Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

n-SiC的电子拉曼散射及二级拉曼谱研究

研究了利用离子注入法得到的掺氮n-SiC拉曼光谱. 理论线形分析表明,与4H-SiC相比,6H-SiC中LO声子等离子体激元耦合模(LOPC模)拉曼位移随自由载流子浓度变化较小. 5145nm激发光下得到的电子拉曼散射光谱表明,k位处由1s(A₁)到1s(E)的能谷轨道跃迁带来的拉曼谱6H-SiC中有四条,4H-SiC中有二条;高频6303及635cm^-1处观察到的谱线被认为与深能级缺陷有关. 最后,利用纤锌矿型结构二级拉曼散射选择定则指认了6 关键词： 碳化硅 电子拉曼散射 轨道能谷分裂 倍频谱     

Competition between Quantum Interference and Stimulated Electronic Raman Scattering Processes via Hybrid Excitation in a Na2-Na System

An experimental observation of the competition between quantum interference and stimulated electronic Raman scattering (SERS) via equal-frequency hybrid excitation and collision-aided radiative excitation in a Na2-Na system is reported for the first time.It is found that the SERS for 3P1/2,3/2-4P are suppressed and the optically pumped stimulated emission (OPSE) for 5S-4P are enhanced dramatically at a particular excitation wavelength where constructive quantum interference between two transitions 3P1/2-5S and 3P3/2-5S in atomic sodium takes place.A theoretical analysis based on the dressed-atom approach is carried out to explain the experimental results.     

Competition between Quantum Interference and Stimulated Electronic Raman Scattering Processes via Hybrid Excitation in a Na_2-Na System

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Raman spectroscopy of the BC3 phase obtained under high pressure and high temperature

A direct transformation of the g‐BC₃ phase to a new diamond‐like d‐BC₃ phase was observed in a diamond‐anvil cell (DAC) at high temperature, 2033 ± 241 K, and high pressure, 50 GPa. Analysis of the peak positions of the d‐BC₃, B₄C, α‐boron, and the boron‐doped diamond leads to the conclusion that the positions of the peaks of the d‐BC₃ are more similar to the peak pattern of the boron‐doped diamond rather than that of boron carbide, α‐boron. Copyright © 2007 John Wiley & Sons, Ltd.     

Origin of the forbidden phonons in Raman scattering spectra of uranium‐doped Ca2CuO3, a spin 1/2 chain system

The Raman scattering spectra of uranium‐doped Ca₂CuO₃ were investigated. The small doping of uranium (≤5%) in this one‐dimensional spin 1/2 chain system induced three new first‐order scattering bands and two new multiphonon bands in the structure of forbidden phonons. The first‐order bands were found to agree well with the existing theoretical results from the ab initio and tight‐binding calculation. Among them, the 470 and 665 cm⁻¹ bands appeared as the basic wavenumbers of which the multiphonon overtones were composed. The grain size effect in this strongly anisotropic system was proposed not to originate from the classical phonon confinement but rather as a result of the segmentation of one‐dimensional spin chains due to doping, which in turn allowed the new vibrational modes and implied the appearance of higher overtones in the scattering spectra. Copyright © 2008 John Wiley & Sons, Ltd.     

Pr掺杂DyFeO3体系的自旋重取向相变、晶格畸变与Raman光谱研究

利用固相反应法制备了Dy_1-xPr_xFeO₃系列化合物. X射线粉末衍射晶体结构分析表明, 随着Pr掺杂量x的增加, 样品晶胞体积逐渐增大, 晶格畸变减弱. Raman光谱测量表明稀土离子有效质量[m_eff=xm_Pr+(1-x)m_Dy] 与晶格结构的变化共同导致该体系Raman光谱的变化. 随Pr掺杂量的增加, 波数小于200 cm^-1的振动模式基本保持不变, 而波数大于200 cm^-1的振动模式(除420 cm^-1处的B_3u模式外)向低频移动. 磁测量结果表明, 由Dzyaloshinsky-Moriya 相互作用导致的宏观磁性随Pr掺杂量增加逐渐减弱. 稀土离子与铁离子磁晶格的耦合作用以及晶格结构畸变的变化共同导致该体系自旋重取向相变温度在一定的掺杂量 (x=0.3)前后先升高后降低. 关键词： 稀土铁氧体 自旋重取向 晶体结构 Raman光谱     

Raman scattering in the magnetically frustrated double perovskite Sr2YRuO6

The spin correlations and excitations of the Sr₂YRuO₆ double perovskite are investigated by means of Raman scattering, complemented by synchrotron X‐ray diffraction measurements. Anomalous softening of a breathing mode of the oxygen octahedra is observed below ~200 K, much above the long‐range antiferromagnetic ordering temperature, T_N1 = 32 K, due to a spin‐phonon coupling mechanism in the presence of magnetic correlations. A diffusive Raman signal is also observed, possibly associated with spin excitations within magnetically correlated regions. Our results point to a characteristic energy and temperature scale of ~25 meV/200 K below which unusual behavior associated with magnetic correlations is observed in this material. Copyright © 2013 John Wiley & Sons, Ltd.     

Strong charge carrier effect on the magnetic coupling of La-doped CaMnO3

The exchange interactions in polycrystalline samples of Ca_1−xLa_xMnO₃ (0.00x0.05) are studied by means of Raman scattering and electron paramagnetic resonance. Dramatic reductions in the spin-phonon interactions and magnetic correlations are observed for La doping levels as small as 2–3%. These results show that the charge carriers play an important role in the overall exchange coupling in the electron-doped manganites, even at very low doping levels.     

Structural relaxation and Jahn-Teller distortion of LaMnO3 (0 0 1) surface

We studied in detail the structural relaxation and Jahn-Teller distortion in LaMnO₃ (0 0 1) surface of the orthorhombic phase by means of classical atomistic simulation. It is found that MnO₂-terminated surface is more energetically favorable than LaO-terminated surface by 0.34 eV. The standard deviation of Mn-O bond lengths of MnO₆ octahedra and Jahn-Teller distortion oscillate in LaMnO₃ (0 0 1) surface. Our simulated atomic displacements in the surface are compared with some ab initio studies.     

自旋梯状化合物Sr14(Cu1-yFey)24O41的拉曼散射谱研究

利用常规的固相合成法制备了三种Sr₁₄(Cu_1-yFe_y)₂₄O₄₁(y=0,0.03,0.05)多晶样品,其中y=0.03是单纯Fe³⁺掺杂,y=0.05是Fe³⁺和Fe²⁺混合掺杂.在100—1500cm^-1频移范围内测量了这三种样品的偏 关键词： 拉曼散射 电输运性质 自旋梯状化合物     

Optical properties of -doped semiconductors: Plasmon–phonon coupling and Raman spectra

Collective excitations and their coupling to optical phonons have been studied for a two-dimensional electron gas in -doped polar semiconductors within the random-phase approximation. The inelastic light scattering spectrum due to the coupled plasmon–phonon modes are calculated for the multisubband two-dimensional electron systems. Our calculation shows that, due to the high electron density in these systems, both intrasubband and intersubband plasmons are strongly coupled to the optical-phonons. On the other hand, due to the high impurity concentration, level broadening modifies the inelastic light scattering spectrum significantly.     

Investigation of thermostability and phonon–phonon interactions in Mo6S3I6 nanowires by Raman scattering spectroscopy

Detailed Raman scattering measurements were performed on molybdenum–sulfur–iodine nanowires (Mo₆S₃I₆). At room temperature, 21 well‐resolved Raman modes were experimentally observed for the first time in this new compound. The phase stability and vibrational properties of the nanowires were investigated by different temperature treatments. High‐temperature Raman spectra showed that the phase separation of Mo₆S₃I₆ nanowires took place between 573 and 673 K, followed by appearance of a new mode at 819 cm⁻¹ characteristic of the MoO₃ phase. Low‐temperature Raman scattering spectra showed a significant difference in phonon–phonon interactions between internal and external Raman modes of Mo₆S₃I₆ nanowires. These interesting vibrational properties can give new insights for improved material preparation and achievement of higher conductivity and other functional properties of these otherwise interesting materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of isotopic composition and microstructure on the crystalline and magnetic phase states in R0.5Sr0.5MnO3

The results of structural neutron experiments on determining crystal and magnetic phase states of perovskite-like manganites R_0.5Sr_0.5MnO₃ (R = ¹⁵²Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) are reported. Experiments are carried out for revealing microscopic factors responsible for the giant oxygen isotope effect that was discovered recently in Sm_1?x Sr _x MnO₃ for x ≈ 0.5. It is shown that separation into two crystal phases P ₁ and P ₂ with the same spatial symmetry but different types of Jahn-Teller distortions in MnO₆ octahedra and magnetic ordering of Mn atoms takes place in all studied compounds at low temperatures. Structural analysis has been carried out successfully owing to exceptionally large differences in the unit cell parameters of the coexisting phases. The P ₁ phase is ferromagnetic and MnO₆ octahedra are distorted only slightly. The P ₂ phase is antiferromagnetic (A-type ordering) and MnO₆ octahedra are strongly compressed in the apical direction. The relative volumes occupied by the P ₁ and P ₂ phases depend on the mean radius of the A cation, and the replacement of ¹⁶O by ¹⁸O results in their redistribution in favor of the P ₂ phase. The results unambiguously point to the percolation nature of the metal-insulator transition in a Sm-containing compound upon isotopic substitution of oxygen due to a sharp decrease (from 65 to 13%) in the fraction of ferromagnetic phase P ₁. In all investigated compounds, the ordered magnetic moment of manganese Mn in the P ₁ and P ₂ phases varies from 1.7μ_B to 3.5μ_B. The data on the evolution of the miscrostructure parameters during a phase transition to the stratified state indicate that the initial spread in the A cation radii, as well as the internal microstrains, produce a critical effect on the formation of mesoscopic phase separation.     

Theory of the evolution of 2D band in the Raman spectra of monolayer and bilayer graphene with laser excitation energy

A double‐resonance process gives rise to the 2D band in the Raman spectra of monolayer and bilayer graphene. Based on the electronic and vibrational dispersion energies of graphene, the wavenumbers of the 2D band were calculated under different laser excitation energies (from 1.0 to 4.4 eV). Calculated results are in good agreement with experimental data and reproduce the experimental dispersion slope of the 2D band very well. The calculated wavenumbers of the 2D band do not show a linear dependence on the laser excitation energies. Moreover, it is explained that the lowest wavenumber peak of the 2D band of the bilayer graphene, which is composed of four components, has the largest slope with laser excitation energy. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman study of phase transition in ferroelectric Ba0.95Ca0.05TiO3

Raman scattering investigation of phase transition in the ferroelectric Ba_0.95Ca_0.05TiO₃ is reported. The results suggest onset of significant dynamic disorder at 105°C. This corroborates findings of recent structural study regarding large positional disorder associated with Ti and O1 atoms well below the tetragonal to cubic transition temperature (∼150°C).     

Bi_(3.15)Nd_(0.15)Ti_3O_(12)纳米棒的拉曼和紫外吸收光谱研究

以Bi(NO3)3.5H2O,Nd(NO3)3.6H2O和Ti(OC4H9)4为原料,加入聚乙烯醇(PVA-124),采用水热法在200℃经48 h合成了铋层状钙钛矿结构掺钕钛酸铋(Bi3.15Nd0.85Ti3O12,BNdT)纳米棒,纳米棒直径约10~200 nm,长度达十几微米。利用Raman散射研究了掺钕对钛酸铋晶格结构的影响。掺钕钛酸铋和钛酸铋的Raman光谱表明,Nd取代了类钙钛矿层中A位的Bi,掺Nd改善了BTO的对称性和减小了TiO6八面体的畸变。利用UV-vis光谱研究了BNdT纳米棒的光吸收特性,BNdT纳米棒存在A(400 nm),B(275 nm),C(210 nm),D(196 nm)四个吸收带,分别对应于电子从Bi3+的基态1S0到激发态3P1,3P2,1P1的跃迁和电子从阴离子团TiO6八面体到带正电的Bi3+离子的跃迁。BNdT的带隙为4.3 eV,大的带隙归因于纳米结构的量子尺寸效应。     

Coherent coupling between vibrational modes of Cben H bonds at different positions studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We performed femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) measurements on liquid toluene and PVK film. For both samples, we selectively excited the CH stretching vibrational modes and observed the expected quantum beat signals. The frequency of the well-defined beats is in good agreement with the energy difference between the two simultaneously excited modes, which demonstrates that a coherent coupling between the vibrational modes of the C-H chemical bonds exists at the different positions of the molecules. The dephasing times of the excited modes are obtained simultaneously.     

B2H6分子在C2v势阱中的频率分解及其各向异性效应

依据杨-泰勒效应理论、量子理论和群论探讨了具有D_3h对称性构型的B2Hs分子的E（?）e′系统在C_2v势阱中的频率分解及其各向异性现象.借助么正平移变换和标度变换计算出了杨-泰勒畸变后的系统振动频率,结果表明,畸变导致系统二重简并的振动模式e′的振动频率发生了分解,对于系统的4个C_2v势阱而言,无论系统处在哪一个势阱中,畸变所导致的频率分解都是相同的;畸变同时还导致系统的振动基态能量比畸变前降低了.正是这种基态能量的降低,使得畸变后系统就达到了一个更加稳定的状态.文中利用群论进一步探讨了系统的频率分解,结果发现,畸变导致系统的二重简并振动态e′分解为两种非简并的振动态,它们分别具有C_2v群下的a₁与b₂对称性.系统的频率分解与基态能量的降低就意味着系统的各向同性遭到破坏而呈现出各向异性.