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The scattering theory for two crossing metallic single-walled carbon nanotubes of finite length exposed to an electromagnetic field has been developed based on a synthesis of the quantum transport formalism and classical electrodynamics. The model of the point contact has been developed to be incorporated into the Hall\'en and Pocklington equations for crossing carbon nanotubes. The influence of the contact conductance and position as well as the angle between the tube axes on the tube polarizability has been analyzed in the range of 1 GHz to 10 THz. The physical mechanisms responsible for the electromagnetic interaction between crossing tubes with zero and nonzero intertube contact conductance are shown and discussed. The influence of the coupling between the tubes on the localized plasmon resonance in them has been demonstrated.

Using density-functional and many-body perturbation theory, we study the electronic, optical, and excitonic properties of indium nitride as single monolayer and bilayer in comparison with the bulk phase. We investigate the stable geometry for the monolayer, the graphenelike unbuckled honeycomb structure, and for the bilayer the AA' stacking geometry. We demonstrate that the quasiparticle and optical gaps, going from bulk to two-dimensional systems, open dramatically due to strong quantum confinement and reduced screening. Large excitonic effects, which survive at room temperature, are predicted. Our results suggest that low-dimensional InN is a promising material for optoelectronic devices in the visible to near-infrared spectral ranges varying with the number of atomic layers and light propagation.

In this paper, we present a new noninvasive and nondestructive approach to recover scattering and absorption coefficients from reflectance measurements of highly absorbing and optically inhomogeneous media. Our approach is based on the Yang and Miklavcic theoretical model of light propagation through turbid media, which is a generalization of the Kubelka-Munk theory, extended to accommodate optically thick samples. We show its applications to paper, a material primarily composed of a web of fibers of cellulose, whose optical properties are strongly governed by light scattering effects. Samples studied were ancient and industrial paper sheets, aged in different conditions and highly absorbing in the ultraviolet region. The recovered experimental absorptions of cellulose fibers have been compared to theoretical ab initio quantum-mechanical computational simulations carried out within time-dependent density functional theory. In this way, for each sample, we evaluate the absolute concentration of different kinds of oxidized groups formed upon aging and acting as chromophores causing paper discoloration. We found that the relative concentration of different chromophores in cellulose fibers depends on the aging temperature endured by samples. This clearly indicates that the oxidation of cellulose follows temperature-dependent reaction pathways. Our approach has a wide range of applications for cellulose-based materials, like paper, textiles, and other manufactured products of great industrial and cultural interest, and can potentially be extended to other strongly absorbing inhomogeneous materials. © 2014 American Physical Society.

We infer the structure of the GaP(001)(2x4) surface from a study of scanning tunneling microscopy (STM) images obtained under UHV conditions on metal-organic vapor phase epitaxy grown samples. STM images are compared with results of first-principles calculations for models energetically most favorable under Ga-rich growth conditions. The comparison shows that the GaP(001)(2x4) surface unit cell consists of a mixed Ga-P dimer on top of a complete gallium layer, hence ruling out the Cia-Ga dimer model.

We present ab initio calculations for both the wave functions and the energies of single quasiparticles. The conventional quasiparticle approach evaluates energy-level corrections to first order in the difference between the self-energy and the Kohn-Sham exchange-correlation potential. Here, we also recalculate the quasiparticle states. At the example of the GaAs(110) surface we show that this nonperturbative treatment is important for surfaces with electronic states close in energy but different with respect to their localization. As a sensitive observable the reflectance anisotropy is studied.

We apply the method of random matrices to a calculation of the optical properties of porous silicon and silicon nanopowders, analyzing the effects of disorder on the visible luminescence properties. The maximum of the luminescence peal; displays a blueshift, which adds up to the effects induced by other mechanisms, such as quantum confinement or surface passivation, and agrees with the experimental findings, The long-wavelength luminescence edge, instead, is redshifted by the presence of disorder, thus partially compensating for the gap-opening effect induced by the other mechanisms.

We apply reflection anisotropy spectroscopy (RAS) to high-quality atomically smooth H/C(100) $2\ifmmode\times\else\texttimes\fi{}1$ surfaces. The optical signal of the H/C(100) ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ flat surface, and of the H/C(100) ${2}^{\ifmmode^\circ\else\textdegree\fi{}}$ and H/C(100) ${4}^{\ifmmode^\circ\else\textdegree\fi{}}$ vicinal surfaces, is investigated in terms of single and double steps. A comparison of experimental and theoretical results obtained from ab initio calculations shows that, in the energy range considered (1--5 eV), the RAS response can be interpreted in terms of single height ${S}_{B}$ and double height ${D}_{A}$ steps.

We present an application of time-dependent density-functional theory (TDDFT) to the study of the optical properties of the $\text{Si}(111)2\ifmmode\times\else\texttimes\fi{}1$ surface from the infrared to the ultraviolet. We have carried out ab initio calculations using different methods, from DFT to Bethe-Salpeter equation (BSE) and, within TDDFT, we have tested the ability of different kernels to describe the optical features in a wide range of energies. We find good agreement between TDDFT and BSE results, by using in TDDFT a long-range frequency-dependent exchange-correlation kernel derived from the many-body formalism (the MB kernel). The agreement between theory and experiment is very good in the whole frequency range studied. Excitonic effects, important in the infrared part of the spectrum, are less pronounced in the visible and UV ranges.

Despite the simplicity of the system, the description of the excited-state properties of the cleavage surface of diamond requires the use of nonstandard treatment of many-body effects. Using a simple tight-binding model, we show that the typical assumption concerning the identity of quasiparticle and Kohn-Sham wave functions fails in an important part of the Brillouin zone. This simple model allows computation of the qualitatively different quasiparticle wave functions. The optical properties calculated with these wave functions at the Bethe-Salpeter level show important excitonic effects. Moreover the line shape of the reflectance anisotropy spectrum is significantly improved when compared to the measured one.

International audience; The interaction between small unsaturated hydrocarbon molecules of C2H4 with a vicinal silicon 001 surface is studied by means of reflectance anisotropy spectroscopy and analyzed with first principles calculations. Our results confirm that ethylene adsorbs without breaking the silicon dimers. Comparison of theoretical optical spectra with experimental data shows that the C2H4 molecules lay on top of the silicon dimers from low to high coverage. This occurs even though, from a purely energetic point of view, a bridge configuration would be favorable at 1 monolayer coverage.

We extend the three-layer model for energy loss calculation to include many-body effects in an ab initio framework. The electron energy loss spectrum of the $\mathrm{C}(100)2\ifmmode\times\else\texttimes\fi{}1$ surface is calculated and compared with the existing experimental results. We show how many-body effects, namely self-energy, local-fields, and electron-hole interaction, deeply influence the dielectric response and how their inclusion is essential to have good agreement with the experiment. A strong anisotropic behavior of local-field effects on the dielectric response has been observed, while the inclusion of the electron-hole attractive interaction produces a surface-state exciton with binding energy of about $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, confirming a recent theoretical-experimental study of the optical spectra of this surface.

We present a first-principles calculation of the geometry and of the electronic structure for the C1112 1 surface. We find that this surface reconstructs with -bonded chains without any significant dimerization or buckling. At the DFT level, it appears semimetallic, in agreement with previous calculations, but in contrast to experimental evidence. Even the introduction of quasi-particle corrections within the usual GW scheme does not lead to the opening of the gap between surface states. Quasiparticle corrections to the surface band structure are hence calculated within a self-consistent GW scheme. A gap of about 1 eV is found between the surface states, thus finally solving the discrepancy between theory and experiment. The 111 face is the cleavage surface of diamond. Understanding its behavior is important from a technological point of view, the 111 plane is one of the growth surfaces for the chemical vapor deposition CVD of diamond, and from a theoretical point of view, in order to elucidate the principles of surface reconstructions in the other group IV semiconductors. Many potential applications of diamond arise from specific properties of its surfaces: while its negative electron affinity and unique surface conductivity make it attractive for electronic and optoelectronic devices, 1 its extreme surface chemical inertness makes it a good candidate for biomedical applications reducing the chances of rejection; moreover, boron-doped diamond electrodes have been used in water purification and in the detection of many substances, espe

The electronic structure of the $\mathrm{InAs}(110)$ surface is investigated using several theoretical tools: semiempirical tightbinding, density functional theory, and perturbative many-body theory within the GW approximation. Comparison is made with available photoemission data. Moreover, we calculate the optical properties of this surface and perform reflectance anisotropy experiments. The tight-binding and especially the GW approximations provide a very good description of the optical spectra and allow for a more detailed analysis of the origin of the spectral features.

We perform first-principles total-energy and electronic structure calculations in order to determine the atomic structure of the P-rich $(2\ifmmode\times\else\texttimes\fi{}1)/(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction commonly observed for GaP(001) surfaces grown by gas-phase epitaxy. A monolayer of buckled phosphorus dimers stabilized by hydrogen adsorbed in an alternating sequence is found to be energetically stable and to be consistent with the experimental data.

We combine low-energy electron diffraction and reflectance anisotropy spectroscopy (RAS) with ah initio calculations of the geometry, band structure, and optical anisotropy to investigate the adsorption of Sb on vicinal Si(001) (1×2). We focus, in particular, on the controversy concerning the Si(001)-(I ×1)-Sb surface. On the basis of total-energy and band-structure calculations, we find that the Sb-undimerised model is unstable and metallic, while experimentally the (1×1) Sb shows no evidence of a Fermi edge. In contrast, the dimerised (2 X 1)-Sb and c(2×2)-Sb reconstructions are found to be semiconducting with a minimal difference in total energy. Furthermore, the RAS spectra calculated for both dimerised reconstructions show strong similarities to one another, and agree well with the experimental RAS data for the Sb-induced (1×1)-Sb surface, with a dominant feature centered at 3.7 eV. We report that these findings are compatible with the (1×1)-Sb surface comprising a mixture of the Sb-dimer-terminated (2×1)-Sb and c-(2×2)-Sb structures.

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.

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.