Importance Of Local Field Effects Research Articles

Macroscopic quantum electrodynamics theory of resonance energy transfer involving chiral molecules

Resonance energy transfer between chiral molecules can be used to discriminate between different enantiomers. The transfer rate between chiral molecules consists of a non-discriminatory and discriminatory parts. We derive these two rate contributions in the framework of macroscopic quantum electrodynamics. We show that their ratio is usually larger in the retarded regime or far-zone of large separation distances and that the degree of discrimination can be modified when considering a surrounding medium. We highlight the importance of local field effects onto the degree of discrimination and predict for general identical chiral molecules the optimum dielectric medium for discrimination. We apply our results on to 3-methylcyclopentanone and show that exotic media can even invert the discriminatory effect.

Strong anisotropic influence of local-field effects on the dielectric response ofα-MoO3

Dielectric properties of {\alpha}-MoO3 are investigated by a combination of valence electron-energy loss spectroscopy and ab initio calculation at the random phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO3 has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-N2,3 edges as well as the low energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of {\alpha}-MoO3 is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak in {\epsilon}2, but only along the [010] direction. Thanks to a detailed analysis at specific k-points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity.

Electronic collective excitations in compressed lithium fromab initiocalculations: Importance and anisotropy of local-field effects at large momenta

According to our ab initio calculations, the recently reported undamped plasmon in compressed fcc lithium emerges not only due to the fact that the dielectric matrix determinant vanishes at long wavelengths at the plasmon energy, i.e., satisfying the ideal condition for an undamped plasmon in a crystal but also extends at large momenta specially along the $\ensuremath{\Gamma}\text{L}$ direction. Different from the case of simple metals, the local-field effects dominate, leading to the striking periodicity exhibited by the low-energy plasmon for momentum transfers beyond the first Brillouin zone. Remarkably, pressure-induced electronic anisotropy and localization increase the impact of local-field effects on the electronic response even in the compact fcc structure. Interestingly, the importance of local-field effects is similar in lithium under pressure and in materials with confined geometries such as ${\text{MgB}}_{2}$.

Screening in semiconductor nanocrystals:Ab initioresults 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.

Structural properties and quasiparticle energies of cubic SrO, MgO and SrTiO3

The structural properties and the band structures of the charge-transfer insulating oxides SrO, MgO and SrTiO3 are computed both within density functional theory in the local density approximation (LDA) and in Hedin's GW-scheme for self-energy corrections, by using a model dielectric function, which approximately includes local field and dynamical effects. The deep valence states are shifted by the GW-method to higher binding energies, in very good agreement with photoemission spectra. Since in all of these oxides the direct gaps at high-symmetry points of the Brillouin zone may be very sensitive to the actual value of the lattice parameter a already at the LDA level, self-energy corrections are computed both at the theoretical and the experimental a. For MgO and SrO, the values of the energies of transition between the valence and the conduction bands are improved by GW-corrections, while for SrTiO3 they are overestimated. The results are discussed in relation to the importance of local field effects and to the nature of the electronic states in these insulating oxides.

On dielectric response and quasiparticles in semiconductors and insulators

Some aspects of dielectric response pertinent to the calculation of quasiparticle energies are discussed. A new generalized plasmon-pole concept to treat the frequency dependence of the dielectric matrix is presented. The scheme based on the framework of the dielectric bandstructure implies a faster convergence when calculating the quasiparticle selfenergy. Moreover it has the merit that the dielectric matrix always has the correct analytical properties. To visualize the polarization cloud around a quasiparticle in a semiconductor the dielectric response for a test charge is studied. This leads to a natural subdivision of the electronic polarization in a semiconductor into charge transfer and dipolar polarization. The relative size of these contributions provides a measure for the importance of local field effects.

Real-space dielectric response in semiconductors

Local field effects in semiconductors are studied through the inversion of the dielectric matrix in real space, using a Wannier function basis. The method will be useful when the translational symmetry is broken, e.g. where there are impurities, dislocations or surfaces. The method is applied to the calculation of the potential produced for a static impurity in diamond. The results show the importance of local field effects, which, in this case, appear as oscillations of the potential with the periodicity of the lattice.