Showing papers on "Brillouin zone published in 1992"

Photonic bands: Convergence problems with the plane-wave method.

Abstract: The problems associated with the poor convergence of the Fourier transform of the hard-sphere dielectric function are discussed. A significant band gap between the eighth and ninth levels has been found for air spheres in fcc. We also consider a periodic array of Gaussian spheres, which converges well and allows a consistent and reliable determination of the general features of photonic bands. It is found that when \ensuremath{\epsilon}(r) is sharply peaked, photonic levels become almost degenerate throughout the Brillouin zone, corresponding to standing waves.

Scattering of electromagnetic waves by periodic structures

Abstract: The authors consider periodic structures made of spheres embedded in a host material with a different dielectric function. They show how to calculate the reflection and transmission of electromagnetic waves by a slab of the material parallel to a given crystallographic plane. The method of calculation is based on a doubling-layer scheme which obtains the reflection and transmission matrix elements for the multilayer from those of a single layer. The reflection and transmission characteristics of the slab are related to the complex band structure of the photon field associated with the given crystallographic plane of the corresponding infinite crystal, which is introduced in the manner of the low-energy electron diffraction theory. They present numerical results which demonstrate the applicability of the method to real systems of current interest and point out some interesting physics which arose from their calculations. They show in particular that the nondegenerate bands of the photon field at the centre of the surface Brillouin zone do not couple to the incident radiation, leading to total reflection at normal incidence.

Ab initio calculation of force constants and full phonon dispersions.

Abstract: We present a method to calculate the full phonon spectrum using the local-density approximation and Hellmann-Feynman forces. By a limited number of supercell calculations of the planar force constants, the interatomic force constant matrices are determined. One can then construct the dynamical matrix for any arbitrary wave vector in the Brillouin zone. We describe in detail the procedure for elements in the diamond structure and derive the phonon dispersion curves for Si. The anharmonic effects can also be studied by the present method.

Remote spectroscopy for raman and brillouin scattering

Abstract: An apparatus and method for performing Raman and Brillouin spectroscopy remotely. A near-infrared laser is used to irradiate a sample of material to be analyzed. Optical fibers transmit incident radiation from a near-infrared radiation source to the sample, and transmit Raman and Brillouin scattered radiation from the sample to the detecting equipment. The incident radiation is carefully determined so as to avoid fluorescence and to limit optic fiber losses. The invention provides useful information with the use of an interferometer.

Electrical conductivity of metallic Si:B near the metal-insulator transition

Abstract: The conductivity has been measured between 55 mK and 4.2 K in zero field and in magnetic fields up to 7.5 T of a series of uncompensated {ital p}-type Si:B samples with dopant concentrations near the critical concentration for the metal-insulator transition. Acceptor wave functions, which are derived in silicon from the degenerate light- and heavy-hole {ital J}=3/2 valence-band maxima at {ital k}=0 and a spin-orbit-split {ital J}=1/2 band, are quite different from donor wave functions associated with the six degenerate conduction-band minima at different equivalent points in the Brillouin zone. Despite this, the conductivity of Si:B is found to be quite similar in many ways to that of Si:P. The critical conductivity exponent for Si:B is close to 1/2 as in Si:P and Si:As, rather than having the expected value of 1. The correction to the zero-temperature conductivity arising from electron-electron interactions is comparable in size, and the temperature dependence of the conductivity in various fixed magnetic fields is also found to be quite similar. For the range of dopant concentrations and experimental parameters of these investigations, the only important experimental difference between the two materials is the sign and size of the magnetoresistance. In contrast with Si:P, whichmore » has both positive and negative components, the magnetoresistance of Si:B is positive for all temperatures and magnetic fields studied. We attribute this to the strong spin-orbit scattering in {ital p}-type silicon associated with the degenerate valence bands.« less

First principles analysis of vibrational modes in KNbO3

Abstract: The vibrational modes in KNbO3 are analyzed using first principles total-energy calculations based on local density functional theory. Normal mode frequencies and eigenvectors are determined for the cubic perovskite structure with wave vectors at the center (Γ) and corner (R) of the Brillouin zone using the frozen-phonon technique. The frequencies are in good agreement with available experimental results with the possible exception of the lowest-frequency zone-center (ferroelectric) mode. Our calculations give a ferroelectric mode which becomes unstable with increasing volume. However, at the equilibrium volume it is stable with a frequency of ∼100 cm−1.

Electronic structure and stability of quasicrystals: Quasiperiodic dispersion relations and pseudogaps.

Abstract: We present calculations of the electronic structure for higher-order rational approximations to quasicrystals up to 12 380 atoms in the unit cell, based on a tight-binding linear-muffin-tin-orbital technique. The dispersion relations of the electronic states of quasicrystals are related to those of quasiperiodic free-electron model. Highly degenerate free-electron states are found close to the Fermi level. Their wave vectors define the analog of a Brillouin zone; the ``Brillouin-zone''--Fermi-sphere interaction leads to the formation of a pseudogap at the Fermi level.

Polarization waves and van der Waals cohesion of C60 fullerite.

Abstract: An approach based on the concept of polarization waves is developed for the computation of the van der Waals cohesive energy of ${\mathrm{C}}_{60}$ fullerite. The dynamical polarizability properties of the ${\mathrm{C}}_{60}$ molecule are simulated by an empirical model consisting of a spherical dielectric shell of finite thickness. The shell material is an isotropic continuum with a dielectric function designed to exhibit the large \ensuremath{\sigma}-plasmon resonance observed in all forms of solid carbon in the vacuum ultraviolet around 20--30 eV. The eigenmodes of the polarization fluctuations on the hollow-molecular model consist of multipolar surface plasmons of angular momentum l. In the solid, the quasistatic interaction between the surface plasmons on different molecules creates multipolar polarization waves. Their dispersion relations are calculated throughout the first Brillouin zones of the fcc and hcp lattices, incorporating high values of the angular momentum. Strong mixing of the successive angular momenta occurs, particularly for low l values. The attractive part of the cohesive energy of the crystal is obtained from the zero-point energy of the polarizaton waves. With values of the shell parameters adjusted to reproduce ultraviolet-spectroscopic data, the van der Waals binding energy is found to be of order 2 eV per molecule and depends rather sensitively on the assumed static polarizability of the shell. No significant difference is found between the cohesion of the two fcc and hcp compact lattices at any multipolar order. The exact polarization-wave cohesive energy is compared to the approximate result obtained by summing the pairwise-additive attractions between two identical molecular shells. The comparison indicates that the multipolar terms of higher l's are crucial in both computations and, as a consequence, that the many-body, nonadditive terms make a relatively weak contribution to the cohesion. The repulsive part of the interactions in the close-contact areas between molecules, which is to be added to the present van der Waals energy, is responsible for the lattice stability and for the orientational ordering of the molecules at low temperature.

Stimulated Brillouin threshold dependence on fiber type and uniformity

Abstract: The effects of fiber uniformity on stimulated Brillouin scattering thresholds have been measured, and the theory for stimulated Brillouin scattering in uniform fibers has been extended to the case of nonuniform fibers. This theory enables the, design of long fiber spans with ten times higher stimulated Brillouin scattering thresholds. >

Temperature-dependent conduction-band exchange splitting in ferromagnetic hcp gadolinium: Theoretical predictions and photoemission experiments.

Abstract: Angle-resolved photoemission is used to determine the temperature-dependent electronic properties of ferromagnetic bulk Gd along Γ-A of the three-dimensional Brillouin zone. The Δ 2 band exchange splitting (0.85 eV) and dispersion (0.5 eV) are in reasonably good agreement with self-consistent local spin-density approximation calculations. The conduction-band magnetic exchange splitting vanishes at the Curie temperature following a conventional power law with a Heisenberg critical exponent

High-Pressure Brillouin Studies and Elastic Properties of Single-Crystal H2S Grown in a Diamond Cell.

Abstract: High-pressure Brillouin spectra of crystalline hydrogen sulfide (H2S) have been measured at up to 7 gigapascals at room temperature. The best fit of the angular dependence of Brillouin acoustic velocities between experimental values and calculations based on Every9s expression for elastic waves of an arbitrary direction yielded the orientation of an H2S cubic crystal grown in the diamond-anvil high-pressure cell. In situ determinations of sound velocities, as a function of pressure, could be made for any direction, the refractive index, the density, and the elastic constants. This method provides a means for the systematic study of elastic properties and phase transitions of condensed gases under ultrahigh pressures.

Applications of Brillouin cells to high repetition rate solid-state lasers

Abstract: A review is given of possible applications for Brillouin cells in solid-state lasers operating at higher pulse repetition frequencies. Brillouin cells are used to reduce beam divergence, improve intensity distribution in the near field, and provide phase locking of light beams pulse compression, and pulse shape control. >

Collective excitations in liquid methanol: A comparison of molecular, lattice‐dynamics, and neutron‐scattering results

Abstract: The collective dynamics of liquid methanol‐d4 is studied by means of molecular‐dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center‐of‐mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron‐scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice‐dynamics calculations for the polycrystalline low‐temperature α phase.

Surface-grating-induced zone folding and hybridization of surface acoustic modes.

Abstract: Brillouin light scattering measurements of surface acousta modes propagating in an ion-milled holographic grating on a Si(001) wafer have revealed clear evidence of hybridization between the Rayleigh wave and the longitudinal resonance which leads to an unexpected gap Other new acoustic features include the folding of the Rayleigh wave about the first two zone boundaries associated with the grating, band gaps which increase with zone number, and different propagation velocities parallel and perpendicular to the grating

Magnetic circular dichroism of zinc-blende-phase MnTe.

Abstract: Temperature dependence of the magneto-optic effect, i.e., magnetic-circular-dichroism (MCD) spectra, of metastable zinc-blende MnTe films has been investigated. MCD peaks from the Γ point of the Brillouin zone as well as from the Mn d-d intra-atomic tranistions were observed. Below 70 K the energy of the Γ peak showed a distinct magnetic blueshift accompanying a rapid increase of the MCD intensity. These anomalies reflect the first-order phase transition into the type-III antiferromagnetic-ordering state of a frustrated Heisenberg antiferromagnet

Stimulated brillouin scattering in the presence of external feedback

Abstract: We investigate theoretically and experimentally the process of stimulated Brillouin scattering (SBS) in the presence of weak feedback in a single-mode optical fiber. The threshold for the occurrence of Brillouin oscillation can be considerably lower than the threshold for normal SBS. The spectrum of the emitted Stokes light exhibits an extreme narrowing near and above the oscillation threshold. The Stokes intensity is found to exhibit various types of periodic and stable behavior.

Topological invariant and the quantization of the Hall Conductance

Abstract: The topological aspects of wavefunctions for electrons in a two dimensional periodic potential with a magnetic field are discussed. Special attention is paid to the linear response formula for the Hall conductance cxY. It is shown that the quantized value of gxY is related to the number of zeros of wavefunctions in the magnetic Brillouin zone. A phase of wavefunctions cannot be determined in a unique and smooth way over the entire magnetic Brillouin zone unless the magnetic subband carries no Hall current.

A general method for determination of Brillouin linewidths by correction for instrumental effects and aperture broadening : application to high-pressure diamond anvil cell experiments

Abstract: A general method for determining true acoustic‐mode linewidths from Brillouin scattering data is presented. The method is specifically applied to diamond anvil cell experiments to obtain accurate hypersonic attenuation data at high pressure. This work was motivated by the noticeable lack of acoustic attenuation data at high pressure in the literature and by our own attempts to obtain relaxation data from Brillouin experiments in the diamond anvil cell. A detailed discussion of both instrumental and finite aperture contributions to the measured acoustic‐mode linewidth is given, as well as specific algorithms for calculating these effects. Fits to Brillouin scattering spectra obtained at high pressures in different organic liquids are shown. Finally, a discussion is given of experimental details for obtaining accurate Brillouin linewidths in high‐pressure diamond anvil cell experiments.

Magnon spin resonance in the Haldane spin chains of Ni(C2H8N2)NO2ClO4.

Abstract: We present a high-frequency electron spin resonance study of the magnetic transitions occurring between excited states in a Haldane spin system. We show that these transitions are induced between the low-energy magnon states, which develop at the boundary (q\ensuremath{\approxeq}\ensuremath{\pi}) of the Brillouin zone. Our study provides therefore, the first evidence that, in the Haldane phase, the excitations associated with the quantum gaps correspond actually to a s=1 magnetic state.

Symmetric conservative form of low-temperature phonon gas hydrodynamics. II : Equations of heat transport and thermal waves in the case of linear isotropic approximation of phonon frequency spectrum

Abstract: In part I of this series the equations of low-temperature phonon gas hydrodynamics based on the drifting approximation of phonon distribution were rearranged to the form of symmetric conservative system and the local properties of this system were determined. At low temperatures, only the phonon modes of relatively short wave vectors (located near the centre of the first Brillouin zone) are appreciably excited. For these modes, the isotropic approximation of the phonon frequency spectrum commonly applied and the domain of phonon wave vectors is approximated by the whole space. In this paper, these additional approximations are taken into account and the corresponding symmetric conservative system of phonon gas hydrodynamics equations is discussed

Interface effects and elastic constants of Ag/Ni superlattices studied by Brillouin scattering

Abstract: The elastic properties of sputter‐deposited Ag/Ni superlattices have been studied by means of the Brillouin light scattering technique. Measurements have been taken on both free‐standing and glass‐supported superlattices, the former deposited at 100 K, the latter at 300 K. The two effective elastic constants c11 and c44 have been determined on specimens 5 μm thick for different superlattice periods; c44 decreases by almost 40% as the bilayer periodicity decreases from 19 to 1.3 nm, while c11 shows no appreciable variation. Measurements performed on supported specimens, 0.5 μm thick, enabled us to get information also on the constant c33, which exhibits a monotonic increase with period. The elastic anomalies found can be related to the appreciable lattice distortion observed by x‐ray diffraction experiments. Both Brillouin scattering and x‐ray analysis have shown that the interface quality improves appreciably when the temperature of deposition is lowered from 300 to 100 K.

Anomalies in the TA-Phonon Branches of SrTiO3 in the Quantum Paraelectric Regime

Abstract: Brillouin and neutron-scattering measurements of TA-phonon branches are reported for SrTiO3 at small wave vectors and at temperatures well below the 105 K structural transition. Anomalous softening in the former measurements, and dispersion in the latter, are found in the quantum paraelectric regime. Furthermore, an unexpected loss of scattered intensity is observed on the TA[100]-phonon branch at temperatures centred around 30 to 40 K.

Band structures of nonmagnetic transition-metal oxides: PdO and PtO.

Abstract: The electronic band structures of PdO and PtO are calculated using the augmented-spherical-wave method and the local-density approximation. Our results are consistent with the widely held view of these materials as conventional band insulators with the crystal-field splitting of metal d states primarily responsible for gap formation. A significant role for correlation effects as well cannot be ruled out. The predicted valence-band structure for PdO agrees well with published photoemission data. The electronic structure of PtO is qualitatively similar. In both cases the calculated gap is direct and occurs at the M point of the Brillouin zone. The magnitude of the gap is found to be larger in PtO, which we attribute to the more relativistic nature of Pt compared to Pd.

Space group approach to the wavefunction of a Cooper pair

Abstract: The method of construction of the wavefunction of a Cooper pair based on the Pauli exclusion principle and the Mackey-Bradley theorem is developed Tables of symmetrized and antisymmetrized Kronecker squares of single- and double-valued irreducible representations of the groups Oh6 and D6h4 are obtained The tables are used to search for points in the Brillouin zone where totally symmetric Cooper pairs can exist It is shown that, in the symmetrical points and directions in a Brillouin zone, a direct connection between the multiplicity and parity of the Cooper pair wavefunction does not exist

Phonons in icosahedral and cubic Al-Li-Cu.

Abstract: We describe the results of inelastic-neutron-scattering measurements on single grains of icosahedral Al-Li-Cu and the related cubic R phase. Measurements along the high-symmetry twofold, threefold, and fivefold axes of the icosahedral phase clearly show the existence of well-defined propagating modes, with isotropic dispersion, near strong Bragg peaks. The spectral weight of the acoustic phonons scale linearly with the integrated intensity of the associated zone centers. In several cases we are able to identify ``quasi''-Brillouin-zones, with stationary points at the ``special'' high-symmetry points of a simple icosahedral lattice. While the phonon dispersion for transverse and longitudinal acoustic modes along the twofold axis of the cubic R phase is quite similar to that found for the icosahedral phase, differences are found in the higher-energy modes; the behavior of the higher-energy modes of the icosahedral phase is consistent with an enhanced degree of spatial localization for these excitations.

Ruderman-kittel-kasuya-yosida exchange interaction in many-valley IV-VI semimagnetic semiconductors

Abstract: The Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect-exchange interaction via free carriers is analyzed in the case of IV-VI semimagnetic semiconductors (diluted magnetic semiconductors) Carriers responsible for the RKKY interaction in these materials originate from the anisotropic band of heavy holes located at the \ensuremath{\Sigma} point of the Brillouin zone (ie, there are 12 equivalent valleys of this band) Both intervalley and intravalley electron processes contribute to the exchange coupling Calculations of the RKKY exchange integral and the paramagnetic Curie temperature (FTHETA) are presented The exchange integral is anisotropic, and its dependence on the interspin distance is significantly modified in such a way that the role of antiferromagnetic couplings is increased As a consequence, the value of FTHETA is reduced