4,182 research outputs found
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
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
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
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
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
- …