Theory of the Exciton-Phonon Coupling

The effect of electron-phonon interactions on optical absorption spectra requires a special treatment in materials with strong electron-hole interactions. We conceptualize these effects as exciton-phonon coupling. Through phonon absorption and emission, the optically accessible excitons are scattered into dark finite-momentum exciton states. We derive a practical expression for the exciton-phonon self-energy that relates to the temperature dependence of the optical transitions and their broadening. This expression differs qualitatively from previous approximated expressions found in literature

Spin Polarization of Photoelectrons from Topological Insulators

We show that the degree of spin polarization of photoelectrons from the surface states of topological insulators is 100% if fully-polarized light is used as in typical photoemission measurements, and hence can be significantly "higher" than that of the initial state. Further, the spin orientation of these photoelectrons in general can also be very different from that of the initial surface state and is controlled by the photon polarization; a rich set of predicted phenomena have recently been confirmed by spin- and angle-resolved photoemission experiments.Comment: A sentence on the effect of the imaginary part of the initial-state electron self energy is added to the published versio

Making Massless Dirac Fermions from Patterned Two-Dimensional Electron Gases

Analysis of the electronic structure of an ordinary two-dimensional electron gas (2DEG) under an appropriate external periodic potential of hexagonal symmetry reveals that massless Dirac fermions are generated near the corners of the supercell Brillouin zone. The required potential parameters are found to be achievable under or close to laboratory conditions. Moreover, the group velocity is tunable by changing either the effective mass of the 2DEG or the lattice parameter of the external potential, and it is insensitive to the potential amplitude. The finding should provide a new class of systems other than graphene for investigating and exploiting massless Dirac fermions using 2DEGs in semiconductors.Comment: 5 pages, 4 figures, significant revision of abstract, text, and figure

First-principles DFT+GW study of oxygen vacancies in rutile TiO2

We perform first-principles calculations of the quasiparticle defect states, charge transition levels, and formation energies of oxygen vacancies in rutile titanium dioxide. The calculations are done within the recently developed combined DFT+GW formalism, including the necessary electrostatic corrections for the supercells with charged defects. We find the oxygen vacancy to be a negative U defect, where U is the defect electron addition energy. For the values of Fermi level below 2.8 eV (relative to the valence band maximum) we find the +2 charge state of the vacancy to be the most stable, while above 2.8 eV we find that the neutral charge state is the most stable