Two-phonon Raman scattering in GaAs

The second-order Raman Spectrum of GaAs has been measured at room temperature by using an argon ion laser with Brewster's angle configuration. A number of critical points have been interpreted by considering neutron inelastic scattering work and detailed critical-point analysis in Ge. The Γ₁ component is the strongest in the spectrum where overtone scattering is dominant. Combination scattering has been observed in the energy region between 300 and 350 cm^?1for both Γ₁ and Γ₁₅ components. The second-order Raman spectrum is compared with the two-phonon overtone Raman intensity, calculated from neutron diffraction data. In the low frequency region, the Raman spectrum shows directly the one-phonon density of states of the TA phonon.     

Elastic and inelastic scattering of 11.8 MEV polarized deuterons from 34S

The elastic and inelastic scattering of 11.8 MeV vector polarized deuterons from ³⁴S has been studied. Angular distributions of the cross section σ(θ) and vector analyzing power iT₁₁(θ) have been measured for the ground state and the first three excited states in ³⁴S. Optical model parameters were first obtained by fitting the elastic deuteron scattering data alone. DWBA calculations with a collective model form factor were performed for the inelastic scattering to the excited states. Coupled-channels calculations were also done both for the one-phonon and the two-phonon excited states. The optical model parameters were suitably modified to obtain simultaneous fit to elastic and inelastic data when channel coupling was introduced. The values of the vibrational parameter β₂ for one- and two-phonon states are discussed and compared with those obtained from γ-decay width studies.     

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.     

Energy transfer between Eu3+ ions in a lattice with two different crystallographic sites: Y2O3:Eu3+, Gd2O3:Eu3+ and Eu2O3

The Eu³⁺ ion occupies two different crystallographic sites in (Y_1−xEu_x)₂O₃ and (Gd_1−xEu_x)₂O₃, with site symmetry S₆ and C₂. Energy transfer over more than 7 Å occurs from Eu³⁺ (S₆) ions to Eu³⁺ (C₂) ions. This is shown to be a direct one-phonon assisted process, in combination with a one-site resonant two-phonon assisted process at higher temperatures. For x = 1 there is energy migration over the Eu³⁺ (C₂) sublattice to quenching impurities. The presence of cooperative absorption points to superexchange interaction between the Eu³⁺ ions.     

Localized exciton-phonon complex as an intermediate state in light scattering

A $\text{sharp}$ LO-phonon replica of the first order Raman spectrum of (Ga_xIn_1-xP and Ga(As_1-xP_x) has been observed for the first time. The experiment shows strong resonance with varying incoming photon energy which only appears in the presence of alloying and disorder. The results can only be understood if the relevant resonant intermediate state is a localized sharp exciton strongly interacting with the LO-phonon system, i.e. a localized exciton-phonon complex. Under those conditions, a simple model yields a ratio of the two-phonon to one-phonon intensities which is given by straightforward Franck-Condon overlaps.     

Raman spectroscopy and Fermi resonance in Mn-doped ZnO bulk single crystal

For the first time the bulk oriented single crystals ZnO:Mn are obtained and the polarized Raman spectra are studied at excitation in the visible and near infrared regions. The resonance enhancing of the Raman scattering by Mn-related modes is found at the visible excitation due to the extra optical absorption in ZnO at the addition of Mn. It is shown that the resonance-enhanced overtone of Mn-related silent modes may be responsible for an appearance of anomalous modes of the A₁ symmetry at 500-600 cm⁻¹. A Fermi resonance between the overtone and one-phonon mode is analyzed.     

Muonium quantum diffusion in cryocrystals

In this paper we review our recent study on muonium (Mu) diffusion in simplest solids — Van der Waals cryocrystals. We give experimental evidence for the quantum nature of Mu diffusion in these matters. The results are compared with the current theory of quantum diffusion in insulators. The first direct observation of theT ⁷ power-law for the Mu hop rate (in solid nitrogen) is taken as confirmation of two-phonon scattering mechanism. In solid xenon the one-phonon interaction is shown to be dominant. Muonium diffusion in solid neon is discussed in terms of band-like motion. Finally, we report a dramatic effect of impurities on Mu quantum diffusion.     

Infrared lattice vibration spectra of tetragonal ZnP2

Infrared reflectivity spectra of tetragonal ZnP₂ are measured in the frequency range from 40 to 600 cm^-1 for both polarization directions E ⊥ c and E 6 c. The parameters of 9 E modes and 4 A₂ modes are determined by a dispersion analysis of the spectra. Three additional A₂ modes are detected by infrared transmission measurements. The results obtained are compared with previous Raman scattering and two-phonon combination mode spectra.     

Hydrogen vibrations in γ-TiH and γ-ZrH under high pressure

Abstract

The effect of high hydrostatic pressure on the vibrational spectra of monohydrides γ-TiH and γ-ZrH was studied by inelastic neutron scattering, the maximum experimental pressure being, respectively, P=15 and 17·5 kbar. There was observed a uniform linear pressure-induced energy shift of each spectrum compared to that at ambient pressure with proportionality factors of 1·02 for y-TiH and 1·03 for γ-ZrH. The observed linear shift of the one-phonon hydrogen modes and the bound multiphonons (biphonons and triphonons) is indicative of a pressure-induced increase of the anharmonicity parameters in the model Hamiltonian representing the strength of two-phonon and three-phonon interactions.     

Resonant multiphonon Raman scattering in AgBr

Resonant Raman scattering in AgBr single crystals is studied at low temperatures. Excitation in resonance with the indirectΓ — L exciton as intermediate state gives rise to selectively enhanced narrow two-phonon Raman scattering. The phonons involved are pairs ofLA andTO phonons of opposite wavevectors near theL-point. A simple model involving one discrete exciton level is developed to explain the resonance behaviour. The temperature dependence of the scattered intensity, that is studied for 1.8 K <T < 35 K, can consistently be interpreted within this model as being due to the lifetime of the intermediate state. Assuming that the excitons predominantly decay by one-phonon scattering with long wavelength acoustical phonons the predicted temperature dependence of the intensity is found in good agreement with the experimental result. Additional scattering peaks are believed to be due to third-order processes involving an acousticalX-phonon in addition to theL-phonons of the second-order scattering. Using an oriented sample the resonant Raman peaks are found to be polarized.     

(CdSe)1(ZnSe)3/ZnSe短周期超晶格多量子阱的共振Ramam谱

在(CdSe)₁(ZnSe)₃/ZnSe短周期超晶格多量子阱中,根据一维线性链模型计算的结果与实验结果的比较表明,我们在不同的共振条件下分别观察到了来自多量子阱的阱中和垒中ZnSe限制纵光学声子模的Raman散射。与GaAs/AlAs量子阱的偏振选择定则不同,在共振条件下,我们在两种偏振配置下都观察到了阱中ZnSe限制模LO₁,并认为这种不同可能来源于样品特殊的电子子带结构和光学声子行为。 关键词：     

Theory of two-phonon absorption processes in liquid paraffins

Calculated absorption spectra are given for linear C₈H₁₈, C₁₀H₂₂, C₁₆H₃₄ and C₂₀H₄₂ chains, based on the assumption of a two-phonon absorption mechanism. In the present work the normal modes of vibration of different finite chains are computed directly, without recourse to the Pitzer sampling approximation and without the assumption of strict k-selection rules. The calculated spectra are found to be much more sensitive to chain length than the experimental spectra. A possible explanation of the relative insensitivity of the observed spectra is suggested in terms of a characteristic chain segment length common to all the chains. Ironically, the apparent importance of chain segmentation in the two-phonon absorption suggests that one-phonon transitions may also play a significant rôle in the absorption process.     

Resonant Raman scattering in mixed GaSxSe1−x crystals

One- and two-phonon resonant Raman scattering around the absorption edge in mixed GaS_xSe_1?x crystals with 0 ? x < 0.23 has been measured. The results can be explained by a simple theory in which the dispersion of RRS is dominate by exciton resonances.     

Antiresonance of Raman Scattering in CdSxSe1-x Mixed Crystals

Abstract

Resonant enhancement of the Raman scattering cross section in II - IV semiconductors has recently received much attention both theoretically and experimentally. All existing theories anticipate a monotonic increase in the scattering intensities when the scattering radiation energy approaches the direct-energy gap. Contrary to them in an early Raman study of CdS¹ a cancellation of scattering efficiencies for the two TO modes prior to the onset of the resonance was pointed out. In a latter work on pure CdS Damen et al.² found even a more pronounced “antiresonance” behavior of the nonpolar E₂ phonon at 41 cm^?1. Thus, this striking feature seems to be rather common for the Raman active modes in CdS for which no electrooptic contribution to the scattering amplitude exists. The experimental data were qualitatively explained by assuming a destructive addition between nonresonant and the weaker resonant terms in LoudoN′s expression for the first-order Raman tensor³. Consequently the cancellation energy difference /E_G - hw_L/ depends on the ratio of the resonant term to the nonresonant terms.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

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 study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO₃) over the frequency range of 100-1500 cm⁻¹ have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO₃. The Raman spectra of BiFeO₃ excited at 532 nm were measured over the temperature range from 77 to 678 K. Besides the abnormal changes of the peak position and the linewidth of the A₁ mode at 139 cm⁻¹, the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250 cm⁻¹ were observed as the temperature increased to Néel temperature (T_N). All these results indicate the existence of strong spin-phonon coupling in BiFeO₃.