Estimation of dispersion curve of transverse acoustic phonons in CdI2 by Raman scattering

Shear type rigid layer modes in various polytypes of CdI₂ have been measured by Raman scattering. The dispersion curve of transverse acoustic phonons in the [0 0 ξ] direction is estimated from the observed frequencies of the rigid layer modes. The analysis of the estimated dispersion curve indicates that interlayer forces in CdI₂ are mainly short range ones.     

Indirect exciton absorption and Raman scattering in GaAs-AlGaAs superlattices

Longitudinal optic (LO) phonon assisted indirect exciton creation (X_LO), hot carrier relaxation ((e-h)_LO) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X_LO, (e-h)_LO and Raman peaks are observed. The X_LO absorption peaks are identified from the observation of a clear threshold in PLE at ℏω_LO (36.4 meV) above the heavy hole exciton peak. The intensity of X_LO is a maximum at 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above ℏω_LO. The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.     

Nuclear spin relaxation in molten CdCdI2 and ICdI2 mixtures

The prevailing opinion is that excess Cd in CdI₂ produces diamagnetic Cd₂²⁺ ions. ¹¹³Cd relaxation data indicates that ～1% of added atoms produce paramagnetic Cd⁺ ions with lifetime ～ 10 ps. The concentration of Cd⁺ ions is depressed by excess iodine.     

Estimation of interlayer interactions in CdI2 by light scattering techniques

The velocity of transverse acoustic wave propagating along the direction perpendicular to the layer and the frequency of the rigid layer mode in CdI₂ have been measured by Brillouin and Raman scattering, respectively, at several temperatures. The interlayer forces have been deduced independently from these two different measurements. A comparison between them has led to the conclusion that the nearest neighbour interlayer interaction dominates the interlayer interaction.     

Raman scattering of NiS2

Raman scattering measurements of NiS₂ is done and five optical phonon peaks are found. These peaks are assigned to A_g, E_g and 3T_g phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS₂ and MnS₂.     

Raman scattering of LiGaO2

The Raman spectrum of LiGaO₂ is reported together with a preliminary identification of the optical modes and their temperature dependence between 6 and 700 K. The observed temperature dependence of the lines gave no indication for second order processes. However, at low temperatures additional lines have been observed indicating a phase transition at approximately 65 K.     

Raman scattering in Ag-intercalated TiS2

Ag-intercalated TiS₂ has been investigated using electron diffraction and Raman scattering. The energies of the E_g-1 and A_1g modes in 1T-TiS₂ at 300 K were found to be 232 and 336 cm^-1, respectively. In Ag_0.3TiS₂, at an ambient temperature of 4.2 K, modes were observed at 207, 239, 277, 311, and 347 cm^-1. Three of these modes have been associated with the formation of a superlattice at low temperatures. The superlattice formation was observed by electron diffraction and is attributed to an ordering of the silver atoms at interlayer interstices.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

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 scattering from 1T-TaS2

Raman measurements on the 1T-polytype of TaS₂ are reported. In the commensurate charge density wave state, a large number of Raman-active peaks are observed below 400 cm^-1. Most of these peaks are attributed to k = 0 optic phonons resulting from superlattice formation.     

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.     

Raman scattering study of interlayer bonding in CdI2 AND SnS2 under hydrostatic pressure: analysis by use of Van Der Waals interaction

The pressure dependence of inter- and intra-layer modes in 4H-CdI₂, 4HSnS₂ and 18R-SnS₂ has been studied using Raman scattering techniques. X-ray diffraction measurements on 4H-CdI₂ give the variation of lattice parameters under hydrostatic pressure. The relationship between interlayer bond-length and bond-strength is obtained from a combination of the Raman scattering and X-ray diffraction results. The results are analyzed by use of a short range 6-exp type potential. Van der Waals constants for the anion-anion interaction and polarizabilities of the anion in CdI₂ and SnS₂ are determined from fitting the calculation to the experimental results.     

Resonant Raman scattering at higher M0 exciton edge in layer compound InSe

Resonant Raman spectra of γ-rich InSe have been measured at the absorption peak, E'₁, which lies 1.2 eV above the lowest direct gap. The resonance behaviors observed indicate that the E'₁ peak is due to the formation of damped M₀ excitons.     

Raman scattering of the MoS2 and WS2 single nanotubes

We report here the results of the first resonance Raman study on single MoS₂ and WS₂ nanotubes and microtubes synthesized by chemical transport reaction. These multiwall tubes represent the longest known inorganic nanotubes grown up to several millimetre lengths with diameters ranging from less than ten nanometers to several micrometers. The nanotubes grown at nearly equilibrium conditions contain extremely low density of structural defects. The selected area diffraction on the thick-wall nanotubes revealing the rhombohedral (3R) stacking, otherwise stable at elevated pressure above 4 GPa, provides indirect evidence of the presence of strain incorporated into the nanotube wall. Results are compared with phonon spectra of plate-like crystals of the same compound. The observed up-shift of Raman peaks in the tubes spectra is explained by the presence of strain. Well preserved crystal structure of tubes is confirmed by comparison with phonon spectra of nanostructured materials from literature.     

Enhanced Raman scattering from nano-SnO2 grains

We present the Raman spectra of nano-SnO_{2} grains with sizes from 4nm to 80nm excited by 532nm and 1.06μm lines. The enhanced Raman scattering of the nanograins is observed for both exciting lines when the grain size is less than 8nm. The less the grain size is, the more intensely the Raman scattering is enhanced. According to our results, the enhancements of the Raman intensity are a few tenfolds and different for different exciting lines when the grain size is 4nm. It can be attributed to enhanced Raman scattering by electron－hole pair excitations in the nanograins that originate from sub-microscopic (10nm) size and other defect- and surface-related features. A critical size that divides respective predominance of bulk properties and the defect-, surface-, and size-related features can be determined to be about 8nm.     

The Raman scattering of Zn3As2

The Raman scattering of Zn₃As₂ crystal is presented within 50–420 cm⁻¹ energy range. The group theory method are used to determine symmetry of the allowed lattice modes, and further to discuss the results obtained.     

Spin-dependent phonon Raman scattering in CdCr2Se4

Influences of ferromagnetic ordering on the phonon Raman scattering are studied for CdCr₂Se₄ through the intensity measurements of Raman spectra between 25 and 300 K with various wavelengths of excitation light (488.0–676.4 nm). Spin-dependent enhancements of Raman cross section are observed for optical phonon lines D(168 cm^?1) and F(238 cm^?1) with excitation wavelengths of about 630 and 550 nm, respectively. This kind of phenomenon in spinel-type chalcogen chromites seems to originate in spin-dependent intermediate interactions in the excited states of specific electronic transitions with which the incident or scattered light is resonant.     

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.     

Raman scattering in CeB6

Raman spectra of phonons in CeB₆ were measured. The observed three peaks were assigned to the A_1g, E_g and F_2g phonon modes. The frequencies of the modes suggest that the Ce ion is in the trivalent state.