Ab Initio calculations of the anisotropic dielectric tensor of GaAs/AlAs superlattices

Anisotropy of W-band complex permittivity in Al2O3

The dielectric anisotropy of Al₂O₃ is studied here by characterizing W-band (75-110 GHz) complex permittivity of four different orientations of sapphire (Al₂O₃ single crystals). This was done using free-space, focused beam methods. Dielectric polarizability ([Formula: see text]) of these orientations is then calculated and these values are related to their complex permittivity. Based on this relationship, a framework is developed for rapid and straightforward estimation of dielectric anisotropy using a known crystal structure and a dielectric permittivity measurement performed on one orientation of the material. This framework can be applied to other materials with dielectric anisotropy (e.g. SnO₂, LiGaO₂) to predict permittivity for different orientations, enabling rapid design of high-frequency systems (e.g. radomes, electromagnetic windows). These permittivity measurements were also used to determine the dominant polarization mechanisms leading to dielectric anisotropy of Al₂O₃ in the W-band; electronic and ionic polarization orthogonal to the direction of the focused beam.

Anomalous angular dependence of the dynamic structure factor near Bragg reflections: graphite

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q, omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.