Phonon-assisted optical absorption in silicon from first principles.

The phonon-assisted interband optical absorption spectrum of silicon is calculated at the quasiparticle level entirely from first principles. We make use of the Wannier interpolation formalism to determine the quasiparticle energies, as well as the optical transition and electron-phonon coupling matrix elements, on fine grids in the Brillouin zone. The calculated spectrum near the onset of indirect absorption is in very good agreement with experimental measurements for a range of temperatures. Moreover, our method can accurately determine the optical absorption spectrum of silicon in the visible range, an important process for optoelectronic and photovoltaic applications that cannot be addressed with simple models. The computational formalism is quite general and can be used to understand the phonon-assisted absorption processes in general.

Role of fluorine in the iron pnictides: phonon softening and effective hole doping.

Using a first-principles approach, we investigate the influence of fluorine doping on the electronic structure, lattice dynamics, and electron-phonon coupling in LaFeAsO. In order to explore properties which are not described by the virtual crystal approximation, we explicitly simulate the F doping using a supercell model. Our analysis reveals that the relaxation of the crystal lattice around the dopant modifies the lattice dynamics in agreement with recent experimental data. In addition, we find that the doped electronic charge does not localize on the two-dimensional Fe plane. The net charge variation in this plane upon doping corresponds instead to a slight hole doping.