Concentration dependence of the mixed crystal modes of vibration

The concentration dependence of the reststrahl absorption in various mixed crystals exhibiting one, two and mixed mode behaviour is investigated using the coherent potential approximation (cpa). The phonon Green’s function, the impurity mode frequencies and the strength of absorption are calculated in the Einstein model from the generalizedcpa proposed by Tripathi and Behera which takes into account both mass and force constant changes. The introduction of a phenomenological concentration dependence of the force constant change parameter is shown to provide a satisfactory explanation of the concentration dependence of the experimental data for the twenty mixed crystal systems analysed. It is conjectured that the nearest neighbour force constant of an impurity atom substituted at a host site is very much different from that of a perfect crystal consisting of these impurity atoms and that both these play an important role in determining the one, two and mixed mode behaviour of the mixed crystals.     

Effect of electric current on the migration of hydrogen interstitial atoms in the region of a crack tip in a crystal

The effect of a constant electric current on the migration of interstitial atoms dissolved in a crystal in the region of a tensile crack tip is estimated. The calculation takes into account plastic deformation that is produced in the vicinity of the crack tip in the loaded sample by dislocation motion in active slip planes of the crystal under the action of mechanically and electrically induced shear stresses, Joule heat release, the Thomson effect, and ponderomotive forces and allows for the effect of gas exchange near the crack edges on the evolution of the distribution of interstitial impurity atoms. The time dependence of the stress intensity factor is found for both the cases of the presence and absence of a constant electric current near the crack tip. Numerical calculations are performed for an α-Fe crystal.     

纳米尺寸CdS半导体微晶的光致发光线宽宽化研究

测量了各种温度下玻璃中掺杂纳米尺寸CdS半导体微晶的光致发光光谱,研究了其光致发光带的峰值能量和线宽对温度的依赖关系.考虑CdS微晶的LO声子的相互作用,拟合了其带隙发光带的线宽随温度的变化曲线,获得样品的非均匀线宽.CdS微晶的非均匀线宽主要是由于微晶的尺寸分布引起的.     

纳米尺寸CdS半导体微晶的光致发光线宽宽化研究

测量了各种温度下玻璃中掺杂纳米尺寸ＣｄＳ半导体微晶的光致发光光谱，研究了其光致发光带的峰值能量和线宽对温度的依赖关系．考虑ＣｄＳ微晶的ＬＯ声子的相互作用，拟合了其带隙发光带的线宽随温度的变化曲线，获得样品的非均匀线宽．ＣｄＳ微晶的非均匀线宽主要是由于微晶的尺寸分布引起的．     

Phenomenological theory of the interaction of electrons with polar phonons in anisotropic crystals

The generalized phenomenological lattice equations of Huang are used to derive the Hamiltonian of the coupled electron phonon system in an anisotropic crystal. The dependence of the coupling constant on the angle formed by the phonon wave vector and the axis of an uniaxial crystal is given explicitely. A significant dependence of the coupling constant on this angle persists even in the case of dominant mechanical or electrostatic forces.     

表面稳定铁电液晶器件灰度特性的测量研究

观察到了多畴结构铁电液晶的灰度显示现象,测量了表面稳定铁电液晶像素透率与驱动电压的依赖关系,研究了液晶盒导电玻璃RC常数以及驱动脉冲持续时间对器件电光响应波形的影响,对这几种情况所得到的光电倍增管的响应波形作了分析和解释,同时对产生这种灰度现象的机理作了讨论。     

Thermal expansion of anharmonic crystals composed of inert gases

The temperature dependence of the lattice constant and linear expansion coefficient for inert gas crystals are determined using the second-order approximation of Bogolyubov's temperature expansion. It is shown that when the temperature dependence of the Gruneisen constant is taken into account the agreement with experiment is improved over the thermodynamic perturbation theory and the theory based on a multiplicative binary distribution function.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 44–47, April, 1976.     

Influence of metastable-metastable collisions on electron distribution function in afterglow

The time dependence of the electron distribution function in helium afterglow, for pressures of 60–260 Pa, is calculated for the case in which metastable-metastable reactions are present. The time variation of the shape of the distribution function shows that careful consideration must be made of the time derivative of the distribution function in the Boltzmann equation for the calculation of the rate constant of such reactions from experimentally determined distribution function.     

Distribution function of relaxation times for a 4-<Emphasis Type="Italic">n</Emphasis>-pentyl-4′-cyanobiphenyl liquid crystal

The distribution function of relaxation times is reconstructed from the dielectric loss spectrum measured over a wide range of frequencies for a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal. It is demonstrated that the distribution function for the isotropic and nematic phases is asymmetric in shape. Comparison shows that the reconstructed distribution function is in qualitative agreement with similar functions that are analytically derived from the Cole-Davidson and Havriliak-Negami empirical equations. The specific features observed in the behavior of the distribution function with a variation in the angle between the direction of polarization of a microwave electric field and the director of the liquid-crystal molecules are analyzed. A complex dependence of the permittivity for the liquid crystal in the range of ultrahigh frequencies is explained in terms of additional relaxation mechanisms associated with different motions of molecular fragments of alkyl chains.     

Optical and dielectric studies of gel grown α-hopeite single crystal

Single crystals of α-hopeite exhibiting high transparency were grown by single diffusion gel growth technique. Single crystal X-ray diffraction analysis reveals that the crystal belongs to orthorhombic system. The values of several structural and physical parameters have been determined for the grown crystal. The optical absorption study reveals the transparency of the crystal and is noticed in the entire visible region and the cut-off wavelength was found to be 230 nm. The optical band gap found to be at 3.25 eV. The dependence of extinction co-efficient (k) and the refractive index (n) on the wavelength was also shown. The dielectric constant and dielectric loss of the crystal was studied as a function of frequency and temperature. Transport properties of the grown crystal have been studied from the Cole-Cole plot.     

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程,利用扫描电镜和原子力显微镜等测试手段,观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值,并利用非线性玻璃棒展宽脉宽,得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系;利用抽运一探针超快探测平台,探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型,并根据晶体材料反射率与材料的介电常量的依赖关系,通过数值计算,模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过程中反射率的变化关系。结果表明,理论结果与实验结果符合较好。讨论了飞秒激光照射下LiF晶体中导带电子数密度的变化规律,并解释了相应的实验结果。     

A model for the spatial distribution and transport properties of defects in mixed ionic-electronic conductors part I: Defect concentration — Pressure relationships and the spatial distribution of defects

The spatial distribution and transport of defect species in a mixed ionic-electronic conductor (MIEC) depend on the activity of the external gas phase at either side of the MIEC, the defect equilibria and mobility of the species within the MIEC. Previous researchers [1, 2] developed models for defect distribution and transport based on various assumptions, the most prevalent of which is that the concentration of the ionic defect species is constant through the range of activities of the external gas phase in which the MIEC finds application. However, the defect equilibria for many MIECs do not support this assumption. It is desirable, therefore, to have explicit, analytical expressions for the spatial distribution of the defects as a function of position and the flux as a function of the activity gradient of the defect species and the gas phase. Towards the development of such a model, the defect concentration-oxygen partial pressure dependence in an oxide MIEC, such as Ce_0.8Sm_0.2O_2−δ, is considered. From the defect equilibria, equations are developed for the dependence of the defect species on the oxygen activity in the oxide MIEC. Then, to relate the defect equilibria to the spatial parameters of the MIEC, an analytic expression for the variation of oxygen activity as a function of position in an electrolyte is derived. Equations relating oxygen activity to position and defect concentration to oxygen activity are then combined to obtain a defect concentration-position relation. All the relations are derived without the assumption of constant vacancy concentration. Finally, the results are compared with those of previous models.     

Finite-temperature quasicontinuum: molecular dynamics without all the atoms

Using a combination of statistical mechanics and finite-element interpolation, we develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nano-indentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter.     

Probability of electron emission in photoemission from doped semiconductor

Electron motion is investigated in a strong-field region —in the region of bending of the band close to the surface of a semiconductor. In this region there is a change in the electron distribution function under the action of the electric field and interaction mechanisms leading to energy loss of the hot electrons. The main characteristics of the external photoeffect are determined by the energy lost by the electron in the region of band bending and the size of the barrier at the semiconductor-vacuum interface. The electron-emission probability from a crystal containing a space-charge region is determined from the solution of the Boltzmann kinetic equation. The energy dependence of the energy scattering length of the hot electrons is taken into account in the calculation. The calculation is made for photoemission from GaAs in conditions when the interaction with polar optical phonons is the most effective energy-scattering mechanism. An expression is obtained for the energy distribution function of the emitted electrons for the case of strong electric fields. The position of the distribution-function maximum depends on the electric field and the effective interaction constant with phonons. The current and quantum yield of the photoeffect are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 81–87, October, 1978.In conclusion, it remains to extend profound thanks to V. L. Bonch-Bruevich for Ms constant interest in the work.     

Pressure and temperature dependences of the dielectric constant,Raman spectra and lattice constant of SnI42

The pressure and temperature dependences of the Raman frequencies, static dielectric constant and lattice constant of the molecular crystal SnI₄, were investigated. These results are combined to evaluate the pure volume and pure temperature (i.e. volume-independent) dependences of the molecular polarizability and of the Raman-active phonon self-energies. The pressure results also allow the separation of the Raman modes into external and internal modes of the crystal; the external modes exhibit a much stronger pressure dependence, and thereby larger Grüneisen parameters, than do the internal modes. The Raman frequencies increase (decrease) with increasing pressure (temperature) whereas the dielectric constant increases with both increasing pressure and temperature, emphasizing the importance of non-volume effects. In fact, it is found that the isobaric temperature dependence of the dielectric constant is dominated by the pure temperature dependence of the molecular polarizability. The pure volume dependence of the polarizability, on the other hand, is relatively small as is perhaps typical of most molecular crystals.     

Possible determination of the emission wavelength of active ruby from its temperature. II

The temperature shift of the ruby R₁ line has been studied on the basis of the absorption, luminescence, and laser-emission spectra above room temperature. When the same method is used to heat the crystal and measure the temperature, the emission and absorption data agree. The curves showing the temperature dependence of the wavelength for a crystal not held at a constant temperature show a low-temperature shift with respect to the curves for a crystal held at a constant temperature, and they have a different slope. This is due to a difference between the surface temperature of the crystal measured experimentally and the actual temperature within the crystal if the crystal is not held at a constant temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 37–40, May, 1969.     

Chiral electrostatic interaction and cholesteric liquid crystals of DNA

Explicit expressions for electrostatic interaction between stiff DNA duplexes of finite length are obtained. These expressions allow for the helical symmetry of charge distribution on DNA molecules and reveal chiral and non-chiral interaction terms. Asymptotic expressions at small twist angles are applied to evaluate the cholesteric pitch and the twist elastic constant and their dependence on the length of DNA fragments. These estimates suggest an explanation for the large value of the cholesteric pitch and its nonmonotonic variation with the density of the liquid crystal. An analysis of biaxial correlations rationalizes the driving force of the transition from the cholesteric to hexagonal phase upon dehydration.     

Ferromagnetic resonance of nonstoichiometric zinc ferrite and cobalt-doped zinc ferrite nanoparticles

Ferromagnetic resonance spectra of zinc ferrite and cobalt doped zinc ferrite nanoparticles, measured at various temperatures, exhibit an invariant point at a given field. This makes it possible to determine the equation relating the resonance field shift to the peak-to-peak linewidth. When particles are frozen in a matrix in a magnetic field, the anisotropy constant of the material can be derived from the angular variation of the resonance field. This procedure is useful to determine the thermal dependence of the anisotropy constant, but is shown to require various freezing temperatures experiments to estimate the accuracy of the deduced anisotropy constant values. It is also shown that the angular dependence of the resonance field is similar for a uniaxial (zinc ferrite) and cubic (zinc ferrite containing 40% cobalt ions) anisotropy. This unexpected result is explained by the weakness of the texturation, leading to a distribution in easy axes directions.     

Dielectric properties of RbCl-RbBr mixed crystals

A systematic measurement of dielectric constant and loss on RbCl-RbBr mixed crystals in various compositions has been carried out in the frequency range 100 Hz to 100 kHz and in the temperature range from room temperature to 320°C. From these measurements the static dielectric constant, the Szigeti charge, the conductivity and the activation energy for conduction are evaluated. All these properties show a nonlinear composition dependence. Semiempirical equations proposed earlier are employed to evaluate the dielectric constant as a function of composition. The validity of these relations is also discussed.     

Manifestations of the higher moments of the orientation distribution function of molecules in the spectral properties of an impurity nematic

The polarized electronic absorption spectra, orientation ordering, and the special local field features were studied for push-pull linear dye molecules with strong donor-acceptor electronic conjugation of terminal fragments in the matrix of a nematic liquid crystal. The temperature-induced inversion of the sign of the splitting of polarized impurity absorption bands was observed. This effect was shown to be caused by the statistical character of orientation ordering of impurity molecules and manifestation of the higher moments of the orientation distribution function. The dependence of local field parameters (Lorentz tensor components) of impurity molecules on their orientation ordering was established. This dependence was used to reproduce the temperature dependence of the orientation order parameter of the matrix.