Showing papers on "Brillouin zone published in 1990"

Electromagnetic wave propagation in periodic structures: Bloch wave solution of Maxwell's equations

Abstract: We examine the propagation of electromagnetic waves in periodic dielectric structures by solving the vector Maxwell equations with the plane-wave method. Contrary to experimental reports, as well as results of scalar-wave calculations, we do not find a true gap extending throughout the Brillouin zone in the fcc structure. However, there is a depletion in the photon density of states, seemingly a remnant of the Mie resonance, giving rise to a pseudogap in the spectrum that is quite strong for dielectric-sphere packing fraction \ensuremath{\beta}\ensuremath{\sim}0.3--0.4. An effect analogous to the Borrmann effect in x-ray diffraction is predicted, where certain photon modes will propagate an anomalously long distance before getting absorbed.

Distributed-temperature sensing using stimulated Brillouin scattering in optical silica fibers

Abstract: Temperature distribution in silica single-mode optical fibers, from -30 to +60 degrees C, is successfully measured by using Brillouin optical-fiber time-domain analysis. A temperature measurement accuracy of 3 degrees C with a spatial resolution of 100 m is attained over a fiber length of 1.2 km.

A technique to measure distributed strain in optical fibers

Abstract: The technique is based on Brillouin optical-fiber time-domain analysis (BOTDA), which uses Brillouin interaction between counterpropagating pump and probe light waves. Experimental results for fibers wound on drums at various tensions are presented. A strain measurement accuracy of better than 2*10/sup -5/ and a spatial resolution of 100 m are achieved. >

Spin-wave calculations for multilayered structures.

Abstract: Results are presented on the calculations of spin-wave frequencies in ferromagnetic layers, double layers, and multilayered structures for small, nonzero wave vectors such as can be investigated by, e.g., Brillouin light scattering. The underlying continuum-type magnetostatic theory includes both dipolar and exchange contributions and fully takes into account magnetic surface and interface anisotropies as well as interlayer exchange coupling. For single magnetic layers the detailed influence of surface anisotropies on both film surfaces is studied. For magnetic double layers the interlayer exchange coupling mechanism is investigated. In the case of multilayers consisting of alternating magnetic and nonmagnetic layers, the crossing regime between dipolar and exchange modes shows a strong dependence of the gap width on the amount of interface anisotropy. For small layer thicknesses the interlayer exchange coupling shifts the spin-wave frequencies of all but the highest-frequency dipolar modes into the exchange-mode regime. In the case of all-magnetic multilayered structures, a new type of collective spin-wave excitations arising from coupled exchange modes is predicted.

High-resolution angle-resolved photoemission study of the Fermi surface and the normal-state electronic structure of Bi2Sr2CaCu2O8

Abstract: High-resolution angle-resolved photoelectron spectroscopic measurements were made of the Fermi edge of a single crystal of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} at 90 K along several directions in the Brillouin zone. The resultant Fermi-level crossings are consistent with local-density band calculations, including a point calculated to be of Bi-O character. Additional measurements were made where bands crossed the Fermi level between 100 and 250 K, along with measurements on an adjacent Pt foil. The Fermi edges of both materials agree to within the noise. Below the Fermi level the spectra show correlation effects in the form of an increased effective mass, but the essence of the single-particle band structure is retained. The shape of the spectra can be explained by a lifetime-broadened photohole and secondary electrons. The effective inverse photohole lifetime is linear in energy.

Thermal effects on the Brillouin frequency shift in jacketed optical silica fibers

Abstract: Brillouin frequency shift in two kinds of jacketed optical single-mode silica fiber, with an ultraviolet curable resin coat and a nylon coat, has been measured at temperatures ranging from -30 to +60 degrees C. It has been found that there are two reasons for the Brillouin frequency shift change in jacketed optical fibers against temperature change. One is the Brillouin frequency shift change for bare fibers. The other is the thermal stress due to the differences in thermal expansion coefficients in bare fiber and its coating material.

Microscopic theory of intervalley scattering in GaAs : K dependence of deformation potentials and scattering rates

Abstract: The ‘‘rigid‐pseudoion’’ model is applied to intervalley scattering processes in GaAs The intervalley deformation potentials (IDPs) that we obtain at high‐symmetry points are in good agreement with previous calculations We find that the IDPs show a strong dependence on the wave vector of the intervalley phonon, therefore a numerical integration over the Brillouin zone (eg, with the tetrahedron method) is necessary to obtain realistic scattering rates that can be compared with those obtained from experiments We calculate the lifetimes of electrons at the L and X valleys as a function of temperature (L: 22±05 ps; X: 130±20 fs at room temperature) and discuss our results in comparison with recent ultrafast laser experiments and Monte Carlo simulations Finally, the IDPs show an anisotropy that might be important when simulating electrical transport in hot‐electron devices

Determination of the bulk band structure of Ag in Ag/Cu(111) quantum-well systems

Abstract: Ag is grown epitaxially on Cu(111) to form quantum wells. The resulting quantum-well states and resonances, observed with angle-resolved photoemission, exhibit shifts in energy for varying emission directions and for changing Ag-film thicknesses. The results are utilized in a determination of the Ag bulk sp-band dispersion relation near the L point in the Brillouin zone. Effective masses and the Fermi surface near the L point are deduced. The Fermi surface agrees well with that obtained earlier from de Haas--van Alphen measurements.

A study of the structural phase transitions in AlF3: X-ray powder diffraction, differential scanning calorimetry (DSC) and Raman scattering investigations of the lattice dynamics and phonon spectrum

Abstract: The cubic-rhombohedral phase transition at 450 degrees C of AlF3 is studied by DSC, X-ray powder diffraction and Raman scattering. It is demonstrated that the transition is of first order with a hysteresis of about 6 degrees. It is established by X-ray powder diffraction patterns that the room temperature space group is R3c. A temperature study of the Raman scattering spectra (that confirms the above conclusion) evidences the presence of two soft modes. It is shown from group theory that the transition can be imputed to the condensation of the R5 mode of the cubic Brillouin zone and the attribution of the Raman lines is deduced on the basis of the compatibility diagram between the cubic and rhombohedral symmetries. The frequencies of the Raman lines are used to adjust the parameters of a rigid ion model and to calculate the phonon spectrum in the cubic phase. The calculated phonon density of states appears to be strongly dependent on the soft phonon frequency.

Thermal effects of Brillouin gain spectra in single-mode fibers

Abstract: Brillouin gain spectra in two 250-m-long single-mode fibers with GeO/sub 2/-doped core/pure-silica cladding (fiber A) and pure-silica core/F-doped cladding (fiber B) were measured at temperatures ranging from -40 to +60 degrees C at a wavelength of 1.32 mu m. The temperature coefficients of Brillouin frequency shift were found to be 1.17 and 1.33 MHz/ degrees C for fibers A and B, respectively. Temperature coefficients of Brillouin gain bandwidth were found to be -0.12 and -0.10 MHz/ degrees C. These measurements provide useful information for applications of stimulated Brillouin scattering. >

A normal vibrational analysis of syndiotactic polystyrene

Abstract: A normal vibrational analysis has been performed for the all-trans conformation of syndiotactic polystyrene. Observed infrared and Raman data, including polarization characteristics, have been satisfactorily assigned. Polarized infrared data show that coextruded samples have nearly perfect orientation of the trans sequences with the plane of the rings normal to the chain axis. The orientation is seen to increase upon annealing at 200°C due to an increase in crystallinity. A theoretical modulus of 67 GPa is calculated from the slope of the dispersion curve of the longitudinal acoustic vibration near the Brillouin zone center

Chapter 2 Effects of Homogeneous Strain on the Electronic and Vibrational Levels in Semiconductors

Abstract: Publisher Summary This chapter describes the effects of homogeneous strain on the electronic and vibrational levels in semiconductors. It reviews the effects of homogeneous strains on the electronic states of the highest valence and lowest conduction bands of diamond- and zincblende-type materials, focusing on the band extrema at the center of the Brillouin zone (BZ). Electronic deformation potentials for a number of relevant bands are summarized in the chapter. The effects of an external stress on the quantum levels of quantum wells and the properties of strained-layer superlattices with built-in strain are discussed in the chapter. The influence of a homogeneous strain on the optic phonons at the BZ center of certain materials is described and also for this situation, vibrational deformation potentials are listed in the chapter. The introduction of a homogeneous strain in a solid produces changes in the lattice parameter and, in some cases, in the symmetry of the material. These in turn produce significant changes in the electronic band structure and vibrational modes. All configurations of homogeneous strain can be divided into the following two contributions: the isotropic or hydrostatic components, which give rise to a volume change without disturbing the crystal symmetry, and the anisotropic component, which in general reduces the symmetry present in the strain-free lattice.

Direct determination of III-V semiconductor surface band gaps

Abstract: Determination des bandes interdites de surface entre l'etat rempli de liaison pendante de l'anion (A 5 ) et de l'etat vide du cation (C 3 ). On se place aux points Γ, X', X et M de la zone de Brillouin de surface. Comparaison des resultats avec les excitations optiques de surface et les donnees de perte d'energie electronique

Optical-phonon modes in a double heterostructure of polar crystals

Abstract: The equation of motion for the polarization vector for a double heterostructure of polar crystals is solved exactly within the framework of the continuum model. There exist only two types of phonon modes, the interface modes and the confined bulk modes, whose eigenvectors are obtained explicitly. Dispersion relations are derived analytically for the interface modes, while the confined bulk modes are dispersionless, a fact consistent with the model. It is also found that in Raman scattering experiments the symmetric interface modes are predominantly longitudinal optical (LO) and the antisymmetric interface modes transverse optical (TO). In the central region of the Brillouin zone, however, they both split into two branches oscillating at LO and TO frequencies, respectively. Possible reinterpretation of various experiments is briefly discussed.

Special points in the reciprocal space of an icosahedral quasi-crystal and the quasi-dispersion relation of electrons

Abstract: It is shown that there exist special points in the reciprocal space of an icosahedral quasi-lattice; they correspond to high-symmetry points in the Brillouin zone of a periodic lattice. The translationally equivalent special points are distributed quasi-periodically with different intensities in the reciprocal space. On the other hand, electronic wavefunctions of the icosahedral quasi-lattice are investigated using a numerical method based on the tight-binding model. Their Fourier spectra are mapped in the energy versus wavenumber plane along several axes in reciprocal space. Dispersion-relation-like patterns are observed. It is found that critical points (stationary points) of the quasi-dispersion relation appear at the special points. The quasi-dispersion relation recurs quasi-periodically all over reciprocal space.

Lattice dynamics of cupric oxide

Abstract: By inelastic neutron scattering we have measured phonon dispersion curves of CuO in several directions. The phonon data are well reproduced by a 22 parameter rigid ion model. Various models with reduced numbers of fit parameters are described.

Direct Observation of Spin-Splitting of the Shubnikov- de Haas Oscillations in a Quasi-Two-Dimensional Organic Conductor (BEDT-TTF)2KHg(SCN)4

Abstract: A clear splitting in the Shubnikov-de Haas oscillation has been directly observed for the first time in a quasi-two-dimensional (2D) organic conductor (BEDT-TTF) 2 KHg(SCN) 4 . Analysis of the oscillations yields a 2D nature of the cylindrical Fermi surface, corresponding to 16% of the first Brillouin zone, with a small warping along the direction normal to the 2D layer. The condition for observation of spin-splitting and the evolution of the splitting pattern with a tilted magnetic field are discussed.

Brillouin and raman scattering from glasses under high pressure

Abstract: Brillouin and Raman Scattering Spectra in SiO2 and GeO2 glasses have been measured in a diamond anvil cell up to pressures of 14 GPa. The elastic properties and equation of state for each glass type were obtained from the Brillouin scattering measurements with respect to pressure. Both elastic constants and compressibility of SiO2 and GeO2 showed anomalous behavior with respect to pressure. This anomalous behavior is reconciled with a model based on the pressure dependent bending of the oxygen angles in both glass types. The Raman measurements corroborate the conclusions from the Brillouin scattering results, namely that the SiO2 and GeO2 bond angles are changing with pressure or the oxygen angle distribution is changed without bond breaking.

Electrons in Metals and Semiconductors

Abstract: 1 The free-electron model.- 1.1 The classical Drude theory.- 1.2 Fermi-Dirac statistics.- 1.3 The Sommerfeld model.- 1.4 The density of states.- 2 Properties of free-electron solids.- 2.1 The electronic heat capacity.- 2.2 The magnetic susceptibility.- 2.3 Transport properties.- 2.4 Hall effect and magnetoresistance.- 2.5 Relaxation effects: high-frequency conductivity.- 2.6 Metals and semiconductors.- 3 Crystal structures and the reciprocal lattice.- 3.1 Crystal structures.- 3.2 The reciprocal lattice.- 4 Electrons in a periodic potential.- 4.1 Bloch's theorem.- 4.2 The Brillouin zone.- 4.3 Nearly free electrons.- 5 Electronic band structures.- 5.1 The band structure of real metals.- 5.2 Metals, semiconductors and insulators.- 5.3 Density of states and heat capacity.- 6 The potential V(r) many-body effects.- 6.1 The one-electron approximation and the choice of V(r).- 6.2 Many-body effects.- 7 The dynamics of Bloch electrons.- 7.1 The velocity and the Lorentz force.- 7.2 Orbits in a magnetic field.- 7.3 Orbit quantization.- 7.4 The de Haas-van Alphen effect.- 8 Collisions.- 8.1 Scattering by static defects.- 8.2 Phonon scattering.- 8.3 Relaxation times and mean free paths.- 9 Electrical conductivity of metals.- 9.1 The basic expression for ?.- 9.2 Temperature dependence of ?.- 9.3 Matthiessen's rule.- 9.4 The Kondo effect.- 10 Metals in a temperature gradient.- 10.1 Thermal conductivity.- 10.2 Thermoelectric effects.- 11 Magnetoresistance and Hall effect.- 11.1 Basic ideas: the free-electron model.- 11.2 Real metals.- 12 Radio-frequency, optical and other properties.- 12.1 Radio-frequency properties.- 12.2 Optical and other properties.- 13 Carriers in semiconductors.- 13.1 The number of carriers.- 13.2 Donors and acceptors.- 13.3 Carrier mobilities and positive holes.- 14 Transport properties of semiconductors.- 14.1 Scattering.- 14.2 Simple transport properties.- 14.3 Cyclotron resonance and optical properties.- 14.4 Orbit quantization and the quantum Hall effect.- 15 Other topics.- 15.1 Charge density waves.- 15.2 Disordered alloys.- 15.3 Localization.- Problems.- Answers to problems.- Further reading.

Magneto-optics near the L point of the Brillouin zone in semimagnetic semiconductors.

Abstract: Utilisation d'une analyse de theorie de groupe pour exprimer les decompositions de Zeeman des transitions optiques interbandes a un point critique Λ ou L en termes d'interactions d'echange des electrons Mn−d et les electrons de bande

Some new aspects of first-order displacive phase transformations: Martensites

Abstract: Over recent years new experimental information from X-ray and neutron scattering, electron diffraction and imaging, computer simulation, and first-principles electronic structure calculations have greatly increased the amount of information we have on the microscopic mechanisms of structural transformations. In this paper, a particular class of martensitic transformation which exhibits both “soft modes” (out in the Brillouin zone, q ≠ 0) and elastic instabilities are discussed. The intent is to provide a heuristic review of some background in phonon theory, symmetry constrained elastic distortions and a pedagogical statistical mechanical model which is appropriate to NiAl and FePd and similar alloy transformations.

Stimulated Brillouin scattering gain variation and transient effects in a CH 4 : He binary gas mixture

Abstract: Experimental measurements are presented that show that the stimulated Brillouin scattering gain of methane, and hence the nature of the competition between stimulated Brillouin scattering and stimulated Raman scattering, can be systematically varied by the addition of helium. The gain variation is explained in terms of a transient gain model in which the dominant effect of the helium is to increase the Brillouin linewidth (acoustic attenuation). Broadening mechanisms are discussed. At methane and helium partial pressures of 19 and 41.8 kTorr (25 and 55 atm), respectively, backward stimulated Raman scattering dominates stimulated Brillouin scattering when the 532-nm pump energy exceeds 160 mJ.

Scattered light moire-brillouin gyroscope

Abstract: A scattered light Moire-Brillouin guided wave gyroscope includes a coherent light source for providing two counter-rotating primary waves; an optical waveguide, responsive to the two primary waves, for generating a Moire fringe from a primary fringe and a Brillouin fringe derived respectively from the counter-rotating primary waves and from the counter-rotating Brillouin waves produced from the primary waves; and means for detecting the rotation of the Moire fringe relative to the waveguide.

A quantum-transport model for semiconductors : the Wigner-Poisson problem on a bounded Brillouin zone

Abstract: — We analyse a quantum-mechanical model for the transport of électrons in semiconductors. The model consists of the quantum Liouville ( Wigner) équation posed on the bounded Brillouin zone corresponding to the semiconductor crystal lattice, with a self-consistent potential determined by a Poisson équation. A global existence and uniqueness proof for this model is the main resuit of the paper. Résumé. — Nous présentons et analysons un modèle quantique de transport des électrons dans un semiconducteur. Le modèle est constitué de l'équation de Liouville quantique (ou équation de Wigner), posée sur un domaine borné en vitesse correspondant à la zone de Brillouin du semiconducteur, couplée à un potentiel déterminé par une équation de Poisson. Dans cet article, nous prouvons l'existence globale et l'unicité des solutions pour ce modèle.

First measurement of strain distribution along field-installed optical fibers using Brillouin spectroscopy

Abstract: The strength of optical fibers is lowered by stress applied to them especially at the point of maximum stress. Hence, it is necessary to measure, not only the average strain value along the whole fiber length(1), but also its spatial distribution, in order to guarantee the long term reliability of optical transmission lines(2). This is the first paper to report on the strain distribution in optical fibers measured using Brillouin Optical-Fiber Time Domain Analysis (BOTDA) (3)(4) both before and after installation into the field conduit.