Analytical study of non-linear transport across a semiconductor-metal junction

In this paper we study analytically a one-dimensional model for a semiconductor-metal junction. We study the formation of Tamm states and how they evolve when the semi-infinite semiconductor and metal are coupled together. The non-linear current, as a function of the bias voltage, is studied using the non-equilibrium Green’s function method and the density matrix of the interface is given. The electronic occupation of the sites defining the interface has strong non-linearities as a function of the bias voltage due to strong resonances present in the Green’s functions of the junction sites. The surface Green’s function is computed analytically by solving a quadratic matrix equation, which does not require adding a small imaginary constant to the energy. The wave function for the surface states is given.     

The Alexander-Anderson problem for two atoms adsorbed on graphene

The Green’s functions for the Alexander-Anderson problem have been obtained using the previously proposed model density of states for graphene. Both the ferromagnetic and antiferromagnetic dimers have been considered. It has been shown that, in order to describe the density of states of the dimer adatom, the density of states of the isolated adatom with two positions of the gravity center of the quasi-level shifted in opposite directions can be used. It has been demonstrated that the approximate method of obtaining the Green’s function of the dimer proposed by us previously and consisting in that the Green’s function of the adatom rather than that of the atom is taken as the seed function gives the same result as the Alexander-Anderson approach. The dependences of the indirect interaction of dimer adatoms on the problem parameters have been evaluated in the limit of low energies.     

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.     

Calculation of the electroelastic Green’s function of the hexagonal infinite medium

The electroelastic 4 × 4 Green’s function of a piezoelectric hexagonal (transversely isotropic) infinitely extended medium is calculated explicitly in closed compact form ((73) ff. and (88) ff., respectively) by using residue calculation. The results can also be derived from Fredholm’s method [2]. In the case of vanishing piezoelectric coupling the derived Green’s function coincides with two well known results: Kröner’s expressions for the elastic Green’s function tensor [4] is reproduced and the electric part then coincides with the electric potential (solution of Poisson equation) which is caused by a unit point charge. The obtained electroelastic Green’s function is useful for the calculation of the electroelastic Eshelby tensor [16].     

Axial Green’s function method for steady Stokes flow in geometrically complex domains

Axial Green’s function method (AGM) is developed for the simulation of Stokes flow in geometrically complex solution domains. The AGM formulation systematically decomposes the multidimensional steady-state Stokes equations into 1D forms. The representation formula for the solution variables can then be derived using the 1D Green’s functions only, from which a system of 1D integral equations is obtained. Furthermore, the explicit representation formula for the pressure variable enable the unique AGM approach to facilitating the stabilization issue caused by the saddle structure between velocity and pressure. The convergence of numerical solutions, the simple axial discretization of complex solution domains, and the nature of integral schemes are demonstrated through a variety of numerical examples.     

A General Solution of the Problem of Elastic-Wave Scattering by a Plane Crack

Doklady Physics - The 3D problem of elastic-wave scattering by cracks in a homogeneous isotropic medium is considered. Using the method of Green’s function and specially introduced auxiliary...     

Optical conductivity in a 2D model of the pseudogap state

A 2D model of the pseudogap state is considered on the basis of the scenario of strong electron scattering by short-range-order fluctuations of the “dielectric” (antiferromagnetic or charge density wave) type. A system of recurrence relations is constructed for a one-particle Green’s function and the vertex part, describing the interaction of electrons with an external field. This system takes into account all Feynman diagrams for electron scattering at short-range-order fluctuations. The results of detailed calculations of optical conductivity are given for various geometries (topologies) of the Fermi surface, demonstrating both the effects of pseudogap formation in the electron spectrum and the localization effects. The obtained results are in qualitative agreement with experimental data for underdoped HTSC cuprates.     

On the spectral function of carriers in the pseudogap state

We consider the evolution of the spectral function of charge carriers for a 2D Kondo lattice depending on the parameters of the model. A self-consistent solution is obtained for the spectral function using the formalism of irreducible Green’s functions. In the low doping level regime, the behavior of the spectral function exhibits suppression of the spectral weight of carriers in the low-frequency range, which is typical of the pseudogap state.     

Computer Simulation of Spin Filtration Properties of Zigzag-Edged Octagraphene Nanoribbon Saturated with Hydrogen Atoms

Russian Physics Journal - The paper deals with the properties of octagraphene nanoribbon determined in terms of the local spin density approximation and nonequilibrium Green’s function,...     

Nonequilibrium Keldysh formalism for interacting leads—Application to quantum dot transport driven by spin bias

The conductance through a mesoscopic system of interacting electrons coupled to two adjacent leads is conventionally derived via the Keldysh nonequilibrium Green’s function technique, in the limit of noninteracting leads [Y. Meir, N.S. Wingreen, Phys. Rev. Lett. 68 (1992) 2512]. We extend the standard formalism to cater for a quantum dot system with Coulombic interactions between the quantum dot and the leads. The general current expression is obtained by considering the equation of motion of the time-ordered Green’s function of the system. The nonequilibrium effects of the interacting leads are then incorporated by determining the contour-ordered Green’s function over the Keldysh loop and applying Langreth’s theorem. The dot–lead interactions significantly increase the height of the Kondo peaks in density of states of the quantum dot. This translates into two Kondo peaks in the spin differential conductance when the magnitude of the spin bias equals that of the Zeeman splitting. There also exists a plateau in the charge differential conductance due to the combined effect of spin bias and the Zeeman splitting. The low-bias conductance plateau with sharp edges is also a characteristic of the Kondo effect. The conductance plateau disappears for the case of asymmetric dot–lead interaction.     

自旋为1的双层平方晶格阻挫模型的相变

采用双时格林函数方法研究了自旋为1的双层平方晶格阻挫模型的相变行为.详细探讨了层间耦合相互作用J_c和单离子各向异性参数D对奈尔态(AF1)和共线态(AF2)之间相转换的影响.结果显示:只要参数J_c和D不同时为零,奈尔态和共线态在J₂=J₁/2(这里J₁和J₂分别描述的是系统自旋间最近邻和次近邻交换作用)时的相变温度相等,两个态共存.在低于相变点的温度范围内,AF1-AF2态之间可以发生相转换,其相变类型为一阶相变.当J₂≠J₁/2时,尽管AF1-AF2态有不同相变温度,但它们也可以共存.如果AF1(AF2)态的相变温度大,在低温,AF1(AF2)态更稳定;在高温,AF2(AF1)态更稳定;在中间温度范围内,AF1-AF2态之间也可以发生一阶相转换.     

Temperature Effects on the Equation of State and Symmetry Energy at Low and High Densities

Thermal properties of symmetric nuclear matter and pure neutron matter are studied in a selfconsistent Green’s function and Brueckner–Hartree–Fock approaches with the inclusion of the contact interaction using CDBONN potential. Also we investigate the temperature dependence of the symmetry energy. The symmetry energy at fixed density is found to generally decrease with temperature. The temperature effects on the nuclear matter symmetry energy are found to be stronger at lower densities while become much weaker at higher densities. The results of several microscopic approaches are compared. Also the results are compared with recent experimental data. There is good agreement between the experimental symmetry energy and those calculated in the Brueckner–Hartree–Fock approach.     

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.     

Magnetic and electric phase transitions in the Hubbard model

Within the Hubbard model, two boson Green’s functions that describe the propagation of collective excitations of the electronic system—magnons (states with a single electron spin flip) and doublons (states with two electrons at one site of the crystal lattice)—are calculated for a Coulomb interaction of arbitrary strength and for an arbitrary electron concentration by applying a decoupling procedure to the double-time X-operator Green’s functions. It is found that the magnon and doublon Green’s functions are similar in structure and there is a close analogy between them. Instability of the paramagnetic phase with respect to spin ordering is investigated using the magnon Green’s function, and instability of the metallic phase to charge ordering is analyzed with the help of the doublon Green’s function. Criteria for the paramagnet-ferromagnet and metal-insulator phase transitions are found.     

Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas drag by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.     

Non-equilibrium approach to transport phenomena in quantum crystals

A pure dielectric quantum crystal subjected to an external mechanical force is described by non-equilibrium Green’s functions. In equilibrium the leading approximation leads to the definition of elementary excitations, the phonons in the renormalized harmonic approximation. Their temperature dependent energies are to be determined as solutions of an integral equation. For hydrodynamic disturbances a generalized transport equation for a phonon number density is derived. A similar approximation for the spectral function yields an integral equation for space and time dependent quasiparticle energies which are expressed as functionals of the displacement field and the phonon distribution. The Boltzmann equation for the latter includes the quasi-particle interaction.     

On the density of states for the Hubbard model: Pseudo-particle Keldysh diagram method—an alternative to DMFT?

It is shown how to construct Keldysh diagram technique for pseudo-particle approach to the Hubbard model. We propose self-consistent equations for pseudo particle and electron Green’s functions in Keldysh diagram technique. Nonlocal effects (spatial dispersion) are included in single impurity problem in this method. Thus we can get rid of the artificial central peak (of Kondo type) in the density of states which is inevitable in Dynamical Mean Field Theory (DMFT). The changes in the density of states for 2D Hubbard model due to variation of Coulomb repulsion U and electron concentration are analyzed.     

Optical Properties of Neutral and Charged Low Band Gap Alternating Copolyfluorenes: TD-DFT Investigation

用含时的密度泛函(TD-DFT)方法研究了低带隙的中性和带电的交替共聚芴(Green 1),该化合物是由烷基取代芴和(1,2,5-噻吩基-3,4-硫重氮基)喹喔啉噻吩(T-TDQ-T)单元交替重复组成,对他们的激发态特性用二维(2D)和三维(3D)实空间分析方法做了进一步分析．对于中性的Green 1,分别得到其带隙、键能、激子结合能和核驰豫能．用3D跃迁密度方法对中性和带电的Green 1的跃迁偶极矩进行比较可显示出跃迁偶极矩的取向和强度;用3D电荷差异密度方法显示出激发后的中性和带电的Green 1电荷重新分布和比较,用2D实空间分析方法(跃迁密度矩阵)来研究中性和带电的Green1处于激发态时的电子空穴相干性．中性Green 1的激发态特性分别用TD-DFT和ZINDO两种方法进行了计算,比较得出电子-电子相互作用(在TD-DFT中)对激发态性质的重要影响．     

Terahertz laser-induced 1D–0D crossover in the density of states for electrons in a cylindrical semiconductor quantum wire

The effects of a linearly polarized, terahertz laser field on the density of states (DOS) for carriers confined in a cylindrical semiconductor quantum wire are investigated here within a nonperturbative scheme, based upon a Green’s function approach. In our model, the functional dependence of the DOS on energy is significantly modified in two ways: (i) an uniform blueshift with respect to the laser-free DOS; and (ii) a strong suppression, with the appearance of Franz–Keldysh-like oscillations. Interestingly, with the increase (decrease) of the laser intensity (frequency) the DOS profile evolves from the usual shape for one-dimensional (1D) systems to a discrete set of sharp peaks that resembles the DOS for quasi-zero-dimensional (quasi-0D) electrons confined in quantum dots.