We report the first ab initio quasiparticle calculation in a real cluster ${\mathrm{Na}}_{4}$ within Hedin's $\mathrm{GW}$ approximation for valence electron self-energy. Our approach avoids summations over empty states, and also eliminates problem of residual interactions between periodic images. Self-energy corrections open local density gap by more than 2 eV; finite-size effects on screening are shown to play an important role. The absorption spectrum calculated including excitonic using...

We develop a gauge-invariant formalism for calculating the optical properties of solids within random-phase approximation and GW approach to electron bands (the independent-quasiparticle approximation). show that, scissors-operator approximation, it is enough shift rigidly spectrum, calculated according density-functional theory--local-density higher energies by gap correction. Moreover, simple formula given valid beyond approximation.

We show by first-principles calculations that, due to depressed screening and enhanced two-dimensional confinement, excitonic resonances with giant oscillator strength appear in hydrogenated Si Ge layers, which qualitatively quantitatively differ from those of graphane. Their large exciton binding energies strengths make them promising for observation novel physical effects application optoelectronic devices on the nanoscale.

The optical properties of GaAs(110) and GaP(110) surfaces are studied by means self-consistent local-density calculations. A very large contribution to the reflectance anisotropy is found be related transitions which do not involve surface states. These give a substantial---yet smaller---contribution also differential reflectivity. Comparison made with relevant experimental data.

The widely used GW approximation for the self-energy operator of a system interacting electrons may, in principle, be improved using an approximate vertex correction \ensuremath{\Gamma}. We estimate \ensuremath{\Gamma} local-density approximation. report results comparable series calculations band structure silicon, which such is (i) excluded entirely, (ii) included only screened Coulomb interaction W, and (iii) both W expression self-energy. also discuss symmetry properties exact how they...

We present a model for the dielectric function of semiconductors. It has been tested successfully Si, Ge, GaAs, and ZnSe. In conjunction with single plasmon-pole approximation it yields plasmon-energy dispersions in fair agreement experiments. allows one, moreover, to deduce an analytical expression Coulomb-hole part static self-energy operator.

We report numerical calculations of the frequency-dependent dielectric function for different gauges electromagnetic field in optical transition operator. Comparing results, we draw conclusions about importance nonlocality effects entering calculations. Apart from spatial inhomogeneity related to atomic structure matter, they are due nonlocal pseudopotentials, quasiparticle self-energies, and incompleteness basis functions. Besides their influence on spectra, effect validity f-sum rule...

Corrections to band gaps calculated within the density-functional local-density approximation are obtained as self-energy differences, computed according GW approximation. Local-field and dynamical screening effects neglected. The use of a model dielectric function simple tight-binding wave functions leads an analytic formula for gap correction valid semiconductors insulators crystallizing in diamond zinc-blende structures. results many materials very good agreement with experiment.

A method is developed to obtain Wannier-Mott --- exciton wave functions in semiinfinite crystals the framework of effective-mass approximation. An analytical approximation shown agree well with numerical and used compute nonlocal polarizability (in closed form) $s$-wave reflectivity. The results are compared normal-incidence reflectivity experiments CdS, ZnSe, GaAs, InP, experimental line shapes reproduced. existence an intrinsic dead layer confirmed a new additional boundary condition...

We formulate the theory of macroscopic dielectric tensor for crystals lower-than-cubic symmetry, using local-orbital approach to response. In case noncubic infinite we give a definition that is simpler than any previous formulation and allows us perform realistic calculations with roughly same computational effort needed in cubic crystals. semi-infinite crystals, first time outline way computing it from two-particle Green's function, which can be computed according method Hanke Sham,...

A microscopic calculation of the surface optical properties Si(111) and Si(110) within randomphase approximation is presented. Surface effects on matrix elements are found important in order to explain anisotropic reflectance observed at surfaces cubic semiconductors. At frequencies above bulk indirect gap, a large contribution differential reflectivity turns out be related structural changes occurring upon oxidation.

The yellowing of paper on aging causes major aesthetic damages cultural heritage. It is due to cellulose oxidation, a complex process with many possible products still be clarified. By comparing ultraviolet-visible reflectance spectra ancient and artificially aged modern papers ab initio time-dependent density functional theory calculations, we identify estimate the abundance oxidized groups acting as chromophores responsible yellowing. This knowledge can used set up strategies selective...

The reflectance anisotropy has been calculated by microscopic tight-binding theory for various configurations of the As-rich GaAs(100) $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructions, based on precise atomic coordinates from ab initio total-energy minimization. comparison to experimental in combination with scanning tunneling microscopy low energy electron diffraction allows one identify correlations between structural units optical...

The inclusion of dynamical quasiparticle effects lifts the underestimation band gaps in semiconductors and insulators derived within density-functional theory. Simultaneously, they give rise to a remarkable reduction strengths peaks that is not observed optical spectra. We show contributions vertex corrections widely compensate peak renormalization due dynamics formation. slight absorption spectrum yields an improvement computed $f$-sum rule.

We present a full application of the finite-temperature ab initio molecular dynamics to Si(100) surface. A clear dynamical picture system has been obtained: coexistence different reconstructions, their temperature stability, surface vibrations and phase transitions. The c(4\ifmmode\times\else\texttimes\fi{}2) structure, with its alternate buckled dimers, explains basic properties, but it does not completely account for fine details experimental results. room structure corresponds mixture...

A microscopic calculation of the optical properties Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface for both asymmetric and symmetric dimers is presented. The comparison with experiments allows us to discard a substantial presence from 40 310 K. An upper bound 10% obtained their instantaneous population. Suggestions are made future experiments.

We present ab initio calculations of the excited state properties liquid water in framework many-body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, results averaged over different configurations. The absorption spectra with inclusion excitonic effects calculated by solving Bethe-Salpeter equation. insensitivity screening a particular configuration make these feasible. resulting which...

Variational wave functions are proposed for the two lowest states of excitons in thin slabs. They shown to be reliable CdS slabs thickness L larger than 2.5 exciton radii ${a}_{B}$. The energy difference between them is close that found by quantizing center-of-mass motion a slab effective L-2d, where d transition-layer depth. This approximate rule becomes exact L>16${a}_{B}$. calculated optical properties show absorptance maxima and transmittance minima correspondence with levels L500...

The optical properties of the chain model Si(111)2\ifmmode\times\else\texttimes\fi{}1 are computed both within a single-particle picture and with inclusion excitonic local-field effects. presence bound exciton states depends on magnitude screening electrons in surface states. For weak singlet is strongly (~ 0.3 eV) gives rise to narrow Lorentzian peak which dominates absorption spectrum. strong screening, excitons not present. resulting line shape asymmetrical, qualitative agreement...

Intra-atomic correlations are shown to lead two separate occupied dangling-bond bands at the buckled Si(111) surface, in agreement with recent photoemission results. A bandwidth of 0.4 eV and spin-polarized obtained when atomic spin periodicities both 2 \ifmmode\times\else\texttimes\fi{} 1. The bandwidths sensitive type ordering surface. Nearly dispersionless result 1 2, respectively. For nonbuckled surface a metastable, antiferromagnetic structure 1.1-eV gap is predicted.

The combination of in situ real-time surface differential reflectivity (SDR) spectroscopy and microscopic calculations has been used for the first time to investigate gas adsorption on a Si surface. optical signatures some structural units Si(111)-( $7\ifmmode\times\else\texttimes\fi{}7$) have identified. development corresponding features SDR spectra upon amount H exposure allowed us demonstrate occurrence two different mechanisms hydrogenation determine their relative kinetics.