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Using a first-principles pseudopotential technique, we have investigated the adsorption of C 2 H 2 on the Si͑001͒ surface. We have found that, at low temperatures, the di-bond configuration is the most stable structure from the energetic point of view. According to our calculations C 2 H 2 adsorbs preferentially on the alternate dimer sites, corresponding to a coverage of 0.5 monolayer. Our calculated surface band structure suggests that the end-bridge configuration, recently pointed out as a more favorable configuration by firstprinciples calculations, presents a metallic character and thus is Peierls unstable. The di-adsorbed system is characterized by symmetric and slightly elongated Si-Si dimers, and by a symmetric CC bond with length close to the double carbon bond length of the ethylene molecule. Our total-energy calculations suggest that other metastable configurations, like the 1,2-hydrogen transfer, the p bridge and the tetra-model are also possible. Available high-resolution electron-energy-loss spectroscopy experimental data are reinterpreted to support the existence of the tetra-model.

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The phonon conductivity in suspended graphene and in graphene nanoribbons has been studied within the framework of Callaway’s effective relaxation time theory. The conductivity expression has been computed by employing analytical expressions for phonon dispersion relations and vibrational density of states based on the semicontinuum model by Nihira and Iwata. It is found that the Normal-drift contribution to the conductivity, arising from the consideration of the momentum conserving nature of three-phonon Normal processes, is very important for explaining the magnitude as well as the temperature dependence of the experimentally measured results for suspended graphene.

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We address the issue of the composition and strain dependence of the piezoelectric effect in semiconductor materials, which is manifested by the appearance of an electric field in response to shear crystal deformation. We propose a model based on expressing the direct and dipole contributions to the polarization in terms of microscopic quantities that can be calculated by density functional theory. We show that when applied to the study of In x Ga 1−x As alloys, the model is able to explain and accurately predict the often-observed discrepancies between the experimentally deduced values of e 14 and those linearly interpolated between the values of InAs and GaAs. The values of the piezoelectric coefficient predicted by our approach compare very well with values deduced from photocurrent measurements of quantum well samples grown on ͑111͒ GaAs substrates by molecular beam epitaxy.