Surface Green Function Method Research Articles

Quantum spin Hall effect in Ta2M3Te5 (M=Pd,Ni)

Quantum spin Hall (QSH) effect with great promise for the potential application in spintronics and quantum computing has attracted extensive research interest from both theoretical and experimental researchers. Here, we predict monolayer Ta$_2$Pd$_3$Te$_5$ can be a QSH insulator based on first-principles calculations. The interlayer binding energy in the layered van der Waals compound Ta$_2$Pd$_3$Te$_5$ is 19.6 meV/A$^2$; thus, its monolayer/thin-film structures could be readily obtained by exfoliation. The band inversion near the Fermi level ($E_F$) is an intrinsic characteristic, which happens between Ta-$5d$ and Pd-$4d$ orbitals without spin-orbit coupling (SOC). The SOC effect opens a global gap and makes the system a QSH insulator. With the $d$-$d$ band-inverted feature, the nontrivial topology in monolayer Ta$_2$Pd$_3$Te$_5$ is characterized by the time-reversal topological invariant $\mathbb Z_2=1$, which is computed by the one-dimensional (1D) Wilson loop method as implemented in our first-principles calculations. The helical edge modes are also obtained using surface Green's function method. Our calculations show that the QSH state in Ta$_2M_3$Te$_5$ ($M=$ Pd, Ni) can be tuned by external strain. These monolayers and thin films provide feasible platforms for realizing QSH effect as well as related devices.

First-principles Green's-function method for surface calculations: A pseudopotential localized basis set approach

We present an efficient implementation of a surface Green's-function method for atomistic modeling of surfaces within the framework of density functional theory using a pseudopotential localized basis set approach. In this method, the system is described as a truly semi-infinite solid with a surface region coupled to an electron reservoir, thereby overcoming several fundamental drawbacks of the traditional slab approach. The versatility of the method is demonstrated with several applications to surface physics and chemistry problems that are inherently difficult to address properly with the slab method, including metal work function calculations, band alignment in thin-film semiconductor heterostructures, surface states in metals and topological insulators, and surfaces in external electrical fields. Results obtained with the surface Green's-function method are compared to experimental measurements and slab calculations to demonstrate the accuracy of the approach.

Regular wave integral approach to the prediction of hydrodynamic performance of submerged spheroid

A free surface Green function method is employed in numerical simulations of hydrodynamic performance of a submerged spheroid in a fluid of infinite depth. The free surface Green function consists of the Rankine source potential and a singular wave integral. The singularity of the wave integral is removed with the use of the Havelock regular wave integral. The finite boundary element method is applied in the discretisation of the fluid motion problem so that the panel integral of the Rankine source potential is evaluated by the Hess–Smith formula and the panel integral of the regular wave integral is evaluated in a straightforward way due to the regularity nature. Present method’s results are in good agreement with earlier numerical results.

Conductance of metallic nanoribbons with defects

The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green's function method. The weak scattering impurities, U ∼ 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons (150 nm). For the narrow armchair graphene nanoribbons, the conductance at the Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U > 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.

A hybrid (finite difference-surface Green's function) method for computing transmission losses in an inhomogeneous atmosphere over irregular terrain

A standard parabolic equation (SPE) is used to approximate the Helmholtz equation for electromagnetic propagation in an inhomogeneous atmosphere. An implicit finite difference (IFD) scheme to solve the SPE is applied between the irregularly shaped ground and an altitude z=z/sub h/, below which all inhomogeneities of the medium are assumed localized. The boundary condition at z=z/sub h/ is obtained by matching the IFD solution to a surface Green's function (SGF) solution within the uniform region above z=z/sub h/. For ground slopes above about 1 degrees , the IFD implementation of the impedance boundary condition at the ground is shown to maintain the validity of the approximation only for vertically polarized waves. Predictions using this hybrid finite difference (FD)-SGF method agree well with results obtained using other computational methods. >

Surface Green function approach to the calculation of tunnelling currents in normal metal-superconductor junctions

A formalism which combines Gorkov's Green function description of the superconducting state and a surface Green function method is proposed to calculate tunnelling currents in normal metal-superconductor junctions. The method allows a straightforward calculation of the reflection and transmission matrices, among other physical magnitudes of interest. A simple example (at T=0), based upon elementary BCS theory, is carried out in detail. The results are in full agreement with previous analyses on this system, which used the matching of wavefunctions at the interface. The formalism is compared with other Green function approaches to the present problem.

Calculation of intravalley and intervalley transition probabilities in the course of electron scattering by a gallium arsenide surface

We employed the surface Green's function method in the discontinuous surface potential model to calculate the coefficients of reflection from (111) and (110) planes for electrons propagating in gallium arsenide. The complex band structure required for the calculation was obtained by the empirical pseudopotential method. The results make it possible to determine the intravalley and intervalley scattering probabilities as the electrons are reflected by the crystal surface. In particular, it is demonstrated that in the case of the (111) gallium arsenide surface γL and Lγ intervalley transitions are more probable than γγ and LL intravelley transitions.

Theory of chemisorption

We determine the electronic structure of a semiinfinite crystal with a general coverage by adatoms, if there exists a partial order described by the long-range order parameter5. The tight-binding formalism is used, the presence of the surface is included via the surface Green function method, and the effect of disorder is described by a version of the coherent potential approximation. The local densities of states at/near the surface as well as the interaction energy of adatoms are calculated as functions of the coverageθ, and the long-range order parameterS. The numerical examples modelling the cases of a gas adsorption on the transition metal surfaces as well as the transition metal atoms on the transition metal surfaces, are investigated.

Surface Green functions approach to planar defects and surfaces in copper: twin faults and (100) and (111) surfaces

A calculation of electronic states at the (100) and (111) surfaces and the twin fault of Cu is presented. The calculation is based upon the surface Green function method of Garcia-Moliner and Rubio (1971). In the study of surface states, overall charge neutrality is ensured by means of a displaced abrupt barrier. The application of the surface Green function method to numerical calculations is discussed.

Surface green function method in surface studies

The advantages of the surface Green function (SGF) method which gives a unified description of electrons at solid surfaces are summarized. A direct parametrization of the SGF is introduced, not requiring an explicit knowledge of the effective potential. This procedure is useful for approximate solution of both the direct and the inversion problem of LEED. Its use is demonstrated on a two-band model: general nonuniformity of erosion of Bragg peaks when inelastic processes are considered and a characteristic influence of resonant levels on the intensity profiles in LEED are observed.

Pseudopotential calculation of the surface band structure of Si(111) faces

The surface Green-function method is used to calculate the surface band structure of Si(111) faces as described by the conventional abrupt step model. The calculation yields a single narrow band located in the absolute thermal gap, in qualitative agreement with previous tight-binding calculations for this face, but differing from experimental data. An approximation suggested by the above method consists in neglecting the plane waves with wavevector lying outside the Jones zone. This approximation is studied analytically, including a careful description of the wavefunctions near the surface.

Electronic surface states in Ge

Using the surface Green function method for the same simplified model already described for Si (see Surface Sci., Vol.29, 540, 1972) the authors have been able to give a density of surface states for the (111) surface orientation of Ge, whose main features are in fair agreement with experimental results.