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We demonstrate the potential of recently developed total-energy and electronic-structure methods for the calculation of the optical properties of real surfaces. The many-body effects are fully taken into account by a solution of the combined Dyson and Bethe-Salpeter equations. We show that an initial-value formulation of the polarization function allows an efficient numerical calculation of the optical susceptibility for large slabs consisting of many atoms. As examples we investigate GaP(001) and Si(001) surfaces covered by hydrogen. In the case of P-rich GaP(001)2 x 2-H surfaces the low-energy region of the reflectance anisotropy (RA) is dominated by electron-hole pair excitations in surface states. Surface-induced modifications of bulk excitons near the E-1 and E-2 transitions are responsible for the RA of the monohydride Si(001)2 x 1-H surface. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

To investigate the early stages of SiC growth on silicon, we performed an ab initio study of the adsorption of C2H2 and other small organic molecules on different Si surfaces. Our calculations, based both on geometry optimization and on finite-temperature molecular dynamics simulations, show that for all the molecules that we have considered the preferred adsorption sites at low temperature are confined at the surface, with no sub-surface penetration. Adsorption occurs through the formation of Si–C bonds, accompanied by a distortion of the adsorbed molecule to adapt the Si–C distance to the SiC bulk bond length. We discuss similarities and differences upon changing the organic molecule and the crystal face. To complete the study with the computation of directly measurable quantities, we analyze the optical reflectance anisotropy of one simulated structure.

We present the results of a first-principles study of the adsorption of acetylene on Si(100). In particular, we focus on a recent controversial issue concerning the occurrence of the ``end-bridge'' configuration in which an acetylene molecule bridges two adjacent surface Si dimers. At variance with recent theoretical calculations, we find that the lowest-energy ``end-bridge'' structure is stable and nonmetallic both at 0.5 ML and at 1.0 ML coverage. This leads to the conclusion that this configuration can be one of those observed in experiments. The results of first-principles calculations of the optical properties of different configurations of acetylene on Si(100) are also presented and discussed.

While the As-rich 2 × 4 reconstruction of GaAs(001) is well explained by the so-called β2 structure, the atomic structure of the Ga-rich 4 × 2 phase has been discussed for a long time. In this review, the most important structural models for the GaAs(001) (4 × 2)/c(8 × 2) surface are compared from different theoretical and experimental points of view. The selected reconstructions include the recently proposed ζ model, a new mixed dimer model, and the well-known β, β2, Cerdà and Skala models. The different structures are compared on the basis of total energy calculations, simulations of STM experimental images and interpretation of X-ray diffraction data. Only the ζ model satisfies all criteria, and provides therefore a satisfactory explanation of the atomic structure of GaAs(001)-(4 × 2).

We compute the optical conductivity of 2D honeycomb crystals beyond the usual Dirac-cone approximation. The calculations are mainly based on the independent-quasiparticle approximation of the complex dielectric function for optical interband transitions. The full band structures are taken into account. In the case of silicene, the influence of excitonic effects is also studied. Special care is taken to derive converged spectra with respect to the number of k points in the Brillouin zone and the number of bands. In this way both the real and imaginary parts of the optical conductivity are correctly described for small and large frequencies. The results are applied to predict the optical properties reflection, transmission and absorption in a wide range of photon energies. They are discussed in the light of the available experimental data.

The discoloration of ancient paper, due to the development of oxidized groups acting as chromophores in its chief component, cellulose, is responsible for severe visual degradation in ancient artifacts. By adopting a non-destructive approach based on the combination of optical reflectance measurements and time-dependent density functional theory ab-initio calculations, we describe and quantify the chromophores affecting Leonardo da Vinci's iconic self-portrait. Their relative concentrations are very similar to those measured in modern and ancient samples aged in humid environments. This analysis quantifies the present level of optical degradation of the Leonardo da Vinci's self-portrait which, compared with future measurements, will assess its degradation rate. This is a fundamental information in order to plan appropriate conservation strategies.

We present an ab-initio study of GaP bulk as a test case to show the importance of going beyond the single particle Density Functional Theory (DFT) scheme in the calculation of excited state properties. The inclusion of many-body effects within the GW approach for the determination of electronic gaps, and of excitonic effects in the calculations of the dielectric function, is discussed. Comparison with one-particle DFT results and with experimental data are presented.

The reflectance anisotropy of the As/InP(110) surface is calculated by using an ab initio plane-wave pseudopotential method. We analyze different models of As coverage, ranging from non-reacted epitaxial layers to exchange-reacted geometries. Comparison with experimental data confirms that the annealed, highly ordered surface phase can be described by an InAs monolayer on the InP substrate (exchange reacted model), whereas the reflectance anisotropy of the as-grown, poorly ordered As/InP surface probably is dominated by disorder effects.

We modify the analytic linear tetrahedron method to calculate the optical properties of surfaces. As st test case, the reflectance anisotropy of InP(110) is calculated within the density-functional theory in the local-density approximation. The convergence with respect to the number of k points and the choice of the triangles is extensively discussed. The resulting spectra are interpreted and compared with experimental results.

We compute the optical properties of the (110) surface of gallium arsenide within the first-principles density-functional theory local-density approximation scheme, using norm-conserving pseudopotentials. Starting from the surface electronic structure calculation, we analyze the imaginary part of the theoretical dielectric function, separating surface and bulk contributions. The effects of the nonlocality of the pseudopotential are studied, by working both in the transverse gauge (neglecting them) and in the longitudinal gauge (where they are automatically included). The two calculations, although giving different dielectric functions, yield the same reflectance anisotropy, which compares well with experimental data and with previous theoretical results.

We report a first principles calculation of gallium arsenide optical properties based on the density-functional theory in the local-density approximation. The nonlocality of the pseudopotential has a sizable effect on the computed spectrum and on the static dielectric constant. The effect of a ``scissors-operator'' shift of the energy bands is discussed.

In this Letter, we investigated the photo-response of multi wall carbon nanotube-based composites obtained from in situ thermal evaporation of noble metals (Au, Ag, and Cu) on the nanotube films. The metal deposition process produced discrete nanoparticles on the nanotube outer walls. The nanoparticle-carbon nanotube films were characterized by photo-electrochemical measurements in a standard three electrode cell. The photocurrent from the decorated carbon nanotubes remarkably increased with respect to that of bare multiwall tubes. With the aid of first-principle calculations, these results are discussed in terms of metal nanoparticle–nanotube interactions and electronic charge transfer at the interface.

Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K' contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology.

We investigate electronic and optical properties of the topological Weyl semimetals TaAs, TaP, NbAs and NbP crystallizing in bct geometry by means of the ab initio density functional theory with spin-orbit interaction within the independent-particle approximation. The small energetical overlap of Ta5d or Nb4d derived conduction and valence bands leads to electron and/or hole pockets near the Fermi energy at the 24 Weyl nodes. The nodes give rise to two-(three-)dimensional Dirac cones for the W1 (W2) Weyl type. The band dispersion and occupation near the Weyl nodes determine the infrared optical properties. They are dominated by interband transitions, which lead to a deviation from the expected constant values of the imaginary part of the dielectric function. The resulting polarization anisotropy is also visible in the real part of the optical conductivity, whose lineshape deviates from the expected linearity. The details of the Weyl nodes are discussed in relation to recent ARPES results for TaAs and NbP, and compared with measured optical spectra for TaAs. The spectral features of the anisotropic and tilted Weyl fermions are restricted to low excitation energies above absorption onsets due to the band occupation.

Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Rytova-Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, confinement effects, and reduced screening. We find exciton radiative lifetimes ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room temperature, thus making 2D nitrides, especially InN, promising materials for light-emitting diodes and high-performance solar cells.

We summarize the theoretical framework and some of the computational methods currently used for the calculation of the optical properties of surfaces within the “ab initio” scheme. Applications and examples are given for calculations including self-energy, excitonic, and local-field effects, with an emphasis on convergence problems and on the approximations related to the pseudopotential approach.

We present the electromagnetic scattering theory for a finite-length nanowire with an embedded mesoscopic object. The theory is based on a synthesis of the integral equation technique of classical electrodynamics and the quantum transport formalism. We formulate Hall\'en-type integral equations, where the canonical integral operators from wire antenna theory are combined with special terms responsible for the mesoscopic structure. The theory is applied to calculate the polarizability of a finite-length single-walled carbon nanotube (CNT) with a short low-conductive section (LCS) in the microwave and subterahertz ranges. The LCS is modeled as a multichannel two-electrode mesoscopic system. The effective resistive sheet impedance boundary conditions for the scattered field are applied on the CNT surface. It is shown that the imaginary part of the polarizability spectrum has three peaks. Two of them are in the terahertz range, while the third is in the gigahertz range. The polarizability spectrum of the CNT with many LCSs has only one gigahertz peak, which shifts to low frequencies as the number of LCSs increases. The physical nature of these peaks is explained, and potential applications of nanoantennas are proposed.

Using ab initio density functional theory the band structure and the dielectric function of a bct Cd3As2 crystal are calculated. We find a Dirac semimetal with two Dirac nodes k± near the Γ point on the tetragonal axis. The bands near the Fermi level exhibit a linear behavior. The resulting Dirac cones are anisotropic and the electron-hole symmetry is destroyed along the tetragonal axis. Along this axis the symmetry-protected band linearity only exists in a small energy interval. The Dirac cones seemingly found by ARPES in a wider energy range are interpreted in terms of pseudo-linear bands. The behavior as 3D graphene-like material is traced back to As p orbital pointing to Cd vacancies, in directions which vary throughout the unit cell. Because of the Dirac nodes the dielectric functions (imaginary part) show a plateau for vanishing frequencies whose finite value is proportional to the Sommerfeld fine structure constant but varies with the light polarization. The consequences of the anisotropy of the Dirac cones are highlighted for the polarization dependence of the infrared optical conductivity.

We discuss an approach to calculate electronic excitations in clusters, which starts from the determination of the ground state within density functional theory and the local density approximation, and subsequently yields electronic spectra from Green's function theory. These methods, which were originally developed and used in extended systems, are shown to work well also in clusters. We discuss the theory and the computational implementation, and illustrate the performance and the physical mechanisms of this approach for the example clusters Na4 ,N a 6 ,a nd SiH 4. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 951-960, 2000

The thermal properties and the electrical conductance are studied for 2D electron gases in doped Xenes -- graphene, silicene, germanene, stanene, and plumbene -- applying a four-band model to describe the low-energy Dirac-fermion-like electronic excitations. Spin-orbit interactions to discriminate the five Xenes and the influence of an electric field in the normal direction, are taken into account. The density of states and the spectral behavior of the current-current correlation function allow the calculation of the Onsager coefficients. They give analytical formulas for the electronic contributions to the heat capacity and thermal conductance. Also, the electrical conductance can be described within the same framework, if the scattering properties of the electron gases are simulated by constant broadening parameters. For all these thermal and transport properties, only a weak variation with the Xene is found, because of their small spin-orbit-induced gaps, with the exception of plumbene. The heat capacity of Xenes does not show a Schottky anomaly. The thermal conductance increases linearly or quadratically with temperature depending on the temperature range. A similar behavior characterizes the electrical conductance. The dominance of Dirac fermions, i.e., linear bands, determines the ratio of electron thermal conductance and electric conductance, which depends on doping level and temperature. It violates the Wiedemann-Franz law known for 3D electron gases with parabolic energy-momentum dispersion. The Lorenz number is generally much larger for 2D electron gases with almost Dirac character of the dispersion relation.