characteristic shared with the reverse process, namely the restoration of the 1× 2 phase after desorption.     

Thermo-elastic Greenʼs functions for an infinite bi-material of one-dimensional hexagonal quasi-crystals

This Letter is concerned with thermo-elastic fundamental solutions of an infinite space, which is composed of two half-infinite bodies of different one-dimensional hexagonal quasi-crystals. A point thermal source is embedded in a half-space. The interface can be either perfectly bonded or smoothly contacted. On the basis of the newly developed general solution, the temperature-induced elastic field in full space is explicitly presented in terms of elementary functions. The interactions among the temperature, phonon and phason fields are revealed. The present work can play an important role in constructing farther analytical solutions for crack, inclusion and dislocation problems.     

Photon Green’s functions for a consistent theory of absorption and emission in nanostructure-based solar cell devices

The light-matter interaction in planar nanostructures with applications in photovoltaic devices is investigated by means of a microscopic quantum-kinetic theory based on the non-equilibrium Green’s function formalism. The Dyson and Keldysh equations for the Green’s functions of photons are solved numerically. The result is used to couple the optical and electronic degrees of freedom via respective self-energies. The numerical approach for the solution of the optical problem is verified against a standard transfer-matrix formalism and applied to the computation of absorption and emission characteristics in ultra-thin-absorber solar cells.