RESUMO
MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200-300 km above the core-mantle boundary (the D" discontinuity) or the polarization anisotropy observed in the lowermost mantle. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase. However, recent in situ X-ray diffraction measurements have revealed a transition to a 'post-perovskite' phase above 125 GPa and 2,500 K--conditions close to those at the D" discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D" layer.
RESUMO
We extend the recently proposed order-N algorithms for calculating linear- and nonlinear-response functions in time domain to the systems described by nonorthonormal basis sets.
RESUMO
We propose an efficient time-dependent algorithm for nonlinear response function that requires CPU time proportional to the system size, and study the size effects in two-photon absorption spectra of Si nanocrystallites by using this algorithm.
