Brillouin zone

About: Brillouin zone is a research topic. Over the lifetime, 13849 publications have been published within this topic receiving 383077 citations.

First-principles pseudopotential study of the structural phases of silicon

Abstract: A first-principles pseudopotential study of 11 phases of silicon is reported: diamond (cd), body-centered cubic (bcc), face-centered cubic (fcc), body-centered tetragonal (bct), simple hexagonal (sh), hexagonal-close-packed (hcp), double-hexagonal-close-packed (dhcp), simple cubic (sc), \ensuremath{\beta}-tin, a body-centered cubic structure with eight atoms per unit cell (bc8), and a simple tetragonal structure with 12 atoms per unit cell (st12) For each structure and for each volume considered we have minimized the energy with respect to all structural degrees of freedom We have used large basis sets and very careful integrations over the Brillouin zone to resolve the small energy differences between structures, and our pseudopotentials incorporate nonlinear core exchange-correlation corrections, leading to more accurate results than those of previous calculations We find good agreement with experiments and with some previous calculations but a few discrepancies remain and we gain some insights into the high-pressure phase diagram of silicon

Structural and vibrational properties of GaN

Abstract: Structural and vibrational properties of device quality pure GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique were studied using x-ray diffraction, Brillouin, Raman, and infrared spectroscopy. Lattice constants were found to be a=3.1896±0.0002 A and c=5.1855±0.0002 A. Comparing the results with those on GaN epilayer directly grown on sapphire substrate, it is shown that the GaN substrate is indeed of high quality, i.e., the lattice is relaxed. However the GaN substrate has a small enough but finite residual strain arising from the pileup of the lateral growth front on SiO2 masks in the course of LEO. It was also found that the elastic stiffness constants C13 and C44, are more sensitive to the residual strain than the optical phonon frequencies. The high frequency and static dielectric constants were found to be 5.14 and 9.04. The Born and Callen effective charges were found to be 2.56 and 0.50.

Energy-Band Structure and Electronic Properties of SnTe

Abstract: The energy-band structure of SnTe is calculated using the augmented-plane-wave method, taking into account the relativistic corrections. The resulting wave functions at $L$ are used to calculate momentum matrix elements. These matrix elements are then used in a k\ifmmode\cdot\else\textperiodcentered\fi{}p perturbation calculation in the presence of a magnetic field to obtain band parameters at point $L$ in the Brillouin zone. Nonparabolic expansions for the conduction and valence bands at $L$ are obtained using Cohen's model. The effect of strain on the energy levels at $L$ is calculated using the deformation-potential theory. The results indicate a complicated shape for the valence band at $L$, with two maxima at each side of $L$ on the face of the Brillouin zone. A record set of apparent maxima is found in the $\ensuremath{\Sigma}$ direction at $\mathbf{k}=(\frac{\ensuremath{\pi}}{a})(\frac{4}{5},\frac{4}{5},0)$. This is consistent with the "two-valence-band" model suggested to explain the experimental results. We are investigating the possibility that these extrema may be saddle points. The conduction- and valence-band-edge symmetries at $L$ are opposite to those of the lead chalcogenides, as are the band-gap deformation potentials at this point (-8.68 eV for SnTe and 11.55-17.24 eV for the others). This is in agreement with the experimentally proposed "band-inversion" model.

Optical properties of bcc transition metals in the range 0 – 40 eV

Abstract: We present a systematic analysis of the optical properties of bcc transition metals in the groups VB: V, Nb, and Ta, and VIB: paramagnetic Cr, Mo, and W. For this we use our formulation of time-dependent currentdensity-functional theory for the linear response of metals. The calculated dielectric and electron energy-loss functions are compared with our ellipsometry measurements and with data reported in literature, showing an overall good agreement. The experimental data of the dielectric functions presented by Nestell and Christy and by Weaver et al. differ mostly in the low-frequency region. However, we found that their reflectivity data are in very good agreement up to about 3 eV. We attribute this apparent discrepancy to the Drude-like extrapolation model used by Weaver et al. in the Kramers-Kronig procedure to extract the optical constants from their reflectivity data. Our experiments are in good agreement with Nestell and Christy’s data. The calculated absorption spectra show some deviations from the experiments, in particular in the 3d metals. We assign the spectra in terms of transitions between pairs of bands and we analyze which parts of the Brillouin zone are mainly involved in the absorption. Our results suggest that the blueshift of some spectral features in our calculations can be attributed mainly to the incorrect description of the virtual d bands by the approximations used for the ground state exchange-correlation functional. These virtual bands are too weakly bound by the local density and generalized gradient approximations, in particular in the 3d metals. We calculate separately the inter- and intraband contributions to the absorption and we show using a k·p analysis that, within the scalar-relativistic approximation, interband transitions contribute to the absorption already at frequencies well below 0.5 eV. This finding makes questionable the Drude-like behavior normally assumed in the experimental analysis of the linear response. We find that the combination of the Drude model in which we use the calculated plasma frequency and an optimized relaxation time, and the calculated interband response can well describe the experimental spectra. The electron energy-loss spectra are very well reproduced by our calculations showing in each metal a dominant plasmon peak at about 22‐24 eV, well above the corresponding Drude-like freeelectron plasma frequency, and additional features in the range 10‐15 eV. We show that the renormalization of the plasma frequency is due to the interplay between inter- and intraband processes, and that the additional features arise from the rich structure in the dielectric function caused by interband transitions.

Evidence of Brillouin scattering in an ytterbium-doped double-clad fiber laser.

Abstract: We have designed and performed an experiment that permitted direct observation of Brillouin backscattering in an Yb-doped double-clad fiber laser. Fifteen Brillouin-shifted frequencies were observed for the first time to our knowledge. We clearly demonstrate that stimulated Brillouin scattering is directly responsible for both fast transient dynamics of the laser and reduction of the laser's pulse width.