All-electron Exact Exchange Treatment of Semiconductors: Effect of Core-valence Interaction on Band-gap and dd-band Position

Exact exchange (EXX) Kohn-Sham calculations within an all-electron full-potential method are performed on a range of semiconductors and insulators (Ge, GaAs, CdS, Si, ZnS, C, BN, Ne, Ar, Kr and Xe). We find that the band-gaps are not as close to experiment as those obtained from previous pseudopotential EXX calculations. Full-potential band-gaps are also not significantly better for $sp$ semiconductors than for insulators, as had been found for pseudopotentials. The locations of $d$-band states, determined using the full-potential EXX method, are in excellent agreement with experiment, irrespective of whether these states are core, semi-core or valence. We conclude that the inclusion of the core-valence interaction is necessary for accurate determination of EXX Kohn-Sham band structures, indicating a possible deficiency in pseudopotential calculations.Comment: 4 pages 2 fig

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 $55^{\circ}$ up to $95^{\circ}$. We point out that such large torsion angles do not destroy the conjugation of the $\pi$ 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.

Band-structure topologies of graphene: spin-orbit coupling effects from first principles

The electronic band structure of graphene in the presence of spin-orbit coupling and transverse electric field is investigated from first principles using the linearized augmented plane-wave method. The spin-orbit coupling opens a gap at the $K(K')$-point of the magnitude of 24 $\mu$eV (0.28 K). This intrinsic splitting comes 96% from the usually neglected $d$ and higher orbitals. The electric field induces an additional (extrinsic) Bychkov-Rashba-type splitting of 10 $\mu$eV (0.11 K) per V/nm, coming from the $\sigma$-$\pi$ mixing. A 'mini-ripple' configuration with every other atom is shifted out of the sheet by less than 1% differs little from the intrinsic case.Comment: 4 pages, 4 figure

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

Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach

The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Δ, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Δ, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices

Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds

The electronic structure of two V-based ladder compounds, the quarter-filled NaV$_2$O$_5$ in the symmetric phase and the iso-structural half-filled CaV$_2$O$_5$ is investigated by ab initio calculations. Based on the bandstructure we determine the dielectric tensor $\epsilon(\omega)$ of these systems in a wide energy range. The frequencies and eigenvectors of the fully symmetric A$_{g}$ phonon modes and the corresponding electron-phonon and spin-phonon coupling parameters are also calculated from first-principles. We determine the Raman scattering intensities of the A$_g$ phonon modes as a function of polarization and frequency of the exciting light. All results, i.e. shape and magnitude of the dielectric function, phonon frequencies and Raman intensities show very good agreement with available experimental data.Comment: 11 pages, 10 figure

Spin-orbit effects in a graphene bipolar pn junction

A graphene $pn$ junction is studied theoretically in the presence of both intrinsic and Rashba spin-orbit couplings. We show that a crossover from perfect reflection to perfect transmission is achieved at normal incidence by tuning the perpendicular electric field. By further studying angular dependent transmission, we demonstrate that perfect reflection at normal incidence can be clearly distinguished from trivial band gap effects. We also investigate how spin-orbit effects modify the conductance and the Fano factor associated with a potential step in both $nn$ and $np$ cases.Comment: 6 pages, 5 figures, conductance and Fano factor plots adde

Fermi surface induced lattice distortion in NbTe2_2

The origin of the monoclinic distortion and domain formation in the quasi two-dimensional layer compound NbTe$_2$ 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