Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Raman and infrared spectra of ZnSiAs2

The optical phonons at k = 0 of ZnSiAs₂ have been investigated by Raman scattering and infrared reflectivity measurements at 300 K. Eleven of thirteen expected optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear in the range from 415 cm^-1 to 75 cm^-1 with predominant polar modes at 400 cm^-1 (gG₅), 389 cm^-1 (Γ₄) and 242 cm^-1 (Γ₄). The dielectric dispersion for E ⊥ c and E 6 c has been determined by Kramers-Kronig integrations.     

Optical phonons in CuAlS2

The optical phonons at k=0 of CuAlS₂ have been investigated by Raman scattering, infrared reflectivity and absorption measurements from 50 to 1000 cm^-1 at T=300 K. Eleven of the thirteen expected optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear in a range from 498 to 76 cm^-1 with predominant polar modes at 445 and 266 cm^-1. The dielectric dispersion for E ⊥ c and E ∥ c has been determined by Kramers-Kronig integrations.     

Directional dispersion of extraordinary phonons in layer compound InSe

The resonant Raman scattering of polar optical phonons has been measured in trigonal γ-InSe at the E'₁ exciton edge. The two-phonon scattering spectrum widely spreads over the frequency range of 50 cm^-1 with an extremely asymmetric lineshape. This spectrum has been interpreted in terms of the directional dispersion of extraordinary LO and TO phonons.     

Raman scattering study of the dynamics of the pressure-induced phase transition in thallium azide (TlN3)

A Raman scattering investigation of the pressure-induced phase transition in tetragonal thallium azide (TlN₃) is reported. The most interesting features of the Raman spectrum of TlN₃ are the anti-resonant line-shapes of two E_g symmetry phonons at 35 and 50 cm^?1 superimposed on a quasi-elastic wing. The scattering data is shown to be consistent with a model in which the two phonons interact via an imaginary coupling term. The phonon at 35 cm^?1 (assigned to a translational shear mode of the Tl⁺ sublattice) softens with increasing pressure and increases in linewidth as P approaches P₀ (=8 kbar) from below. At the same time, the quasi-elastic scattering component (associated with large amplitude N₃^? librational fluctuations) becomes less damped. A displacive structural transition from tetragonal to monoclinic is indicated by the eigenvector of the soft phonon.     

Raman scattering study of reduced Er:LiNbO3 crystals

Raman spectra of as-grown and reduced (or annealed) Er:LiNbO₃ crystals, which have different cut orientations, varied Li/Nb ratio, and different Er-doping levels of 0.2, 0.4, 0.6, 1.0, and 2.0 mol%, were recorded at room temperature over a wavenumber range of 50–1000 cm^-1 by use of backward scattering geometries. The spectra are assigned on the basis of their Raman scattering features and previous relevant work. A weak but well-resolved peak around 633 cm^-1 appears in the E(TO) spectra that were recorded under the configuration of X(ZY)X̄(for an X-cut sample) or Y(ZX)Ȳ(for a Y-cut sample) for all crystals studied. The appearance of this peak in the E(TO) spectrum provides further evidence for a previous attribution of this peak to E(TO₉) mode. Some additional peaks distributed in the low wavenumber region ranging from 101–137 cm^-1 are attributed to Er³⁺ fluorescence with a wavelength range of 490.41–491.3 nm. The reduction effects include a significant drop of the Raman scattering intensity and a slight narrowing instead of a broadening in the linewidth. The reduction procedure hardly affects the spectral shape and the wavenumber of most of the phonons. The anneal effect is similar to the reduction effect and both effects are not as obvious as the vapor transport equilibration (VTE) effect. In addition, the present Raman scattering result provides evidence for our earlier reported individual result on light-induced diffraction from strongly reduced Er:LiNbO₃ crystals. PACS 42.70.Hj; 81.05.-t; 63.20.-e; 78.30.-j.     

Raman scattering in layered compound 2H-WS2

Raman scattering of layer-type compound 2H-WS₂ has been studied at room temperature. The first-order Raman peaks are observed at 27.4, 357 and 423 cm^-1. The low-frequency peak at 27.4 cm^-1 is a rigid-layer mode, from which the interlayer shear force constant is estimated. The central force model is applied to the high-frequency phonons. The interlayer shear force constant is much smaller than the intralayer force constants. We also find several peaks due to the second-order processes.     

Monte Carlo calculation of hot electron drift velocity in silicon (100)-inversion layer by including three subbands

A Monte Carlo calculation of the drift velocity of hot electrons in quantized silicon inversion layers for (100)-oriented surface has been performed by considering the three lowest subbands. The intersubband and intervalley phonons conform to the surface Brillouin zone structure and are assumed to have bulk values of deformation potential constants. It is found that most of the electrons tend to occupy the E_0′ subband at about 10 kV cm^-1. The effect of surface-oxide-charge scattering is found to be quite important. The calculated curves show a change of slope at about 10 kV cm^-1 and do not show clear saturation. This is in contrast with the experimental curve which shows first a smooth variation and then tends to saturate.     

Reactive scattering of a supersonic oxygen atom beam O + I2

Reactive scattering of O atoms with I₂ molecules has been studied at an initial translational energy E = 43 kJ mol^-1 using a supersonic beam of O atoms seeded in He and E = 18 kJ mol^-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross section were obtained which show predominantly rebound scattering at both initial translational energies. Scattering in the forward direction has a product translational energy distribution similar to that predicted for a long-lived collision complex but scattering in the backward direction has a much higher product translational energy. The greater predominance of rebound scattering observed for O + I₂ compared with O + Br₂ may be attributed to the greater exoergicity of the O + I₂ reaction. However, comparison with the O + IBr reaction which exhibits a long-lived collision complex mechanism indicates that the predominance of backward scattering for O + I₂ also arises from diminished forward scattering from larger impact parameter collisions.     

Resonant Raman scattering of CdCr2Se4

The scattering cross section of the Raman-active phonons at 156 cm^?1 (Eg) and 169 cm^?1 (F2g) in the ferromagnetic semiconductor CdCr₂Se₄ (T_c=130 K) has been measured as a function of incident photon energy between 1.55 and 2.81 eV, both in the ferromagnetic and paramagnetic phases. The resonance curve peaks sharply near 2 eV and shows a broadening for temperatures below the Curie point. The relative line intensities change significantly with photon energy. The results show that the concept of spin-dependent Raman scattering in the ferromagnetic spinels has to be revised in terms of exchange-splitting-induced resonant Raman scattering.     

Disorder-induced Raman scattering of In1−xGaxP in the transverse acoustic phonon region

Raman scattering measurements have been performed on In_1?xGa_xP (0.62?x?1) over the entire frequency range of first and second order scattering. Besides the already known disorder activated band between the LO(Γ) and TO(Γ) modes a new disorder activated band is found in the region of transverse acoustic phonons around 87 cm^-1. The position of the new band shifts only slightly with composition while its strength and line-shape change.     

Anisotropy of the Raman scattering measured in the xy plane of a nontwinned YBa2Cu3O7−x single crystal

The temperature dependence of anisotropy of the phonon and electron Raman scattering components was studied by measurements in the xy plane of a nontwinned YBa₂Cu₃O_7?x single crystal. It is shown that the sign of the orthorhombicity parameter γ=(I _yy ?I _xx )/(I _yy +I _xx ) for the full-symmetry phonons (150, 340, and 435 cm^?1 modes) generated by the displacements of ions in the CuO₂ plane is opposite to the sign of this parameter for the phonons generated by the out-of-plane barium and bridging oxygen displacements (120 and 500 cm^?1 modes). In the superconducting state, the γ value decreases in the region of low frequencies, but the frequency renormalization of the 340 cm^?1 mode measured in the xx and yy spectra is the same to within the experimental error.     

Raman scattering on intermediate valent TmSe

Defect induced first order resonance Raman spectra have been measured on cleaved single crystals of intermediate valent Tm_xSe, where the Tm to Se ratio x varied between 0.87 and 1.05. A softening of the zone boundary LO phonons linear in the degree of valence mixing was observed. On polished samples an additional scattering peak at 60 cm^-1 was found. Its intensity varies with stoichiometry concomitant with the degree of valence mixing and it disappears in the trivalent Tm³⁺_0.87Se.     

Raman scattering from phonons and electronic excitations in UO2 with different oxygen isotopes

Comparison of the phonons and electronic excitations of UO₂ with different oxygen isotopes (¹⁶O, ¹⁸O), two purely electronic excitations at 4162 and 5778 cm⁻¹ have been evidenced by their limited shift, when ¹⁶O is replaced by ¹⁸O. The bands observed at 570, 1144 and 2300 cm⁻¹ in U¹⁶O₂ shift to 545, 1091 and 2190 cm⁻¹ for U¹⁸O₂. The wavenumbers and the isotope shifts of these bands exhibit multiple relationships. It supports the assignments of the 1LO (~570 cm⁻¹) and 2LO (~1144 cm⁻¹) phonons for U¹⁶O₂. Both bands exhibit marked decrease for increasing O/U ratio. The observed phenomenon is regarded as a good estimation of the stoichiometry of a uranium oxide. In addition, by analyzing the intensity ratios of the 2LO to 1LO phonons for various UO₂ samples, the 2LO band is assigned to the Fröhlich interaction. Defect‐induced and/or deformation potential mechanisms play a dominant role for the 1LO band. Copyright © 2015 John Wiley & Sons, Ltd.     

The composition dependence and new assignment of the Raman spectrum in lithium tantalate

The Raman scattering spectra of lithium tantalate crystals with different compositions were investigated. The comparison of the Raman data obtained for the congruent and the near-stoichiometric crystals reveals some differences in the shape and the number of Raman peaks, which lead to a new assignment of the long-wavelength optical phonons. And quantitative relationships between the linewidth of Raman peaks (142 cm⁻¹ for E-phonon and 861 cm⁻¹ for A₁-phonon) and the crystal composition were firstly presented. Two local Raman lines, 278 and 750 cm⁻¹, were found for the congruent crystals and attributed to the intrinsic defects, Li vacancy and anti-site Ta ion, respectively.     

Elastic scattering of \sf{59.54}-keV \sf{\gamma}-rays in elements with \sf{22 \leq {Z} \leq {92}} at momentum transfer \sf{0.4 \leq {x} \leq 4.7~{\AA}^{\sf -1}}

Differential cross sections for elastic scattering of the 59.54 keV γ-rays in elements with 22 ≤Z ≤92 have been measured over the angular range 10^○–160^○corresponding to the momentum transfer 0.4 ≤x ≤4.7 Å^-1. The measurements at forward and backward angles were performed using the ²⁴¹Am radioactive point-source, target and the Ge detectors in the transmission and reflection arrangements, respectively. The measured differential scattering cross sections are compared with those based on the form-factor (FF) formalism and state-of-the-art S-matrix calculations to differentiate between their relative efficacies and to check angular-dependence of the anomalous scattering factors (ASF) incorporated as correction to the modified form-factor (MF). The S-matrix values exhibit agreement with the measured data at backward angles and differences ～10% at forward angles. The scattering cross sections based on the MF including ASF’s are in general lower at various angles by up to 20% for medium- and high-Z elements. The observed deviations being higher at the forward angles infer possibility of angular-dependence of ASF’s.     

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A₁ LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A₁ LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E₁ LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.     

Magnetoelectric Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> and relativity theory

Based on the dielectric continuum phonon model, uniaxialmodel and force balance equation the mobility of two dimensional electrongas in wurtzite Al_xGa_1-xN/GaN/Al_xGa_1-xN quantum wells isdiscussed theoretically within the temperature range dominated by opticalphonons. The dependences of the electron mobility on temperature, Al molarfraction and electron sheet density are presented including hydrostaticpressure effect. The built-in electric field is also taken into account. Itis found that under normal pressure the main contribution to the mobility isfrom the scattering of interface optical phonons in narrow (for well widthd < 12 Å) and wide (for d > 117 Å and d > 65 Å for finitelythick barriers and infinitely thick ones, respectively) wells, whereas thatis from the scattering of confined optical phonons in a well with anintermediate width. It is shown that the electron mobility decreases withincreasing Al molar fraction and temperature, whereas increases obviouslywith increasing electron sheet density. The theoretical calculated electronmobility is 978 cm²/V?s which is higher than an available experimentaldata 875 cm²/V?s when x equals to 0.58 at room temperature. Theresults under hydrostatic pressure considering the modification of strainindicate that the mobility increases slightly as hydrostatic pressureincreases from 0 to 10 GPa.     

Effect of dislocations on the lattice thermal conductivity of superconducting niobium

Thermal conductivity measurements on single crystal Nb samples in the superconducting state have demonstrated a resonant scattering of thermal phonons at roughly 5 × 10¹⁰Hz. The assumption of a mechanical resonance associated with the dislocation structure accounts for the present data and is consistent with other data found in the literature. The thermalization of phonons at an abraded sample surface, and the attendant failure of the relation l^?1 = ∑_jl_j^?1 for phonon mean free paths, was also observed.     

Raman scattering in magnetic US3

A Raman scattering investigation of magnetic US₃ has been made from 7 to 300 K. Comparison of room temperature spectra with those of the non magnetic isostructural sulfide HfS₃ allowed the assignment of most of the lines to $\textcolor{red}{}\text{=}\text{0}$ optical phonons. Drastic changes take place in the 10?150 cm^?1 range when lowering the temperature down to 7 K : four equally spaced lines appear at 54, 72.5, 91 and 109.5 cm^?1. Three of them broaden significantly with increasing temperature and disappear near 50 K, at which previous measurements indicate a maximum in the magnetic susceptibility and suggest a magnetic phase transition. The stronger fourth line is still observed at 100 K and merges into a phonon line at higher temperature. These four lines are attributed to electronic transitions within the 5f² configuration of U⁴⁺. Their temperature dependences appear to involve a spin-dependent scattering mechanism and are consistent with antiferromagnetic ordering.