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The thermal conductivity kappa(T) of Si nanostructures containing impurities is calculated from first-principles using nonequilibrium molecular dynamics simulations in thermally "prepared" periodic supercells. For a given concentration of impurities, kappa exhibits strongly nonlinear variations with the mass of the impurity. There is a narrow range of conditions for which kappa is substantially reduced relative to that of the pure material. This suggests that the kappa of nanowire could be controlled with impurities and that nanoregions with a desired kappa could be implanted on chips.

The vibrational lifetimes and decay channels of local vibrational modes are calculated from first principles at various temperatures. Our method can be used to predict the temperature dependence of the lifetime of any normal mode in any crystal. We focus here on the stretch modes of H2*, H+(BC), and VH x HV in Si. The frequencies are almost identical, but the lifetimes vary from 4 to 295 ps. The calculations correctly predict the lifetimes for T > 50 K and illustrate the critical importance of pseudolocal modes in the decay processes of high-frequency local vibrational modes.

Pseudolocal vibrational modes (pLVMs) are defect-related vibrational modes which are localized despite being below the phonon maximum. Such modes are sometimes seen as phonon replicas in photoluminescence spectra. The pLVMs associated with two copper-related defects are calculated from first-principles density-functional theory in periodic supercells. The localization of the pLVMs is quantified using the magnitude of the eigenvectors of the dynamical matrix.