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Agradecimentos: This work was supported by IN/MCT, THEO-NANO, NANOMAT, LNLS, CNPq, and FAPESP. The authors thank P. C. Silva and J. Bettini for sample preparation and data treatment assistance Abstract: We report high resolution transmission electron microscopy and ab initio calculation results for defect formation in sharp pyramidal Pt nanocontacts. Our results show that there is a size limit to the existence of twins (extended structural defects). These defects are always present but blocked away from the tip axes. They may act as scattering planes, influencing the electron conductance for Pt nanocontacts at room temperature and Ag/Au nanocontacts at low temperature (< 150 K) CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Aberto

Agradecimentos: We thank Professor M. J. Aziz for useful discussions throughout this work and helpful comments on the manuscript. A. A. wishes to thank Harvard University for its hospitality during the course of this work, and to acknowledge partial support from the Brazilian funding agencies Fundo de Apoio ao Ensino e à Pesquisa (FAEP/UNICAMP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Computer calculations were performed at the facilities of the Centro Nacional de Processamento de Alto Desempenho em São Paulo (CENAPAD-SP). The work at Harvard University was supported the Materials Science and Engineering Center, which is funded by the National Science Foundation Abstract: The structure of the single vacancy in silicon. one of the most common point defects and an important mediator of atomic diffusion, is examined through extensive first principles calculations. We find a hitherto unexpected result, namely that there exist two distinct distortions associated with the vacancy with essentially identical formation energies at zero pressure. The two distortions are distinguished by their different relaxations, volumes of formation, and :formation enthalpies. We discuss how, at finite pressure, one of the two distortions should become dominant, and suggest experimental tests of this effect FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Aberto

Agradecimentos: One of us (A.A.) wishes to thank Harvard University for its hospitality during the course of this work, as well as to acknowledge partial support from the Brazilian funding agencies Fundo de Apoio ao Ensino e à Pesquisa (FAEP/UNICAMP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). This work was supported in part by Harvard’s Materials Science and Engineering Center, which is funded by NSF Abstract: The free energy of the concerted exchange mechanism for self-diffusion in silicon is estimated using the thermodynamic integration method and Monte Carlo (MC) simulations with an interatomic potential fitted to reproduce local-density-approximation calculations. Anharmonicity and relaxation are fully taken into account in the calculations, since the phase space is extensively explored by the MC simulations. The results indicate that the concerted exchange mechanism can have a significant contribution to the self-diffusion constant in silicon CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Aberto