Showing papers on "Brillouin zone published in 1994"

Tunneling spectroscopy of the (110) surface of direct-gap III-V semiconductors.

Abstract: Results of tunneling spectroscopy measurements of the (110) cleaved surface of GaAs, InP, GaSb, InAs, and InSb are presented These materials form the family of direct-gap III-V binary semiconductors Spectroscopic measurements are performed in ultrahigh vacuum, using a scanning tunneling microscope (STM) Techniques based on variable tip-sample separation are used to obtain high dynamic range (six orders of magnitude) in the measured current and conductance Detailed spectra are obtained for all the materials, revealing the conduction- and valence-band edges, onset of the higher lying conduction band at the L point in the Brillouin zone, and various features associated with surface states The precision and accuracy in determining energetic locations of spectral features are discussed In particular, limitations in the accuracy due to tip-induced band bending is considered

Combined distributed temperature and strain sensor based on Brillouin loss in an optical fiber

Abstract: We present a novel distributed sensor that utilizes the temperature and strain dependence of the frequency at which the Brillouin loss is maximized in the interaction between a cw laser and a pulsed laser. With a 22-km sensing length, a strain resolution of 20 µ? and a temperature resolution of 2°C have been achieved with a spatial resolution of 5 m.

Translation-rotation coupling, phase transitions, and elastic phenomena in orientationally disordered crystals

Abstract: Many of the properties of orientationally disordered crystals are profoundly affected by the coupling (known as translation-rotation coupling) between translation displacements and molecular orientation. The consequences of translation-rotation coupling depend on molecular and crystal symmetry, and vary throughout the Brillouin zone. One result is an indirect coupling between the orientations of different molecules, which plays an important role in the order/disorder phase transition, especially in ionic orientationally disordered crystals. Translation-rotation coupling also leads to softening of elastic constants and affects phonon spectra. This article describes the theory of the coupling from the point of view of the microscopic Hamiltonian and the resulting Landau free energy. Considerable emphasis is placed on the restrictions due to symmetry as these are universal and can be used to help one's qualitative understanding of experimental observations. The application of the theory to phase transitions is described. The softening of elastic constants is discussed and shown to be universal. However, anomalies associated with the order/disorder phase transition are shown to be restricted to cases in which the symmetry of the order parameter satisfies certain conditions. Dynamic effects on phonon spectra are described and finally the recently observed dielectric behavior of ammonium compounds is discussed. Throughout themore » article examples from published experiments are used to illustrate the application of the theory including well known examples such as the alkali metal cyanides and more recently discovered orientationally disordered crystals such as the fullerite, C[sub 60].« less

Optical properties and temperature dependence of the interband transitions of cubic and hexagonal GaN.

Abstract: The optical properties of cubic and hexagonal GaN thin films, grown by electron-cyclotron resonance microwave plasma-assisted molecular-beam epitaxy on silicon and sapphire substrates, respectively, have been studied at photon energies up to 25 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The fundamental gaps of the two polytypes are located at different energies, namely at 3.25 and 3.43 eV for cubic and hexagonal GaN. Analysis of the dielectric function of the two phases in the region 4.5--9.5 eV with appropriate models yields the energy location and broadening of the observed critical points. These critical points are assigned to specific points in the zinc-blende and wurtzite Brillouin zones, respectively, making use of the latest published band-structure studies and a comparison is made between the corresponding results for GaN, GaAs, and GaP. Measurements in the temperature range from 80 to 650 K provide the temperature dependence of these parameters. The features observed in the reflectivity spectra of hexagonal GaN are discussed in relation to other works. Kramers-Kronig analysis of the reflectivity between 0 and 33 eV of the hexagonal polytype verifies the existence of a broad feature centered at 14 eV. Finally, average properties, such as the effective ir dielectric constant and the effective number of valence electrons per atom are calculated for the two polytypes and compared to GaAs and GaP.

Coherent self-heterodyne Brillouin OTDR for measurement of Brillouin frequency shift distribution in optical fibers

Abstract: Time domain reflectometry of spontaneously Brillouin scattered lightwaves in a single-mode optical fiber is demonstrated with a coherent self-heterodyne detection system employing a recently proposed frequency translator, a DFB laser diode, and erbium-doped fiber amplifiers. Since the probe pulse frequency is up-converted by the translator by an amount approximately equal to the Brillouin frequency shift, the self-heterodyne beat frequency can be reduced to a sufficiently low frequency in the IF band. The system enables one-end measurement of the Brillouin frequency shift distribution in optical fibers with a single way dynamic range (SWDR) of 16 dB and a frequency resolution of 5 MHz for a spatial resolution of 100 m. >

Electronic structure and properties of AlN.

Abstract: The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

Method and apparatus for analyzing optical fibers by inducing Brillouin spectroscopy

Abstract: For the noninvasive investigation of optical fiber lines by means of heterodyne Brillouin spectroscopy light from a pumplaser (10) is directed into one end of the optical fiber line under investigation (16), so that spontaneous Brillouin backscattering is induced in the fiber (16). The light, which is backscattered by spontaneous Brillouin scattering, is superimposed to a lightwave, which is derived from the pumplaser (10), and heterodyned at the detector (46). The output signal from the detector is fed into a spectrum analyzer (50). A part of the light from the pumplaser (10) is directed into a ring resonator (38). The lightwave, which is derived from the pumplaser (10) and which is superimposed with the light, which is backscattered from the optical fiber line under investigation (16), is generated by stimulated Brillouin scattering inside the ring resonator (38).

Surface polaritons on gold-wire gratings.

Abstract: A resonance phenomenon in the zeroth diffraction order of a gold-wire grating is explained by the excitation of surface polaritons. This effect is connected with a strong enhancement of the electromagnetic fields on the wire surface and consequently with a peak of power losses in the grating material. Measurements of the zeroth-order transmittance have been performed on gold gratings with periods of 1 and 2 micrometers in the near-infrared region which are in agreement with theoretical results. Furthermore, dispersion relations of the first-order coupling mode are presented having large energy gaps in the center of the Brillouin zone. It is shown that this energy gap strongly depends on the wire profile. In this coupling branch, however, practically no dispersion could be observed for optical wavelengths less than the grating period.

Phonon dispersions of silicon and germanium from first-principles calculations

Abstract: We present the calculation of the full phonon spectrum for silicon and germanium with the pseudopotential method and the local-density approximation without using linear-response theory. The interplanar-force constants for three high-symmetry orientations [(100), (110), and (111)] are evaluated by supercell calculations using the Hellmann-Feynman theorem. By considering the symmetry of the crystal, three-dimensional interatomic-force-constant matrices are determined by a least-squares fit. Interactions up to the eighth nearest neighbors are included. The dynamical matrix, which is the Fourier transform of the force constant matrix, is hence constructed and diagonalized for any arbitrary wave vector in the Brillouin zone, yielding the phonon dispersion. In this paper we will present the calculation details and discuss various aspects of convergence. Phonon dispersions of Si and Ge calculated are in excellent agreement with experiments.

Ab initio determination of the bulk properties of MgO

Abstract: Ab initio periodic Hartree-Fock (HF) theory was used to determine the elastic constants and selected phonon frequencies of bulk MgO; the accuracy of a posteriori correlation corrections to the periodic Hartree-Fock calculations is also discussed. Inclusion of diffuse atomic orbitals in the MgO basis was necessary to accurately describe elastic distortions and phonon vibrations of the solid. The computed HF lattice constant (4.195 \AA{}) agrees with experiment (4.19 \AA{}) and the elastic constants are within +15% of the observed values. Correlation corrections to these energetics shorten the lattice parameter to 4.09 \AA{} and further stiffen the elastic constants. The HF phonon frequencies at the (\ensuremath{\Gamma}, X, and L) points in the Brillouin zone were within 15% of experiment and the correlation corrections softened the modes improving agreement with experiment. These data will be used to parametrize electrostatic shell models of MgO.

Size-consistent Brillouin-Wigner perturbation theory with an exponentially parametrized wave function: Brillouin-Wigner coupled-cluster theory.

Abstract: Size consistency of the Brillouin-Wigner perturbation theory is studied using the Lippmann-Schwinger equation and an exponential ansatz for the wave function. Relation of this theory to the coupled cluster method is studied and a comparison through the effective Hamiltonian method is also provided.

Quasiparticle dispersion in the cuprate superconductors and the two-dimensional Hubbard model

Abstract: The momentum dispersion of the peak structure of the single-particle spectral weight A(p,\ensuremath{\omega}) of the two-dimensional Hubbard model is discussed. Using results obtained from Monte Carlo simulations on lattices up to 12\ifmmode\times\else\texttimes\fi{}12 in size, we determine the low-lying quasiparticle dispersion relation. This dispersion relation is anomalously flat near the (\ensuremath{\pi},0) and (0,\ensuremath{\pi}) points of the Brillouin zone, similar to the results of recent angular resolved photoemission measurements of the hole-doped cuprates. We argue that the generic nature of the quasiparticle dispersion relation observed in these materials arises from the strong Coulomb interaction and reflects the hole-spin correlations rather than the one-electron interactions which customarily determine the band structure.

Ab initio lattice dynamics and charge fluctuations in alkaline-earth oxides

Abstract: We present an ab initio calculation of lattice-dynamical properties of the alkaline-earth oxides MgO, CaO, and SrO in the rocksalt structure. We employ the density-functional perturbation theory within the local-density approximation. We show phonon dispersion curves along several symmetry directions in the Brillouin zone. The calculated frequencies agree well with the available experimental data. The investigation of the phonon-induced charge-density fluctuations of MgO and CaO at the L point of the Brillouin zone partially supports the breathing-shell model of lattice dynamics and rules out the charge-transfer model for this class of materials. Moreover, our calculations show that the breathinglike charge-density response is more pronounced for oxygen than for the cations in these compounds.

Band theory of solids : an introduction from the point of view of symmetry

Abstract: 0. Notation 1. The free-electron picture 2. Symmetry and group theory 3. Space groups 4. The reciprocal lattice and the Fourier series 5. Block functions and Brillouin zones 6. Space group representations 7. The representation of space groups: an example 8. Brillouin zones and energy bands 9. Bands and Fermi surfaces in metals and semiconductors 10. Methods of calculation of band structures 11. Phonons and conductivity 12. Phase stability 13. Wannier functions and Lowdin orbitals 14. Surface and impurity states 15. Solutions to the problems References Index

Ab initio lattice dynamics and charge Huctuations in alkaline-earth oxides

Abstract: within the local-density approximation. We show phonon dispersion curves along several symmetry directions in the Brillouin zone. The calculated frequencies agree well with the available experimental data. The investigation of the phonon-induced charge-density jIuctuations of MgO and CaO at the I point of the Brillouin zone partially supports the breathing-shell model of lattice dynamics and rules out the charge-transfer model for this class of materials. Moreover, our calculations show that the breathinglike charge-density response is more pronounced for oxygen than for the cations in these compounds.

Stimulated Brillouin scattering for improvement of microwave fibre-optic link efficiency

Abstract: Stimulated Brillouin scattering is used to increase the modulation depth of a weakly-modulated optical signal by rejecting the forward travelling carrier power. For an input power of eight times the stimulated Brillouin threshold, an improvement of 8 dB in link loss from 0.1 to 20 GHz is achieved.

Suppression of brillouin scattering in lightwave transmission system

Abstract: Optical nonlinear effects, such as stimulated (Brillouin) scattering, cause disproportionate attenuation of transmitted optical signals and usually occurs only when the optical power exceeds a certain level. One of the most important types of nonlinear scattering which occurs in an optical fiber as the power of the optical signal is increased above a certain level is stimulated Brillouin scattering. Brillouin scattering limits the power density of an optical signal that can be injected into an optical fiber. This invention suppresses Brillouin scattering and, by so doing, permits the power density of the optical signal which is injected into an optical fiber to be increased approximately threefold before the Brillouin scattering threshold is reached. More specifically, in a lightwave AM-VSB CATV transmission system having an external modulator, the optical beam which is amplitude modulated with the CATV signals is also phase modulated with a sinusoidal signal having a frequency which is not less than twice the frequency of the highest CATV signal. The phase modulation of the optical beam suppress Brillouin scattering. With this invention the power density of the CATV signal can be increased approximately three fold before the threshold of Brillouin scattering is reached.

Fabrication of Two-Dimensional Photonic Band Structure with Near-Infrared Band Gap

Abstract: We have succeeded in fabricating a two-dimensional photonic band structure with a near-infrared band gap for p-polarization in a two-dimensional Brillouin zone.The sample consists of circular air rods constituting a triangular lattice with a lattice constant of 1.17 µm and a background dielectric constant of 2.6. The transmission spectra were measured over a range from 1.4 to 5.0 µm for both p- and s-polarizations. The energies of complete reflection regions observed in the Γ- X direction were in good agreement with the theoretical calculation for both p- and s-polarizations. This agreement indicates that the above band gap should exist.

Magnetic anisotropies of ultrathin Co films on Cu(1 1 13) substrates.

Abstract: All magnetic anisotropy contributions in single-crystalline (1 1 13)-oriented Co films on Cu were determined at room temperature using Brillouin light scattering. An in-plane uniaxial anisotropy contribution, not found in the reference system Co/Cu(001), is identified as magnetoelastic in origin. From the thickness dependence of the in-plane anisotropies of both twofold and fourfold symmetry about the film normal it is concluded that elastic strain fields are very likely to provide the driving mechanism for stabilizing ferromagnetic order in ultrathin Co(1 1 13) films.

Calculated and measured uv reflectivity of SiC polytypes

Abstract: A systematic study of the vacuum uv reflectivity (4--10 eV) for an electric field perpendicular to the c-axis is presented for SiC polytypes. Experimental results for the 4H, 15R, 6H, and 3C SiC polytypes are compared to theoretical calculations within the local-density approximation in conjunction with a muffin-tin-orbital basis set for the 2H, 4H, 6H, and 3C polytypes. Agreement to \ensuremath{\sim}0.2 eV is obtained between theoretical and experimental peak positions assuming a single, k-point-, energy-, and polytype-independent gap correction, the value of which agrees well with recent calculations based on Hedin's GW approximation. The main features and their trends with degree of hexagonality of the polytype are interpreted in terms of interband transitions involving extended regions in the Brillouin zone where valence and conduction bands are nearly parallel.

Understanding far-infrared absorption in the s=1 antiferromagnetic chain compound nenp

Abstract: Infrared transmission measurements on the S=1 antiferromagnetic chain compound NENP [Ni(C2H8N2)2NO2ClO4] in applied magnetic fields show a sharp absorption line at the field-shifted Haldane gap. This violates a wave-vector selection rule of the Hamiltonian normally used for NENP, as the gap excitations occur at the Brillouin zone boundary. We argue that the crystal structure admits terms which can explain the absorption lines. The observation of a finite gap at all applied fields can be explained by the presence of an induced staggered magnetic field, originally proposed by Chiba et al. © 1994 The American Physical Society.

Electronic structures of disordered Ag-Mg alloys.

Abstract: We have calculated the electronic structures of the valence bands in a series of [alpha]-phase (disordered fcc) Ag-Mg alloys over the range 0--30 at. % Mg using the Korringa-Kohn-Rostoker method within the coherent potential approximation (KKR-CPA). We find that the variation of the equilibrium lattice constant with composition is in good agreement with experimental measurements. The bandwidth of the Ag-related [ital d] states decreases with the addition of Mg although the position of the bottom of the band remains roughly fixed in energy with respect to the Fermi level; an observation that is consistent with photoemission measurements. The electronic spectral densities are very sharply peaked at the Fermi energy with widths that are [lt]1% of the Brillouin zone dimension and so the Fermi surfaces of the alloys are well defined throughout the zone. The radius of the neck at the [ital L] point and the belly radii in the [Gamma][ital X] and [Gamma][ital K] direction increases approximately linearly with increasing Mg content. Taking into account some previous work on the origin of the short range order in Ag-rich alloys, we conclude that the local-density approximation KKR-CPA method provides a realistic description of the electronic structure in [alpha]-phase [ital Ag]-Mg alloys.

High-accuracy measurement of the linewidth of a Brillouin fibre ring laser

Abstract: A high resolution spectral analysis of the emission of a stimulated Brillouin scattering fibre ring laser has been performed, demonstrating the single frequency operation of such lasers and a 3 Hz emission linewidth.

Mean-field analysis in the p-d model of oxide superconductors.

Abstract: A highly correlated [ital p]-[ital d] model of cuprate oxide superconductors is analyzed by use of composite operators on CuO[sub 2] clusters. A minimal set of composite operators to describe the electronic state is identified from the criterion that the order of the approximation is determined by coupling constants multiplied by the weight of their respective operators. It is shown that the intensity transfer among bands with hole doping is well described within the mean-field approximation. Effects of the nearest-neighbor spin correlation are investigated and it is shown that the band dispersions are strongly affected. By the local antiferromagnetic correlation, the band dispersion of the upper Hubbard band is flattened in the whole Brillouin zone, while that of the [ital p] dominant band near the Fermi level is flattened only around a certain region centered at the zone boundary. In the moderately doped region, the Fermi surface is consistent with a large Fermi surface.

The dynamics of poly(n-butyl acrylate) above the glass transition

Abstract: Measurements of the dielectric constant of poly(n-butyl acrylate) have been taken in the frequency range 100 Hz-3 GHz, for temperatures between 254 and 359 K. Analysis of dielectric data and their comparison with results recently obtained on the same system by Brillouin light scattering have evidenced a two-step relaxation process. Dielectric spectra account for the behaviour of the slower process, i.e. the structural relaxation, while Brillouin spectra are particularly sensitive to the faster process, i.e. the secondary relaxation. The temperature behaviour of these processes is not in accordance with the predictions of the mode-coupling theory, while it can be suitably interpreted by a model that relates the high-frequency dependence of the dielectric loss curve to the local chain dynamics and the low-frequency dependence to the intermolecular correlation.

Band structure and x-ray resonant inelastic scattering.

Abstract: We present the results of a tight-binding calculation of the soft-x-ray emission spectra obtained in an earlier study of resonant inelastic scattering of x rays at the carbon k edge of diamond. The close agreement between the calculated spectra and the experimental observations lends support to a picture of momentum conservation restricting the emission process to the same point in the Brillouin zone sampled by the initial excitation. We thus demonstrate qualitatively the feasibility of band-structure determinations with this technique.