Search for inelastic scattering of thermal neutrons by planar magnetic excitations

The experimental technique of searching for high-frequency (～10¹³ Hz) surface magnetic excitations by neutron reflectometry is considered. The first experimental results are reported.     

Raman scattering in sillenite-type crystals

The Raman spectra of Bi₂₄AlPO₄₀ and Bi₃₈ZnO₅₈ crystals have been studied for the first time. The polarization Raman spectra of Bi₁₂GeO₂₀, Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ crystals are also studied. The lines in the Raman spectra of Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ are identified and compared with the lines of the Bi₁₂GeO₂₀ spectra studied earlier. It is shown that the differences in the Raman spectra of the crystals studied are associated with the specific features of their atomic structures.     

Sugar bioprotective effects on thermal denaturation of lysozyme: Insights from Raman scattering experiments and molecular dynamics simulation

The bioprotective properties of trehalose, sucrose and maltose on lysozyme protein are studied using Raman spectroscopy and molecular dynamics simulation. The temperature dependencies of the α-helical unfolding processes are determined in presence of different sugars and at various concentrations using the amide I band. The energies of stabilization and acting forces are discussed. The problem of the threshold sugar concentration needed to protect proteins is addressed. The results point out that the three sugars exhibit very similar bioprotective behaviors in the investigated temperature and concentration ranges and suggest that the difference between their respective effects arise mostly from the different denaturated states of lysozyme in the three sugar solutions. Indeed, trehalose gives rise to a significantly more important shift of the denaturation temperature T_m and of the Gibbs net free energy Δ(ΔG_ND) of stabilization of lysozyme with respect to the two other sugars.     

Surface enhanced Raman scattering in an amorphous matrix for Raman amplification

In order to improve the efficiency of Raman Amplifiers, the Surface Enhanced Raman Scattering (SERS) effect of an amorphous matrix of TiO₂ was studied. First, optimisation of the amorphous layer quality was performed by depositing thin films on glass substrates at different temperatures. Then, thin films of amorphous TiO₂ were deposited on silicon commercial gold SERS substrates (Klarite®) by a dip-coating process. The SERS effect was demonstrated by the great difference of Raman intensities of the amorphous TiO₂ matrix dip-coated on active and inactive parts of Klarite® substrate under 633 nm and 780 nm laser excitations in the tail of the Surface Plasmon Resonance band of gold nanoparticles.     

Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

Raman scattering and structural order in liquid GeSe2

High temperature polarized and depolarized Raman spectra have been obtained in liquid GeSe₂ near the melting point of 770 C. The spectra indicate a number of similarities to amorphous GeSe₂, including the presence of the “companion” line which has been associated with intermediate range order in the amorphous state. The Raman spectra, as well as previous low angle diffraction studies, indicate that intermediate range structural order is also present in liquid GeSe₂.     

Inelastic neutron and low-frequency Raman scattering in a niobium-phosphate glass for Raman gain applications

We present measurements of the vibrational spectrum of a binary niobium-phosphate glass in the THz frequency range using inelastic neutron and Raman scattering. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (“boson peak”). Using a recently developed theory of vibrational excitations in disordered solids we are able to reconcile the measured neutron and Raman spectra using fluctuating elastic and Pockels constants as a model concept. As the spontaneous Raman susceptibility is a key parameter for Raman amplification our results suggest a significant gain profile for application of niobium-phosphate glasses in Raman amplifiers.     

Raman scattering in low wavenumber region as a new probe to structural properties of microcrystalline silicon

The structural properties of microcrystalline silicon (μc-Si) are studied by Raman scattering. It is found that the intensity of each Raman band closely correlates with the absorption coefficient in the interband region and that the Raman band at ca. 150 cm^?1 is a sensitive probe to randomness of Si-Si bonding structure in μc-Si.     

Flexible polyvinyl chloride neutron guides for transporting ultracold and very cold neutrons

The transmission of ultracold neutrons (UCNs) through flexible polyvinyl chloride (PVC) tubes with lengths of up to 3 m and an internal diameter of 6–8 mm has been studied. High UCN transmission is found even for arbitrarily bent tubes (single bend, double bend, triple bend, figure eight, etc.). The transmission can be improved significantly by coating the inner surface of the tube with a thin layer of liquid fluorine polymer. The prospects of these neutron guides in fundamental and applied research are discussed.     

Low-frequency Raman scattering and small-angle X-ray scattering of glasses inclined to phase decomposition

Low-frequency (<1000 cm⁻¹) Raman scattering of lithium aluminosilicate (12Li₂O : 15Al₂O₃ : 73SiO₂ with 4 mol% TiO2) glasses with addition of titanium dioxide has been studied. With a heat treatment at temperatures 660°C, 700°C, 720°C and 820°C and for various times and sequences of temperature, our samples decompose into nanometer sized dispersed aluminotitanate particles. In Raman spectra of these glasses an evolution of a boson peak was observed. The width of the relatively broad boson band decreases as does the frequency of the band. From small-angle X-ray scattering data we conclude that the boson peak is connected with elastic vibrations of amorphous or crystalline regions of inhomogeneity with a dimension of ∼1.7 nm in initial glasses or larger depending on the heat treatment sequences.     

Polarized neutron scattering investigation of excitations at low momentum transfer in liquid Ga: The mystery continues

New polarized neutron scattering experiments are presented on liquid gallium just above the melting transition of 303 K in order to shed light on the origin of the observed increased Ga cross-section at small scattering angles that has previously been reported in the literature. Our polarized neutron scattering experiments show that this increased cross-section cannot be linked to any magnetic or incoherent process, a linkage that was needed to justify the interpretation of this broad mode as being part of the cage-diffusion process. Instead, the increased cross-section has to be attributed to a coherent process, in violation of the measured sum-rules.     

Raman scattering and nonlinear refractive index measurements of optical glasses

The absolute Stokes-Raman scattering cross section is measured for optical glasses of varying composition, from which the nuclear contribution to the nonlinear polarizability is determined. The nonlinearities increase progressively from fluoroberyllate compositions to fluorophosphate to phosphate to silicate and finally to heavy-metal oxide glasses. Although values of the total nonlinear polarizability vary by nearly two orders-of-magnitude it is determined that the ratio of the nuclear to the electronic contribution is nearly constant and is independent of the glass composition. The electrostrictive contribution to the nonlinear polarizability is also calculated using previously measured elasto-optic constants. Correlations in the atomic structure were determined by measuring the position of the lowest-frequency Raman scattering peak and it is found that the correlation length for pure one-component glasses is larger than multi-component glasses. Examination of the low-frequency scattering region shows that there is an 0xcess light-scattering below 30 cm^?1 in all glasses tested. This excess scattering in LaSF-7 was fitted between 2 to 20 cm^?1 to a Lorentzian centered at zero frequency with a halfwidth of 2.5 ± 1 cm^?1, while the fit in pure silica between 5 and 30 cm^?1 yields a halfwidth of 8 ± 3 cm^?1.     

Raman scattering and electron spin resonance studies of fluorozirconate glasses

Electron spin resonance and Raman spectra are reported for fluorozirconate glasses which contain transition metal impurities. From the observed g values at 2.0 and a broad signal from 1 to 10, it is concluded that the dominant contribution to the ESR signal is due to Fe³⁺ in sites with rhombic distortion. The Raman spectra are shown to be superposed by luminescence emissions for paramagnetic impurity concentrations as low as ∼10¹⁶ spin/mol of ZrF₄. However, the amplitude and frequency of the dominant polarized Raman mode near 580 cm⁻¹ are observed to be relatively independent of impurities.     

Raman scattering study of dopant homogeneity in GaP and GaAs single crystals

Application of Raman scattering as a non-destructive method with high spatial resolution suitable for the determination of charge carrier profiles connected with dopant segregations in polar semiconductors is reported. Single crystals of GaP and GaAs are investigated. The results are in good agreement with electrical measurements.     

XRD,TEM and Raman scattering studies of PbTiO3 nanopowders

Lead titanate nanopowders were fabricated by mechanochemical synthesis. The obtained materials were characterized by X‐ray diffraction, high resolution electron microscopy and Raman scattering. The X‐ray diffraction showed the perovskite structure of the powder. It was found that: (1) the powder consists of loosely packed grains with a broad distribution of sizes between a few nm and 45 nm, (2) the grains of sizes larger than about 30 nm exhibit well developed crystalline structure, (3) the Raman lines of PbTiO₃ nanopowder exhibit a conspicuous broadening in comparison to Raman lines of the bulk material, and 4) in the spectrum of nanopowder the lines related to A₁(1TO), E(2TO) and A₁(3TO) modes are shifted into lower frequencies and their spectral linewidth increase and there is no significant change in frequency shift for the remaining modes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Raman coupling function in v-GeO2 and v-SiO2: A new light and neutron scattering study

We report inelastic Raman and neutron scattering spectra for the network glass formers vitreous silica (v-SiO₂) and vitreous germania (v-GeO₂) measured at temperature from 10 to 300 K. The ability to determine the temperature dependence of the luminescence background in Raman scattering has allowed to obtain the Raman coupling function C(ω) and in particular, its low-frequency limit. This study indicates that C(ω) has a linear behavior near the Boson peak maximum and below.     

Broadband terahertz time-domain spectroscopy and low-frequency Raman scattering of glassy polymers: Boson peak of PMMA

The low-energy vibrational properties of polymethyl methacrylate (PMMA) were studied by terahertz time-domain spectroscopy and Raman scattering. The real and imaginary parts of a complex dielectric constant were accurately determined in the frequency range from 0.2 to 5.0 THz. The imaginary part of the dielectric constant shows a line-shape similar to the imaginary part of the Raman susceptibility measured by Raman scattering. These two spectra show broad peaks at about 2.3 THz. We also separately determined the Raman and far-infrared light-vibration coupling constants of the PMMA using the vibrational density of states determined by cold neutron inelastic scattering in the literature.