Lattice dynamics of cubic PbTiO3 by inelastic neutron scattering

High-temperature dispersion relations of the phonon modes in a cubic PbTiO₃ single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T _c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed.     

Dispersion relations of hybrid waves in dielectric media

The dispersion relations of hybrid waves—polaritons, scalar bosons (hiddennons) and pseudoscalar bosons (axinons)—in one-dimensional dielectric media are derived with allowance for the resonant interaction between mechanical excitations of crystal lattices and elementary excitations of vacuum. It is shown that in the range of small wave vectors hybrid quasi-particles are formed in dielectric media: polaritons, hiddennons and axinons. For sodium nitrite crystal, dispersion relations of such hybrid quasiparticles are calculated near the center of the Brillouin zone. The resulting dispersion relations of hybrid quasi-particles provide conditions that allow observation of conversion of vacuum photon-like particles into hybrid quasi-particles in dielectrics, and also of parametric light scattering accompanied by the generation of hybrid quasi-particles.     

Nonrelative dispersion relation in crystals with a diamond structure

The dispersion equations introduced in a previous paper are solved and the analytical expression of the dispersion relations thus obtained are investigated. The latter is in fair agreement with experiment for the heavy and light hole bands in the <100>, <110> directions and for electrons of the lower conduction band in the <100> directions. An iteration procedure is proposed for the solution of the dispersion equations in arbitrary directions of the wave vector up to the Brillouin zone boundary.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–6, July, 1982.     

Spatial–frequency selection of magnetostatic waves in a two-dimensional magnonic crystal lattice

Features of the propagation of magnetostatic waves in a tangentially magnetized magnonic crystal structure based on iron–yttrium garnet with a two-dimensional array of grooves on the surface are studied. Numerical simulation is performed by the finite element method and the dispersion characteristics and the spatial distribution of fields of eigenmodes of surface magnetostatic waves propagating in this structure are calculated. The characteristics of waves in the magnonic crystal structure are experimentally studied by means of Brillouin scattering. It is shown that the formation of waveguide channels is possible when the frequency of the input signal is close to the frequency of the band gap of the structure.     

Properties of optical and acoustic phonons in diamond crystals

The dependence of the energy on the quasi-momentum of optical and acoustic phonons in diamond-type crystals is analyzed. It is shown that the dispersion relation of optical branches near the Brillouin zone center in such crystals can be derived based on the modified Klein-Gordon equation corresponding to quasiparticles (optical phonons) with negative effective mass. Analytical expressions are derived for dispersion curves of acoustic and optical branches in the entire Brillouin zone, based on the sinusoidal approximation. The obtained analytical dependences of dispersion relations are compared to the experimental results of the study of inelastic neutron scattering spectra in diamond crystals. The law of the coordinate and time transformation for optical phonons with negative effective rest mass when going to the new inertial reference frame, similar to Lorentz transformations in relativistic mechanics, is determined.     

Phonon dispersion in aluminium arsenide and antimonide

The phonon dispersion curves for aluminium arsenide and antimonide have been investigated by using a deformation bond approximation model. The results obtained from this model are compared with the experimental values wherever it is available. Since there is no complete experimental phonon dispersion curves for AlAs, we could not compare our calculated results, but the results of AlSb have been compared with the inelastic neutron scattering measurements at 15 K. However, we compare the phonon frequencies of AlAs and AlSb at critical points of the Brillouin zone obtained by our calculations and Raman spectroscopy measurements. This model predicts the phonon modes satisfactorily in all the symmetry directions of the Brillouin zone (BZ). The spectrum has similar features as observed in other III–V compound semiconductors.     

Mixed acoustic phonons and phase modes in an aperiodic composite crystal

Aperiodic crystals which are long range ordered materials present original dynamics features due to the lack of translational symmetry formally implying the nonvalidity of the Brillouin zone concept. This Letter reports the observation by neutron scattering of an overdamped acousticlike mode at a Bragg peak position in a n-alkane-urea inclusion crystal. This result implies the existence of a gap in the dispersion branch. The gap and anomalous damping of these collective modes are discussed in terms of specific dynamics and interaction in aperiodic materials.     

Experimental observation of optical bandgaps for surface electromagnetic waves in a periodically corrugated one-dimensional silicon nitride photonic crystal

Dispersion curves of surface electromagnetic waves (SEWs) in 1D silicon nitride photonic crystals having periodic surface corrugations are considered. We experimentally demonstrate that a bandgap for SEWs can be obtained by fabricating a polymeric grating on the multilayered structure. Close to the boundary of the first Brillouin zone connected to the grating, we observe the splitting of the SEW dispersion curve into two separate branches and identify two regions of very low group velocity. The proper design of the structure allows the two folded branches to lie beyond the light line in a wide spectral range, thus doubling the density of modes available for SEWs and avoiding light scattering.     

On interlay between the magnetic susceptibilities of localized and itinerant electrons in hole-doped HTSCs

In the framework of the singlet-correlated motion of holes over oxygen sites in CuO₂ layers, a formula for the dynamic spin susceptibility has been derived taking into account the strong correlation between the magnetizations of the spins of the collective holes and localized moments on copper sites. The calculated behavior of the imaginary part of the susceptibility as a function of the frequency and wave vector is consistent with the available experimental data on the inelastic neutron scattering. The plot of the dispersion of the collective spin modes over the entire Brillouin zone is proposed.     

Gapless spin-1 neutral collective mode branch for graphite

Using the standard tight binding model of 2D graphite with short range electron repulsion, we predict a gapless spin-1, neutral collective mode branch below the particle-hole continuum with energy vanishing linearly with momenta at the Gamma and K points in the Brillouin zone. This spin-1 mode has a wide energy dispersion, 0 to approximately 2 eV, and is not Landau damped. The "Dirac cone spectrum" of electrons at the chemical potential of graphite generates our collective mode, so we call this "spin-1 zero sound" of the "Dirac sea." Epithermal neutron scattering experiments and spin polarized electron energy loss spectroscopy can be used to confirm and study our collective mode.     

Rayleigh scattering-assisted narrow linewidth Brillouin lasing in cascaded fiber

We report a single frequency lasing phenomenon with a narrow linewidth of ～3 kHz in cascaded fiber that is composed of three types of low-loss communication fibers. The Rayleigh scattering of the Brillouin Stokes light created in the middle fiber section along both directions is enhanced by the other two fiber sections. When the Brillouin gain of the middle fiber exceeds the effective loss of the Brillouin stokes light in a roundtrip, a narrow linewidth lasing is observed on the top of the Brillouin spectrum line of the middle fiber. To the best of our knowledge, it is the first report on Rayleigh scattering-assisted Brillouin lasing with single frequency and narrow linewidth in cascaded low-loss communication fibers.     

光纤中基于双布里渊增益线的受激布里渊散射快光

为了解决受激布里渊散射快光在高吸收区产生损耗的问题,通过分析普通单模光纤中双线泵浦产生的双布里渊增益线特性及在增益峰间实现脉冲的超光速传输理论,利用有限元法数值模拟了双布里渊增益线处受激布里渊散射引起的快光特性。结果表明,当频率分离因子大于0.596时,可以观察到双增益峰;当频率分离因子在1~5.25范围内时,两个泵浦波产生的双增益峰之间可以明显地产生快光;当频率分离因子为1.75时,在双布里渊增益线之间的最大时间提前可达25 ps。当频率分离因子为2.42时,三阶色散所对应的归一化色散长度为无穷大,三阶色散可以得到完全补偿;当频率分离因子大于2.464时,脉冲展宽因子趋近于1,可以实现无畸变传输,但时间提前量小于13.52 ps。本文的研究结论对于在布里渊增益区实现快光具有一定的理论意义,并对设计基于受激布里渊散射快光器件具有理论指导作用。     

CuA1S2 crystalline lattice dynamics

Using the model of rigid ions bound by short-distance forces the lattice dynamics of the CuA1S₂ crystal with chalcopyrites structure are investigated. The model parameters are determined from experiments on infrared reflection and combination scattering of light. The phonon spectrum is calculated in the center of the Brillouin zone and in directions of high symmetry. Oscillator amplitudes for frequencies active in IR reflection and components of the static dielectric permeability tensor are calculated.     

Brillouin light scattering on the two quasi-transverse waves in rotated sample of the LiNbO3 crystal

The paper presents results for the Brillouin light scattering experiment, using a LiNbO₃ crystal, and identifies Brillouin scattering switch from one quasi-transverse acoustic branch to the second one during sample rotation. Measured hypersonic frequencies of quasi-transverse acoustic waves observed in different crystallographic directions are examined. A selection rule for the switching phenomenon is described.     

Determination of the lattice vibrations of imidazole by neutron scattering

The external molecular vibrations of deuterated imidazole have been investigated at 12 K for wave vectors along the three principal directions of the Brillouin zone by neutron scattering and by Raman scattering at the zone center. A complete symmetry assignment is obtained. The dominant role of the N-H … N hydrogen bond within the intermolecular interactions is reflected in a relatively simple manner in the shape of the dispersion curves because of the chain-like arrangement of hydrogen bonds. The experiment yields directly a stretching force constant of 0.33 mdyn Å^?1 for this bond. This value is compatible with monopole-dipole and dipole-dipole interactions as major contributions to the attractive interactions of the hydrogen bond of this length (2.86 Å). Relatively strong static monopoles and dipoles are confirmed by a $\text{CNDO}\text{2}$ calculation and are consequently incorporated into the interaction model.     

Low-energy charge-density excitations in MgB2: Striking interplay between single-particle and collective behavior for large momenta

A sharp feature in the charge-density excitation spectra of single-crystal MgB2, displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer (q) along the c* axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-q collective mode residing in the single-particle excitation gap of the B pi bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB2.     

Transient effects in stimulated light scattering

Transient stimulated scattering of light is treated numerically for situations with arbitrarily high conversion of laser into scattered light. New, transient effects are predicted and will be dicussed for scattering in the backward and in the forward directions. The computations are found to be in good agreement with time-resolved, experimental studies of stimulated Brillouin scattering in several liquids.     

Optical sum rules that relate to the potential energy of strongly correlated systems

A class of sum rules for inelastic light scattering is developed. We show that the first moment of the nonresonant response provides information about the potential energy in strongly correlated systems. The polarization dependence of the sum rules provides information about the electronic excitations in different regions of the Brillouin zone. We determine the sum rule for the Falicov-Kimball model, which possesses a metal-insulator transition, and compare our results to the light scattering experiments in SmB(6).     

基于光相干外差检测的布里渊散射DOFS的研究

对布里渊散射分布式光纤传感器(DOFS)检测原理进行了分析, 针对布里渊散射光信号的特点, 应用光相干和外差技术来检测布里渊散射光信号. 具体采用微波电光调制产生频率可调的参考光, 和布里渊散射光进行相干检测, 根据布里渊频移特性取出布里渊散射光信号, 从而得到分布式传感信号. 分别实现了25 km光纤的分布式温度和应变传感, 达到3℃的温度分辨率、100 με的应变分辨率和10 m的空间分辨率.     

微波电光调制的布里渊散射分布式光纤传感技术

在分析布里渊散射分布式光纤传感器检测原理的基础上,针对布里渊散射光信号比较微弱且和瑞利散射光之间存在频移的特点,采用微波电光调制产生频率可调的参考光,和后向布里渊散射光进行相干检测,得到放大的布里渊散射光信号,再用高速模拟数字转换和存储叠加进行时序信号处理,最后得到分布式传感信号。给出了各部分实验过程和结果,得出与理论分析一致的布里渊散射光频谱和光强随温度升高而产生变化。实现了25km的分布式温度传感,初步实验结果证明了方案的可行性。