Neutron scattering in impure anharmonic solids

The Debye-Waller factor and the coherent inelastic differential scattering cross section for the one- and two-phonon band modes have been investigated for an anharmonic crystal containing randomly distributed substitutional impurities of low concentration. The method adopted in the evaluation is that of double time temperature dependent Green's functions. The quantities thus evaluated have been separated into diagonal and nondiagonal terms. The diagonal terms are further separated in three contributions, namely the contributions from defects, anharmonicity, and simultaneous involvement of impurity and anharmonicity. A new mode, the impurity-anharmonicity interference mode, is the salient featue of the theory, and arises due to the interaction between local and anharmonic fields.     

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.     

Theory of Raman light scattering by nanocrystal acoustic vibrations

A theory of Raman scattering of light by acoustic phonons in spherical nanocrystals of zinc-blende and wurtzite semiconductors has been developed with the inclusion of the complex structure of the valence band. The deformation-potential approximation was used to describe the exciton-phonon interaction. It is shown that this approximation allows only Raman scattering processes involving spheroidal acoustic phonons with a total angular momentum F=0 or 2. The effect of phonon quantum confinement on linewidth in Raman scattering spectra and scattered polarization is analyzed. An expression for the shape of the spectral line corresponding to nonresonant scattering from F=0 phonons was obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 1473–1483 (August 1999)     

Anisotropic attenuation of transverse ultrasound in cubic crystals of Ge,Si, and diamond with various isotopic compositions

The attenuation of transverse ultrasound in germanium, silicon, and diamond crystals is considered with allowance for competing isotopic and anharmonic scattering processes. The dependence of the attenuation of transverse ultrasound on the direction of the wave vector of quasi-transverse phonons is analyzed within an anisotropic continuum model. The Landau—Rumer mechanism is considered for anharmonic scattering processes. Given the second-and third-order elastic moduli, the parameters are found determining ultrasonic absorption in the above crystals with various degrees of isotopic disorder. The attenuation coefficients of transverse ultrasound associated with isotopic and anharmonic scattering processes are shown to have qualitatively different angular dependences. Therefore, from studying the anisotropic attenuation of ultrasound in cubic crystals, one can determine the dominant mechanism of ultrasonic absorption in isotopically modified crystals.     

2 + 1 Flavors QCD Equation of State in NJL Model with Proper Time Regularization

Totally symmetric A_1g phonons are studied for the equilibrium and coherent states of a Bi₂Te₃ lattice. Equilibrium phonons were investigated in the frequency domain by the method of spontaneous Raman scattering, whereas coherent phonons were studied by the method of active femtosecond spectroscopy in the time domain. In the latter case, femtosecond laser pulses were used both for generating and detecting coherent A_1g phonons having a well-defined phase allowing the selective optical control of the lattice dynamics. A comparison of the results obtained in the frequency and time domains suggests that diagonal and nondiagonal elements of the density matrix of lattice excitations relax with the same characteristic time to the equilibrium and zero values, respectively.     

Raman scattering of light in silicon nanostructures: First-and second-order spectra

First-and second-order Raman scattering spectra in Si nanocrystals have been studied. The shift to lower frequencies and the substantial broadening of first-order Raman scattering lines observed to occur with decreasing nanoparticle size were established to correlate with those in second-order spectra. It is shown that the experimentally observed shifts of peaks and their broadening cannot be predicted based only on the phenomenological model of strong phonon wave function localization. The anharmonic effect originating from the heating of the nanoparticle surface by laser radiation should also be included. Proper fitting of experimental data revealed that the anharmonic constants depend strongly on nanoparticle size. The shape and spectral positions of maxima in second-order Raman scattering spectra have been theoretically described.     

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.     

Pressure dependence of phonon energies at critical points in the Brillouin zone in KTaO3 measured with differential second order raman scattering

Wavelength modulated Raman scattering was used to study the effects of pressure on the second order spectrum of KTaO₃. The pressure results lend support to previous identifications of structures in the second order differential Raman spectrum in terms of pairs of phonons from critical points in the Brillouin zone. Using these identification mode Grüneisen parameters are obtained for several vibrational modes. Strong anharmonic behavior of of the TA(X) mode is observed when we correlate the results from pressure measurements with previous studies of the effects of temperature.     

Resonant raman scattering at the E1 energy gap of semiconductor crystals

We present a discussion of resonant Raman scattering by optical phonons at the E₁ energy gap of group IV and groups III–V compound semiconductor crystals (e.g., Ge and InSb). For allowed scattering by TO and LO phonons, the q-dependent “double resonant” two-band calculation of the Raman tensor may display destructive interference effects when the intermediate electron-hole pairs are uncorrelated. We also discuss the Franz-Keldysh mechanism of resonant electric field induced Raman scattering by LO phonons. The double resonance terms due to this mechanism will, for large electric fields, broaden and have its largest resonance enhancement at the energy gap.     

Role of disorder and anharmonicity in the thermal conductivity of silicon-germanium alloys: a first-principles study

The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.     

Longitudinal-phonon relaxation mechanisms in cubic crystals of germanium,silicon, and diamond

The relaxation rates of thermal and high-frequency longitudinal phonons are calculated using an anisotropic-continuum model. Three-phonon scattering mechanisms (L ? L + L, L ? T + L) for the phonon relaxation are considered. Anisotropic anharmonic phonon scattering in cubic crystals is described in terms of the second-and third-order elastic moduli. The parameters determining the longitudinal-phonon relaxation rates are found for germanium, silicon, and diamond crystals. The long-wavelength limit and the transition to the isotropic-medium model are considered, and the dependences of the relaxation rates of thermal and high-frequency phonons on temperature and phonon wave vector are analyzed for these crystals.     

稀土发光和光谱理论的研究进展

简述了近20多年以来我们在稀土发光和光谱理论方面所做的主要工作、取得的进展和成果。包括:(1)关于(4f)^N组态内单光子跃迁强度:推广Judd-Ofelt三参量公式,在其中计入"J混合"(偶宇称晶场成分造成的)导致的修正;澄清了国际上关于Judd-Ofelt全参量公式中"静力学项"和"动力学项"相对正负号的争论(仍为负号)。(2)关于(4f)^N组态内双光子过程强度:提出直接计算强度方法(放弃缔合近似,利用Ce³⁺的5d和左邻RE³⁺的(4f)^N能级-波函资料,近似模拟出最重要的虚中间组态(4f)^N-15d的能级-波函结构)来讨论和改进Judd-Ofelt缔合近似。(3)固体中声子辅助的稀土离子间能量传输理论:发现HLO理论关于单声子过程中的"干涉相消"实际只适合于对角过程,而非对角过程则是"干涉相加",因而,后者首要的是单声子而非双声子过程;同时,用"晶体格波模"取代"Debye模"描述能量传输中的声子,给出了新模型下计算对角和非对角过程速率等物理量的整套公式。并由之看出:波矢q=0和对应色散曲线扁平段的光学声子对单声子非对角过程速率有完全不能忽略的贡献,这些都不同于或超出于HLO理论。(4)提出和发展了分析指认4f-5d跃迁光谱的简单理论模型,用能量和跃迁强度同时进行近似拟合,成功地分析指认了从(4f)³到(4f)¹³离子在几种基质中的4f→5d吸收(激发)谱,也成功用于若干5d→4f辐射谱。(5)对跃迁的初末晶场态同时引用有效自旋哈密顿描述稀土离子间超交换作用,成功地解释了低温10 K时立方结构的纯稀土冰晶石晶体Cs₂NaLnCl₆(Ln=Er,Yb)等的吸收谱中0声子线(量级为几个cm^-1)的精细分裂。     

Evolution of the 128-cm-1 Raman phonon mode with temperature in Ba(Fe1-xCox)2As 2 (x = 0.065 and 0.2)

We report electronic Raman scattering measurements on Ba(Fei_1-xCo_x)₂As₂（x = 0.065 and 0.2） single crystals with Raman shifts from 9 cm^-1 up to 600 cm^-1 in the symmetry of B_lg with respect to 1 Fe unit cell.When the crystals are cooled down,the evident quasielastic peaks of Raman spectra occur only in the crystal with x = 0.065,which is due to the contribution of orbital ordering between xz and yz Fe 3d orbitals,as we reported in another work.Here,we analyze the E_g phonon at 128 cm^-1,which has the same function form of its Raman tensors as those of xz and yz Fe 3d orbitals in these two crystals respectively.Unlike their electronic continuums,no anomalies are found in the E_g phonons of these two samples,which simply follows the expressions corresponding to the anharmonic phonon decay into acoustic phonons with the same frequencies and opposite momenta.Our results indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping and there is not any indication of coupling between charge nematicity and E_g phonon mode from our experimental results,which is consistent with the results in our previous work.     

Dispersive phonon linewidths: the E2 phonons of ZnO

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.     

Angular dependence of Raman scattering selection rules for long-wavelength optical phonons in short-period GaAs/AlAs superlattices

The angular dependence of Raman scattering selection rules for optical phonons in short-period (001) GaAs/AlAs superlattices is calculated and experimentally studied. Experiments are performed using a micro-Raman setup, in the scattering geometry with the wavevectors of the incident and scattered light lying in the plane of superlattices (so-called in-plane geometry). Phonon frequencies are calculated using the Born model taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra are calculated in the framework of the deformation potential and electro-optical mechanisms. Calculations show an angular dependence of the selection rules for optical phonons with different directions of the wavevectors. Drastic differences in the selection rules are found for experimental and calculated spectra. Presumably, these differences are due to the Fröhlich mechanism in Raman scattering for short-period superlattices.     

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.     

高温超导体的喇曼光谱研究

<正> 本文评述高温超导体的喇曼光谱研究工作,较详细地介绍了超导体中声子、电子和磁子的喇曼散射及其与高温超导性的关联.一、引言二、晶体结构与晶格振动谱晶体结构与对称性晶格动力学     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.