29 research outputs found
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
First-principles calculation of x-ray dichroic spectra within the full-potential linearized augmented planewave method: An implementation into the Wien2k code
X-ray absorption and its dependence on the polarization of light is a
powerful tool to investigate the orbital and spin moments of magnetic materials
and their orientation relative to crystalline axes. Here, we present a program
for the calculation of dichroic spectra from first principles. We have
implemented the calculation of x-ray absorption spectra for left and right
circularly polarized light into the Wien2k code. In this package, spin-density
functional theory is applied in an all-electron scheme that allows to describe
both core and valence electrons on the same footing. The matrix elements, which
define the dependence of the photo absorption cross section on the polarization
of light and on the sample magnetization, are computed within the dipole
approximation. Results are presented for the L2,3 and M4,5 egdes of CeFe2 and
compared to experiments
Investigation of A1g phonons in YBa2Cu3O7 by means of LAPW atomic-force calculations
We report first-principles frozen-phonon calculations for the determination
of the force-free geometry and the dynamical matrix of the five Raman-active
A1g modes in YBa2Cu3O7. To establish the shape of the phonon potentials atomic
forces are calculated within the LAPW method. Two different schemes - the local
density approximation (LDA) and a generalized gradient approximation (GGA) -
are employed for the treatment of electronic exchange and correlation effects.
We find that in the case of LDA the resulting phonon frequencies show a
deviation from experimental values of approximately -10%. Invoking GGA the
frequency values are significantly improved and also the eigenvectors are in
very good agreement with experimental findings.Comment: 15 page
Stacking-Fault Energy and Anti-Invar Effect in FeMn Alloys
Based on state-of-the-art density-functional-theory methods we calculate the
stacking-fault energy of the paramagnetic random Fe-22.5at.%Mn alloy between
300-800 K. We estimate magnetic thermal excitations by considering longitudinal
spin-fluctuations. Our results demonstrate that the interplay between the
magnetic excitations and the thermal lattice expansion is the main factor
determining the anti-Invar effect, the hcp-fcc transformation temperature, and
the stacking-fault energy, which is in excellent agreement with measurements.Comment: 5 pages, 3 figure
Fermi surface induced lattice distortion in NbTe
The origin of the monoclinic distortion and domain formation in the quasi
two-dimensional layer compound NbTe is investigated. Angle-resolved
photoemission shows that the Fermi surface is pseudogapped over large portions
of the Brillouin zone. Ab initio calculation of the electron and phonon
bandstructure as well as the static RPA susceptibility lead us to conclude that
Fermi surface nesting and electron-phonon coupling play a key role in the
lowering of the crystal symmetry and in the formation of the charge density
wave phase
Transport, optical and electronic properties of the half metal CrO2
The electronic structure of CrO_2 is critically discussed in terms of the
relation of existing experimental data and well converged LSDA and GGA
calculations of the electronic structure and transport properties of this half
metal magnet, with a particular emphasis on optical properties. We find only
moderate manifestations of many body effects. Renormalization of the density of
states is not large and is in the typical for transition metals range. We find
substantial deviations from Drude behavior in the far-infrared optical
conductivity. These appear because of the unusually low energy of interband
optical transitions. The calculated mass renormalization is found to be rather
sensitive to the exchange-correlation functional used and varies from 10%
(LSDA) to 90% (GGA), using the latest specific heat data. We also find that
dressing of the electrons by spin fluctuations, because of their high energy,
renormalizes the interband optical transition at as high as 4 eV by about 20%.
Although we find no clear indications of strong correlations of the Hubbard
type, strong electron-magnon scattering related to the half metallic band
structure is present and this leads to a nontrivial temperature dependence of
the resistivity and some renormalization of the electron spectra.Comment: 9 Revtex 2 column pages, including 8 postscript figures. Two more
figures are included in the submission that are not embedded in the paper,
representing DOS and bandstructure of the paramagnetic CrO
Self-consistently renormalized quasiparticles under the electron-phonon interaction
Combining ab initio techniques and the analytic properties of the electron Green’s function, we outline a method for calculating quasiparticle properties under the electron-phonon interaction. The presented scheme is a generalization of the work by Engelsberg and Schrieffer [Phys. Rev. 131, 993 (1963)] to finite temperatures and is suitable for being applied to complex materials, where the electronic and vibrational properties are calculated from first principles. We show that under some circumstances, the low-energy dynamical properties are well described by quasiparticles, but at the same time the renormalization effects on quasiparticle lifetimes and energies can be very important. The bare second-order perturbative (such as Fermi’s golden rule) results for the self-energy are compared with self-consistent ones. The theory is first illustrated with the simple Einstein and Debye models at finite temperatures. Thereafter we consider realistic materials such as the 1×1 hydrogen-covered (deuterium-covered) W(110) surface and the superconductor MgB2.This work was supported by Gipuzkoako Foru Aldundia and Donostia International Physics Center DIPC as well as by the Austrian Science Fund, Project No. P16227.Peer reviewe
noloco: An efficient implementation of van der Waals density functionals based on a Monte-Carlo integration technique
The treatment of van der Waals interactions in density functional theory is an important field of ongoing research. Among different approaches developed recently to capture these non-local interactions, the van der Waals density functional (vdW-DF) developed in the groups of Langreth and Lundqvist is becoming increasingly popular. It does not rely on empirical parameters, and has been successfully applied to molecules, surface systems, and weakly-bound solids. As the vdW-DF requires the evaluation of a six-dimensional integral, it scales, however, unfavorably with system size. In this work, we present a numerically efficient implementation based on the Monte-Carlo technique for multi-dimensional integration. It can handle different versions of vdW-DF. Applications range from simple dimers to complex structures such as molecular crystals and organic molecules physisorbed on metal surfaces