1,721,069 research outputs found
Surface versus crystal-termination effects in the optical properties of surfaces
No full textWe prove, by realistic microscopic calculations within the sp(3)s(*) tight-binding method for GaAs(110) and GaAs(100), that the surface optical properties are not influenced by long-range crystal termination effects, and hence that they can be consistently studied considering slabs of limited thickness (20-30 Angstrom). The origin of derivative like and bulklike line shapes in reflection anisotropy spectra is also discussed, analyzing the effects arising from possible surface-induced reduction, broadening, and shifting of the bulk spectrum near the surface. [S0163-1829(99)01123-61]
Plane-wave DFT-LDA calculation of the electronic structure and absorption spectrum of copper
No full textWe present an accurate, first-principles study of the electronic structure and absorption spectrum of bulk copper within density functional theory in the local density approximation, including the study of intraband transitions. We construct norm-conserving pseudopotentials (PP's) including the 3d shell (and optionally the underlying 3s and 3p shells) in the valence and requiring a relatively small plane-wave basis (60 and 140 Ry cutoff, respectively). As a consequence, these PP's are strongly nonlocal, yielding macroscopically wrong results in the absorption spectrum when momentum matrix elements are computed naively. Our results are compared with experimental photoemission, absorption, and electron energy loss data, and suggest nontrivial self-energy effects in the quasiparticle spectrum of Cu
Spontaneous Formation of Surface Antisite Defects in the Stabilization of the Sb-Rich GaSb(001) Surface
No full textThis Letter solves the long-standing puzzle [Phys. Rev. Lett. 79, 693 (1997)] of why GaSb(001) apparently violates the electron counting rule (ECR) in forming a reconstruction featuring long Sb-dimer chains, rather than the c(4 x 4) reconstruction found in all other arsenide and antimonide III-V compounds in the V-rich regime. We find that an alternative strategy, that in fact satisfies the ECR, is followed by the Sb-rich GaSb(001) surface, whereby long Sb-dimer chains are stabilized by randomly distributed subsurface Ga antisite defects. The excess of surface Sb drives the defect formation that in turn stabilizes the surface in a metastable phase. The transition to the c(4 x 4) reconstruction, where the ECR is instead satisfied through missing dimers, is therefore inhibited. Our conclusions are supported by ab initio simulations of experimental reflectance anisotropy spectra
Role of surface structural motifs on the stability and reflectance anisotropy spectra of Sb-rich GaSb(001) reconstructions
Full text linkThe structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 x 6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2 x 6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this "doping" effect considerably lowers the total energy of the long chain c(2 x 6) reconstruction model, making it competitive with the more stable short-chain (4 x 3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4 x 3) reconstructions are also present
Optical properties of GaSb(001)--c(2x6): the role of surface antisite defects
No full textWe consider the formation of surface antisite defects on a previously proposed model for the GaSb(001)-c(2 x 6) surface. Based on ab initio total energy calculations, we show how these defects stabilize the otherwise metallic surface and how their formation is driven by the excess charge associated with the Sb-rich surface conditions. The surface-sensitive optical technique of reflectance anisotropy spectroscopy is shown to be crucial for detecting the defects, and computation of spectra yields a good agreement with experiment when defects are included in the surface reconstruction
Structure and optical properties of the Sb-stabilized GaSb(001) surface
No full textWe suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms in the second layer are partially substituted by Ga. This ‘doping’ effect enables the surface to fulfil the electron counting rule whilst maintaining a reconstruction based on long dimer chains.Total energy calculations verify that the suggested reconstruction is relatively stable, and calculations of the reflectance anisotropy spectra confirm that it comprises a major component of the largely disordered surfac
Phenomenological approximations to the self-energy operator by a generalized X alpha method
No full textThe generalized X alpha method, based on a local, energy-independent operator derived from the exchange-correlation potential of the local-density approximation by changing the weight of the exchange term, is revisited with the aim of building an optimized starting point for band-structure calculations within the GW method. We find that the optimal choice coincides with the unmodified local-density approximation potential, i.e., with alpha = 2/3. Moreover, we show that the use of an X alpha method to mimic the self-energy effects in the calculation of absorption spectra leads to worse results than the simpler "scissors operator" approach
Optical Properties of real surfaces from microscopic calculations of the dielectric function of finite atomic slabs
No full textWe present a critical analysis of the procedures and approximations which are generally used to obtain the optical properties of a real solid surface starting from microscopic calculations of the dielectric function of a finite atomic slab. We show that relatively thin slabs-often the only ones accessible to modern ab initio methods-are indeed adequate provided that the proper procedure is followed for the extraction of the actual surface contributions, and we point out some possible sources of artifacts that should be avoided
First-principles calculation of the plasmon resonance and of the reflectance spectrum of silver in the GW approximation
No full textWe show that the position and width of the plasmon resonance in silver are correctly predicted by ab initio calculations including self-energy effects within the GW approximation. Unlike in simple metals and semiconductors, quasiparticle corrections play a key role and are essential to obtain electron energy loss in quantitative agreement with the experimental data. The sharp reflectance minimum at 3.92 eV, that cannot be reproduced within density-functional theory (DFT) in the local-density approximation (LDA), is also well described within GW. The present results solve two unsettled drawbacks of linear-response calculations for silver
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