Brillouin zone

Tight-binding electronic structure calculations for periodic systems are often carried out in non-primitive unit cells, or for imperfect (eg random-alloy) nanostructures, such as nanowires. In the first case bands exist but they are difficult to identify due to the choice of unit cell for the calculation, while in the second case bands only exist in an approximate sense. The Brillouin zone unfolding technique applied to tight-binding calculations provides a powerful tool for extracting the true primitive-cell bands from non-primitive cells. Also, it ...

Using first-principles density functional calculations, the structural, electronic, elastic and optical properties of cubic spinel SiGe2N4 were studied by means of the full-relativistic version of the full-potential augmented plane wave plus local orbitals. We employed both the generalized-gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy, and the Engel–Vosko formalism, which optimizes the corresponding potential for band structure calculations.The calculated bulk properties, including lattice parameters, bulk modulus and their pressure derivatives, are in reasonable agreement with the available data. We have determined the full set of first-order elastic constants and their pressure dependence, which have not been calculated and measured yet. Band structure, density of states and pressure coefficients of energy band gaps are given. The obtained results for band structure using EV-GGA are larger than that of GGA. We calculated the frequency dependent complex dielectric function ε2(ω) for radiation up to 30 eV. The assignment of the critical points to the band structure difference at various points of the Brillouin zone was made. The pressure and volume dependence of the static dielectric constant and the refractive index were calculated.

We have performed a first principles study of structural, mechanical, electronic, and optical properties of orthorhombic Sb2S3 and Sb2Se3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data.

We then use these ideas to discuss focusing of geodesics in orbifolds of constant curvature. In the particular case of the Riemann surfaces H^2/Gamma(k), (k=2,3, or 5), we explicitly count the number of geodesics of length t that connect the point i to itself.

We have performed ab initio density functional calculations of thin Ag films on the Pd(111) surface. We have calculated the structural properties and the electronic bands of the Ag/Pd systems. There is a band gap in the electronic density of states around the centre of the two-dimensional Brillouin zone of the Pd(111) surface, which makes possible the formation of localised states in the adsorbed silver films. We find that quantum well states may form at binding energies around 4 eV.

Based on identical sphere (atom) models, a comprehensive study of SH and HCP has been carried out for the first time by constructing the unit cells (e.g. the conventional, primitive, Wigner–Seitz, reciprocal and Brillouin zone) of HCP and comparing them with their SH counterparts to resolve the existing ambiguities between them. This has been further supported by a comprehensive crystallographic calculation. Similarly, a thorough study of symmetry of the two unit cells has been carried out to decide their positions in the final crystal system table.
The geometrical study suggests the existence of rhombohedral close packing (RCP) and its inherent structural connection with HCP throughout (i.e. as its primitive cell, etc.). Similarly, the study of symmetry suggests the existence of a Trigonal unit cell and its inherent association with SH throughout as a symmetry derivative. The present study resolves clearly that SH, Trigon, HCP and RCP are exclusive and independent lattices where HCP and RCP are exclusively associated with the close packing of identical atoms and hence proposed to be included as independent members of the space lattices. Henceforth, the two new lattices will be called as Wahab lattices (or W lattices) and the sixteen lattices together as Bravais-Wahab lattices (or BW lattices) or simply space lattices as before. The sixteen lattices have been divided into eight crystal systems in the order of their increasing symmetry. The results obtained from the present study help explain the existence of 73 symmorphic space groups in 3-dimensional crystal system. These findings will have immediate as well as far reaching implications.

The quantum size effect (QSE) and temperature dependence of the low-energy electronic properties of thin bismuth crystals are studied by means of the high-resolution electron-energy-loss spectroscopy (HREELS) technique. Electronic interband transitions, taking place at the points L and T ...

Fast-light phenomena can enhance the sensitivity of an optical gyroscope of a given size by several orders of magnitude, and could be applied to other optical sensors as well. MagiQ Technologies has been developing a compact fiber-based fast light Inertial Measurement Unit (IMU) using Stimulated Brillouin Scattering in optical fibers. We will report on our findings, including repeatable fast-light effects in the lab, numerical analysis of noise and stability given realistic optical specs, and methods for optimizing efficiency, size, and reliability with current technologies. The technology could benefit inertial navigation units, gyrocompasses, and stabilization platforms, and could allow high grade IMUs in spacecraft, unmanned aerial vehicles or sensors, where the current size and weight of precision gyros are prohibitive. By using commercially mature technologies, we believe that our design is appropriate for development without further advances in materials or components.

An ab-initio pseudopotential calculation has been performed by using density functional methods within the local density approximation (LDA) to investigate the band structure and optical properties of the ferroelectric-semiconductor SbSI in the para- and ferroelectric phases. It has been shown that SbSI has an indirect gap in both phases (1.45 eV and 1.49 eV in the para- and ferroelectric phases respectively) and that the smallest direct gap is at the S point of the Brillouin zone (1.56 eV and 1.58 eV in the para- and ferroelectric phases respectively). Furthermore, it is shown that first-order phase transition, from the paraelectric phase to the ferroelectric phase (the transiton temperature is about 22 °C), does not change the nature of the band gap. Moreover, the linear frequency dependent dielectric function, including self-energy effects, has been calculated along the c-polar axis in the para- and ferroelectric phases.

Photonic band gap templating using optical interference lithography. Timothy YM Chan, Ovidiu Toader, and Sajeev John Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada. Received 23 June 2004; published 7 April 2005 ...

We characterize the nature of the Van Hove singularity near the Fermi energy (EF) in YBa2Cu4O8 using ultrahigh energy resolution (10 meV) angle-resolved photoemission spectroscopy, together with corresponding first-principles spectra intensity computations. The singularity, related to CuO2 planes, is located about 19 meV below EF at the Y point in the Brillouin zone and arises from a band which is