Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

On the drastic temperature broadening of hard mode Raman lines of ferroelectric KDP type crystals near Tc

The drastic broadening of a few A₁ Raman lines of ferroelectric KH₂PO₄(KDP) and KH₂AsO₄(KDA) crystals have been measured at various temperatures near T_c. The results in KDP are in fair agreement with two different simplified expressions of linewidths, which are based on pseudospin-phonon interaction arguments. No such agreement is found between the results and a simplified linewidth expression derived from a theory based on soft mode-hard mode interaction arguments. The linewidths and the reciprocal of the proton spin-lattice relaxation time in KDP exhibit a very similar temperature dependence below T_c but behave differently above T_c. This phenomena is presented in detail, and discussed.     

Temperature-dependent Brillouin scattering studies of surface acoustic modes in Nd0.5Sr0.5MnO3

Brillouin scattering experiments are carried out to study the surface acoustic waves in Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 40-300 K covering the metal-insulator and charge-ordering phase transitions. The surface modes include surface Rayleigh wave, pseudo-surface acoustic wave (PSAW) and high velocity PSAW. The observed softening of the sound velocities for the surface modes below paramagnetic to ferromagnetic transition, T_c is related to the softening of the C₄₄ elastic constant. The subsequent hardening of the sound velocity below the charge ordering transition temperature T_co is attributed to the coupling of the acoustic phonon to the charge ordered state via long range ordering of the strong Jahn-Teller (JT) distortion.     

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.     

Time-domain measurements of S-branch N2–N2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy

Time-resolved dual-broadband picosecond pure rotational CARS has been applied to measure self-broadened S-branch N₂–N₂ Raman linewidths in the temperature range 294–1466 K. The coherence decays were detected directly in the time domain by following the J-dependent CARS signal decay as a function of probe delay. The rotational Raman N₂–N₂ linewidths were derived from these time-dependent decays and evaluated for thermometric accuracy. Comparisons were made to the energy-corrected sudden (ECS) and modified exponential gap (MEG) dynamical scaling laws, and the results were used to quantify the sensitivity of nanosecond rotational CARS thermometry to the linewidth model employed. The uncertainty based on the linewidth model used in pure N₂ was found to be 2 %. The merits and limitations of this rapid method for the determination of accurate Raman linewidths are discussed.     

Normal vibrational modes in benzil crystals

A group-theoretical analysis of the vibrational spectrum of benzil crystals was carried out. The selection rules and normal coordinates were found. Raman scattering spectra of benzil single crystals were studied in polarized light within a broad temperature range from 100 K to the melting point (T _m=96°C). The experimental data were found to correlate with theory. The temperature dependences of the spectral-line half-widths were used to separate the observed vibrations into translational and librational modes, and the correspondence between the experimentally observed vibrations and their normal coordinates was established in some cases.     

FMR linewidth and eddy-current losses in Ag-doped CdCr2Se4 single crystals

We report FMR linewidth data on Ag-doped CdCr₂Se₄ single crystals, with 0.1, 0.75 and 5.4% of Ag doping, in the range 70 K < T < 170 K. The contribution of eddy-current losses to the linewidth is analyzed for samples of spherical and slab shapes. Together with two-magnon scattering due to surface and volume pits, these losses explain the observed linewidths. The microwave conductivities obtained are consistent with the model for conduction and the results for n-type CdCr₂Se₄ and EuO.     

后向泵浦高功率掺镱光纤放大器的非弹性散射效应

 在考虑受激布里渊散射和受激喇曼散射效应的基础上,数值求解了双包层掺镱光纤放大器的速率-传输方程,分析了后向泵浦高功率掺镱光纤放大器在不同信号光线宽、光纤长度和纤芯直径下的输出特性,研究了受激布里渊散射和受激喇曼散射的抑制条件。计算结果表明:在后向泵浦方式下,若掺镱光纤长度小于0.68 m,纤芯直径大于26 μm,信号光线宽大于0.04 nm,则放大过程中的受激布里渊散射和受激喇曼散射能同时得到有效抑制。     

Microstructural evolution of pure copper during friction-stir welding

The microstructural evolution of pure copper during friction-stir welding was found to be principally influenced by welding temperature. At temperatures below ~0.5 T_m (where T_m is melting point), the microstructure was shown to be essentially determined by continuous recrystallization, leading to significant grain refinement and related material strengthening in the stir zone. In contrast, grain structure development at temperatures above ~0.5 T_m was dominated by discontinuous recrystallization producing a relatively coarse grain structure in the stir zone and giving rise to material softening.     

Scattering tensors and Clebsch-Gordan coefficients in crystals: Brillouin and morphic effects

In this paper we demonstrate that the scattering tensors for one phonon Brillouin scattering and for “morphic-induced” Raman scattering can be simply related to known Clebsch-Gordan coefficients. Tensors for crystals with point group symmetry T_d and C_6v are given. Tensors for higher order processes can be found similarly.     

Multiphonon selection rules for cuprous oxide type crystalline systems

Experimental data on the IR and Raman spectra along with the slow coherent neutron scattering data for Cu₂O were examined for assignments of frequencies to phonon modes not only at the center but those at the boundary of the Brillouin zone. The inconsistency in assignments for the zone center modes and the lack of not only assignments but identifications of the zone boundary phonon required the use of second order Raman and IR spectra as the useful probes. The basis for the interpretation of these spectral data is given in terms of reduction coefficients for ordinary products and symmetrized squares and cubes of representations in the “cuprite” structure O_h⁴-Pn3m. They are presented for the R, X, M and Γ points of the Brillouin zone. They will serve for obtaining selection rules, invariants, and covariants for this structure. Additional tables are given that enable one to identify what pair of phonons correspond to electric dipole, magnetic dipole, electric quadrupole, or Raman transitions, respectively.     

Soft mode and magnetic phase transition in PrNi

The spectrum of magnetic excitations in a single crystal of intermetallic compound PrNi was studied by inelastic neutron scattering. Experiment showed the substantial softening of some collective magnetic excitation modes near the ferromagnetic ordering temperature T _c≈20 K. The result is analyzed within the framework of a model that describes the magnetic phase transition in systems with induced magnetic moment.     

Stimulated Raman scattering in carbon dioxide and a measurement of time for V-T energy transfer

A weak Q-switched ruby laser (1 MW, 40 nsec, 0.015 cm^-1 linewidth) was focused into a cell (CO₂, 50 atm). The spectrum of the total output energy (40–50 MW, 25 nsec) was studied. In addition to powerful Brillouin scattering, the first stimulated Raman Stokes line of CO₂ (10 MW) and the first three anti-Stokes Raman lines were observed for the first time in pure CO₂. It was also possible to obtain coherent Raman excitation in a second CO₂ cell for pressure down to 3 atm. This last experimental result has been applied to the measurement of the time for V-T transfer in CO₂.     

Specific heat related to the Raman frequency shifts for the rotatory mode in ammonia solid I close to the melting point

This study gives an evidence for the validity of our spectroscopic modification of the Pippard relation as applied to ammonia solid I close to the melting point. We use our calculated frequencies for the rotatory lattice (librational) mode in ammonia solid I for the pressures of 0, 1.93 and 3.07 kbars. We obtain that the specific heat C_p varies linearly with the frequency shifts 1/ν(∂ν/∂T)_p for this Raman mode at those pressures studied in this crystalline system. Our values of the slope dP_m/dT that we deduced from the linear plots, are not in satisfactory agreement with the experimental values in ammonia solid I near the melting point.     

Radiation damping of magnetostatic modes in yig

The radiation induced linewidth of magnetostatic modes in spheres of Yttrium Iron Garnet has been measured in microwave experiments. The linewidths of the lowest order modes (n m 0) have a ω²ⁿ⁺¹ frequency dependence which can be explained by multipole radiation.     

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.     

Anisotropic polariton dispersion in red HgI2

We have measured resonant Brillouin Scattering near the A-exciton region of a layer-semiconductor, red-HgI₂. The Brillouin shift is interpreted in terms of a scattering process between the two polariton states by an acoustic phonon. The estimated exciton masses for the translational motion are (0.68 ± 0.1)m₀ and (1.2 ± 0.1)m₀ for the (100 and (001) directions, respectively.     

A Brillouin study of the temperature-dependence of the acoustic modes across the insulator-metal transitions in V2O3 and Cr-doped V2O3

Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V₂O₃. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (T_IM=T_N=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below T_IM in the V₂O₃ system. The softening of the SRW frequency from 330 K to T_IM can be qualitatively understood on the basis of the temperature-dependence of C₄₄, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below T_IM suggests that C₄₄ must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.     

Phase diagram of the Kondo necklace model with planar and local anisotropies

We present a detailed temperature-dependent Raman light scattering study of optical phonons in molecular-beam-epitaxy-grown films of the electron-doped superconductor La_{2 -x} Ce _x CuO₄ close to optimal doping (x ~ 0.08, T _c = 29 K and x ~ 0.1, T _c = 27 K). The main focus of this work is a detailed characterization and microstructural analysis of the films. Based on micro-Raman spectroscopy in combination with X-ray diffraction, energy-dispersive X-ray analysis, and scanning electron microscopy, some of the observed phonon modes can be attributed to micron-sized inclusions of Cu₂O. In the slightly underdoped film (x ~ 0.08), both the Cu₂O modes and others that can be assigned to the La_{2 -x} Ce _x CuO₄ matrix show pronounced softening and narrowing upon cooling below T ~ T _c . Based on control measurements on commercial Cu₂O powders and on a comparison to prior Raman scattering studies of other high-temperature superconductors, we speculate that proximity effects at La_{2 -x} Ce _x CuO₄/Cu₂O interfaces may be responsible for these anomalies. Experiments on the slightly overdoped La_{2 -x} Ce _x CuO₄ film (x ~ 0.1) did not reveal comparable phonon anomalies.     

Electronic structure and lattice dynamics of Li2Ni(WO4)2

We combined spectroscopic ellipsometry and Raman scattering measurements to explore the electronic structure and lattice dynamics in Li₂Ni(WO₄)₂. The optical absorption spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents a direct optical band gap at 2.25 eV and two bands near 5.2 and 6.0 eV, which are attributed to charge-transfer transitions from oxygen 2p states to nickel 3d or tungsten 5p states. The Raman scattering spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents seventeen phonon modes at approximately 112, 143, 193, 222, 267, 283, 312, 352, 387, 418, 451, 476, 554, 617, 754, 792, and 914 cm⁻¹. When the temperature is decreased to 20 K, the frequency, linewidth, and normalized intensity of all phonon modes exhibited almost no temperature dependence. Upon cooling across 13 K, which is the antiferromagnetic phase transition temperature, the oxygen octahedra stretching mode at 914 cm⁻¹ exhibited a softening and an increase in intensity, thus suggesting a coupling between the magnetic and lattice degrees of freedom. The spin-phonon coupling constant was estimated to be 0.94 mRy/Ų, indicating a weak spin-phonon interaction in Li₂Ni(WO₄)₂.