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.     

Raman scattering in impurity-induced anharmonic crystals

The theory of first-, second-, and third-order Raman scattering is investigated for isotopically disordered anharmonic crystals. The theory of time-dependent thermodynamic Zubarev Green's functions is adopted to obtain the Raman tensor, intensity of Raman lines, and differential cross sections of various orders of scatterings. It is observed that each class of scattering can be separated into diagonal and nondiagonal parts. The first-order and nondiagonal parts are absent in the case of chemically pure crystals. The diagonal parts are separated into anharmonic and interference terms. The interference terms arise due to the interactions of anharmonic phonons with the local phonons. The temperature and defect dependencies are discussed in detail along with the nature of continuous and line spectra. It is proposed that very high-power laser sources will reveal the third-order spectra, and that the resulting structure can be explained with the help of temperature-dependent one-, two-, and three-phonon density of states.     

Modern Approach to Optical Absorption in Cuprate Superconductors

Adopting the many body quantum dynamical theory of phonon Green’s functions via a general Hamiltonian which includes the effects of electron-phonon interactions, anharmonicities and impurities (but not those of BCS terms) the theory of infrared (i.r.) absorption has been investigated which separates out into (i) diagonal and (ii) non-diagonal contributions. The non-diagonal contribution emerges only in the impure crystals and becomes extinct in pure crystals. The critical investigations divulges that the infrared (i.r.) absorption coefficients not only depends upon electron-phonon coupling coefficient but also on temperature, which appears as the unique feature of the present theory and provide us a plat form to study a system in superconducting and normal regions. The Y B a ₂ C u ₃ O _7−δ superconductor has been taken for the purpose of analysis and comparison with the well-established experimental results.

     

Electric field dependence of the Curie temperature and microwave absorption in displacive ferroelectrics with impurities II

Expressions for the complex dielectric constant, microwave absorption and the Curie temperature in doped displacive ferroelectrics, subjected to an external electric field are discussed, using the approach made in our previous study. A cross-term of defect parameters with electric field and anharmonic parameters is obtained. The intrinsic parametersB andC are modified by impurity terms. The qualitativeE ² dependence of tan δ is discussed.     

The flavor problem and discrete symmetries

In this Letter we propose a multi-Higgs extension of the standard model with Abelian and non-Abelian discrete symmetries in which the mass matrices of the charged fermions obtained from renormalizable interactions are diagonal. However, non-diagonal contributions, that are important for obtaining the CKM matrix in the quark sector, arise from non-renormalizable dimension five interactions. Active neutrinos acquire mass only from non-renormalizable interactions, the non-diagonal entries arising through dimension five operators, while the diagonal entries comes from dimension six operators. Realistic mixing matrices in the neutrino and the quarks sectors are obtained.     

24Mg集体态的三体非对角玻色相互作用修正

引入三体非对角玻色相互作用计算了²⁴Mg的能谱,克服了以往理论谱中0过低的困难,改进了理论和实验的符合程度.说明在某些情形下三体非对角玻色相互作用有可能是重要的.     

Correction of 24Mg Collective States With Three-Body Non-diagonal Boson Interaction

Although it has been verified that the interacting boson model (IBM) can be applied to light nuclei in sd shell successfully, the deficiency shll exists. The level of experimental spectrum of ²⁴Mg is much higher than 3. The similar result can not be obtained by using of conventional IBM, in which only one-body and two-body interactions are concerned. Three-body interactions have been discussed by some authors, but the authers mainly considered the diagonal interactions. Moreover,the defect mentioned above can not be solved with these interactions. Introducing three-body non-diagonal interaction, the energy levels of ²⁴Mg are calculated, the difficulty of low 0 theoretical level is overcome and the coincidence betWeen theory and experiment is improved. This work illustrates that in some cases the three-body non-diagonal boson interaction is important.     

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.     

Electric field dependence of the Curie temperature and microwave absorption in displacive ferroelectrics with impurities

The electric field dependence of the complex dielectric constant in an anharmonic defect ferroelectric crystal is calculated in its paraelectric phase from the Silverman-Joseph-Hamiltonian augmented with fourth-order phonon coordinates using double-time Green’s functions. The field and impurity dependence of the shift in the Curie temperature is discussed. An expression is obtained for the electric field dependence of dielectric loss at microwave frequencies in defect ferroelectrics. An erratum to this article is available at .     

Structure constants for rational conformal field theories

Exploiting the associativity of the operator product algebra, we obtain the structure constants for the affine SU(2) complementary and exceptional series, uniquely determined in terms of the diagonal ones. The result extends to the non-diagonal unitary minimal series with c<1. We investigate the more general cases of class I (extended diagonal) and class II (non-diagonal) algebras and find a relation between them based on the symmetries of their fusion rules.     

用一维光子带隙结构增强硫化镉双光子吸收研究

用真空镀膜方法制备了含有单个CdS缺陷层的具有不同周期和结构参量的TiO2／SiO2一维光子晶体。用抽运一探测技术研究了CdS缺陷层的双光子吸收（TPA）现象。实验结果表明：一维光子晶体中CdS缺陷层的双光子吸收显著增强。不同周期和结构参量的一维光子晶体中CdS缺陷层的双光子吸收系数不同。双光子吸收的增强来源于由光局域化导致的缺陷层的电场强度的增加。缺陷层电场强度与一维光子晶体的结构有关,如周期,光子带隙的位置与宽度及缺陷模式等因素都会影响缺陷层电场强度。采用四分之一波长的高低折射率介质层和与入射波长匹配的缺陷模可以得到最大的缺陷层电场强度。     

Resonant and localised modes due to boron in gallium arsenide

Silicon-doped n-type GaAs crystals pulled from a melt encapsulated with boric oxide exhibit an IR absorption band at 123 cm^?1 as well as a number of localised-mode bands associated with boron and silicon. Evidence is presented which leads us to attribute the band at 123 cm^?1 to a resonant mode associated with isolated B atoms on Ga sites. Other crystals containing high concentrations of silicon or phosphorous did not show such an absorption band. The results are discussed briefly in terms of the simple mass defect model for impurity vibrations.     

Thermodynamics of impure anharmonic crystals

The vibrational thermodynamic properties of doped anharmonic solids are investigated using thermodynamic Green's functions. The study uses cubic and quartic anharmonic interactions, and different masses and force constants for the impurity and host lattice atoms. The explicit expressions so derived for the partition function, free energy, entropy, and lattice heat capacity for these solids in the low-impurity-concentration limit are discussed for various situations. Our results are modified due to the presence of interaction terms between the anharmonic and localized fields and reduce to those obtained by earlier workers as limiting cases.     

The lattice anharmonicity-induced broadenings of defect spectral lines

Summary The effect of the anharmonicity of the host lattice to the defect optical absorption and emission is studied. Anharmonicity contributes to the defect optical linewidths via phonon lifetime effect and defect-phonon scattering mediated by a second phonon. Numerical estimates of these contributions indicate that the anharmonic broadenings may not be negligible in some insulating crystalline hosts.     

Advances in theoretical and experimental XAFS studies of thermodynamic properties,anharmonic effects and structural determination of fcc crystals

Thermodynamic properties, anharmonic effects and structural determination of fcc crystals have been studied based on the theoretical and experimental Debye–Waller factors presented in terms of cumulant expansion up to the third order, thermal expansion coefficient, X-ray absorption fine structure (XAFS) spectra and their Fourier transform magnitudes. The advances in these studies are performed by the further development of the anharmonic correlated Einstein model primary only for approximating three first XAFS cumulants into the method using that all the considered theoretical and experimental XAFS parameters have been provided based on only the calculated and measured second cumulants. The obtained cumulants describe the anharmonic effects in XAFS contributing to the accurate structural determination. Numerical results for Cu are found to be in good agreement with the experimental values extracted by using the present advanced method and with those obtained by the other measurements.     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

Infra-red absorption due to localized modes of vibration of impurity complexes in ionic and semiconductor crystals

A brief account is given of the theory of the vibrations of an imperfect crystal containing isolated impurities and pairs of impurities, and the strength of the induced infra-red absorption is discussed. An analysis is then made, by the methods of group theory, of the energy levels and expected optical transitions of an anharmonic localized oscillator vibrating in a static potential well with various point symmetries. These results are compared with the available experimental data for pairs of impurities and more complicated impurity aggregates in ionic crystals with the fluorite and rocksalt structures, and also for similar defects in the covalent crystals silicon, germanium and some III–V compound materials.     

Fe：BSO晶体的缺陷能级及光折变效应的研究

在硅酸铋（Ｂｉ１２ＳｉＯ２０，ＢＳＯ）晶体中掺入少量Ｆｅ２Ｏ３，生长出Ｆｅ：ＢＳＯ晶体，由于Ｆｅ杂质在ＢＳＯ晶体禁带中造成了一些缺陷能级，使得晶体的衍射效率和四波混频位于共轭反射率等光折变性能指标有较大提高，文中根据吸收光谱，初步确定了晶体中缺陷能级。     

Fe∶BSO晶体的缺陷能级及光折变效应的研究

在硅酸铋（Bi12SiO20，BSO）晶体中掺入少量Fe2O3，生长出Fe：BSO晶体，由于Fe杂质在BSO晶体禁带中造成了一些缺陷能级，使得晶体的衍射效率和四波混频位相共轭反射率等光折变性能指标有较大提高。文中根据吸收光谱，初步确定了晶体中缺陷能级。     

Theory of surface force-constant changes in body-centered cubic lattices

A calculation has been made of force-constant changes at the (100) and (110) surfaces of body-centered cubic crystals. A model consisting of first, second and third-neighbor Lennard-Jones interactions together with harmonic angle-bending interactions is employed. The parameters characterizing the interactions are chosen to fit the equilibrium lattice spacing, the elastic constants, and certain phonon frequencies at high symmetry points in the Brillouin zone. The procedure consists of first calculating the static displacements produced by the creation of the surface and then calculating the change in force constants produced by the cubic and quartic anharmonic terms in the potential energy. Specific results have been obtained for chromium, iron, tungsten, and molybdenum.