34 research outputs found
Dynamical Exchange Effects in a Two-Dimensional Many-Polaron Gas
We calculate the influence of dynamical exchange effects on the response
properties and the static properties of a two-dimensional many-polaron gas.
These effects are not manifested in the random-phase approximation which is
widely used in the analysis of the many-polaron system. Here they are taken
into account by using a dielectric function derived in the time-dependent
Hartree-Fock formalism. At weak electron-phonon coupling, we find that
dynamical exchange effects lead to substantial corrections to the random-phase
approximation results for the ground state energy, the effective mass, and the
optical conductivity of the polaron system. Furthermore, we show that the
reduction of the spectral weight of the optical absorption spectrum at
frequencies above the longitudinal optical phonon frequency, due to many-body
effects, is overestimated by the random-phase approximation.Comment: 9 pages, 7 figure
Screening in semiconductor nanocrystals: \textit{Ab initio} results and Thomas-Fermi theory
A first-principles calculation of the impurity screening in Si and Ge
nanocrystals is presented. We show that isocoric screening gives results in
agreement with both the linear response and the point-charge approximations.
Based on the present ab initio results, and by comparison with previous
calculations, we propose a physical real-space interpretation of the several
contributions to the screening. Combining the Thomas-Fermi theory and simple
electrostatics, we show that it is possible to construct a model screening
function that has the merit of being of simple physical interpretation. The
main point upon which the model is based is that, up to distances of the order
of a bond length from the perturbation, the charge response does not depend on
the nanocrystal size. We show in a very clear way that the link between the
screening at the nanoscale and in the bulk is given by the surface
polarization. A detailed discussion is devoted to the importance of local field
effects in the screening. Our first-principles calculations and the
Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field
effects are dominated by surface polarization, which causes a reduction of the
screening in going from the bulk down to the nanoscale. Finally, the model
screening function is compared with recent state-of-the-art ab initio
calculations and tested with impurity activation energies
Dynamical exchange effects in the dielectric function of the two-dimensional electron gas
Dynamical exchange interactions can be introduced in the dielectric function via a dynamic local field factor. We study the effects of this inclusion on both the static and the frequency dependent dielectric function of a two-dimensional electron gas, using the dynamic local field factor that we derived recently via the dynamical exchange decoupling method. The results are compared with the dielectric function in the Random Phase Approximation and with different dynamic and static approximations of the local field factor. Copyright Springer-Verlag Berlin/Heidelberg 2003
The rutile TiO2 (110) surface: Obtaining converged structural properties from first-principles calculations
We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile TiO2 (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first theoretical results on the displacement of the surface atoms in agreement with experiment. (c) 2006 American Institute of Physics
Screening in semiconductor nanocrystals: Ab initio results and Thomas-Fermi theory
A first-principles calculation of the impurity screening in Si and Ge nanocrystals is presented. We show that isocoric screening gives results in agreement with both the linear response and the point-charge approximations. Based on the present ab initio results, and by comparison with previous calculations, we propose a physical real-space interpretation of the several contributions to the screening. Combining the Thomas-Fermi theory and simple electrostatics, we show that it is possible to construct a model screening function that has the merit of being of simple physical interpretation. The main point upon which the model is based is that, up to distances of the order of a bond length from the perturbation, the charge response does not depend on the nanocrystal size. We show in a very clear way that the link between the screening at the nanoscale and in the bulk is given by the surface polarization. A detailed discussion is devoted to the importance of local field effects..
Thomas-Fermi model of electronic screening in semiconductor nanocrystals
Using first-principle density-functional theory in the GGA approximation we have studied the electronic screening in semiconductor nanocrystals.
Combining simple electrostatics and the Thomas-Fermi theory
it is shown that an analytical and general form
of a model position-dependent screening function can be
obtained. Taking as a case study silicon nanocrystals,
the relative weights of the nanocrystal core and surface polarization
contribution to the screening are thoroughly discussed.
The connection between the screening at the nanoscale and
in the bulk is clarified
The energy of step defects on the TiO2 rutile (110) surface: An ab initio DFT methodology
We present a novel methodology for dealing with quantum size effects (QSE) when calculating the energy per unit length and step–step interaction energy of atomic step defects on crystalline solid surfaces using atomistic slab models. We apply it to the TiO2 rutile (110) surface using density functional theory (DFT) for which it is well-known that surface energies converge in a slow and oscillatory manner with increasing slab size. This makes it difficult to reliably calculate step energies because they are very sensitive to supercell surface energies, and yet the surface energies depend sensitively on the choice of slab chemical formula due to the dominance of QSE at computationally practical slab sizes. The commonly used method of calculating surface energies by taking the intercept of a best fit line of total supercell energies against slab size breaks down and becomes highly unreliable for such systems. Our systematic approach, which can be applied to any crystalline surface, bypasses such statistical estimation techniques and cross checks and makes robust what is otherwise a very unreliable process of extracting the energies of steps. We use the calculated step energies to predict island shapes on rutile (110) which compare favorably with published scanning tunneling microscopy (STM) images