The electro-optic kerr effect in noncentrosymmetric KH2PO4 and KD2PO4 monocrystals

Experimental and theoretical results of the determination of the quadratic electro-optic coefficients in noncentrosymmetric KH₂PO₄(KDP) and KD₂PO₄(DKDP) monocrystals are presented. The coefficientR _c=R ₁₁₂₂-R ₁₁₁₁ for the X-cut crystals has been measured by a dynamic method in the a.c. electric field. The theoretical value of the coefficient has been calculated by the Kurtz-Robinson model. It has been found that the coefficientR _c was less than 10×10^–20m²V^–2.     

Analysis of quadratic electrooptic response of BaTiO<Subscript>3</Subscript> crystal in ferroelectric phase

Using the example of BaTiO₃ in a ferroelectric phase it is shown that a large difference in magnitudes of individual linear electrooptic coefficients may be a reason of additional indirect quadratic contributions that are independent of real quadratic coefficients. For some directions of the applied field and light, the indirect contributions may be even larger than the real quadratic ones. Independently of nonlinear distortions that can affect applicability of technical devices, a large difference between linear electrooptic coefficients may lead to serious problems in measurements of the real quadratic electrooptic effect. The analysis is based on the extended Jones matrix calculus applied to a Gaussian beam.     

High Accuracy and High Sensitivity Measurements of the Electrooptic Effect in Undoped and MgO-Doped LiNbO3 Crystals

Accurate measurements of the composite electrooptic (EO) coefficients r_c consisting of the EO coefficients r₁₃, r₃₃, and the piezoelectric constant d₃₁ in undoped and MgO-doped LiNbO₃ (LN) were made by extracting the fundamental and third harmonic components from photo-detection signals of the parallel Nicol optics including the LN crystal driven by minimal sinusoidal voltage. The analysis of relationship between multiple-reflection rays in the EO crystal and the accuracy of measurement indicates that anti-reflection films must be coated on the end faces of the crystal for highly accurate measurements. Measured values of r_c of undoped and MgO-doped LN crystals at a wavelength of 632.8 nm were 19.8 ± 0.1 pm/V and 19.2 ± 0.1 pm/V, respectively.     

Influence of transport current and thermal fluctuations on the resistive properties of HTSC+CuO composites

Measurements of the temperature dependence of the electrical resistance R(T) below the superconducting transition temperature have been performed at different values of the transport current in HTSC+CuO composites modeling a network of weak S-I-S Josephson junctions (S—superconductor, I—insulator). It has been shown experimentally that the temperature dependence R(T) at different values of the transport current is adequately described by means of the mechanism of thermally activated phase slippage developed by Ambegaokar and Halperin for tunnel structures. Within the framework of this model we have numerically calculated the temperature dependence of the critical current J _c(T) as defined by various criteria. Qualitative agreement obtains between the measured and calculated temperature dependences J _c(T). Fiz. Tverd. Tela (St. Petersburg) 41, 969–974 (June 1999)     

Gap distribution and multigap-coupling in highT c 's

Gap coupling interaction is a necessary consequence if a superconducting system has a gap distribution, as found for the high-T _c oxocuprates, since the interaction has to result in a single critical temperatureT _c . This paper shows with the help of BCS gap-coupling equations and values of gap distribution, including gap weights from experiments, thatT _c values as observed are easily achievable with reasonable diagonal and off-diagonal coupling coefficients. Of course our phenomenologically derived coupling coefficients have to be verified from purely theoretical grounds. Most importantly, it is found that the highest gap contributes dominantly toT _c in spite of its low weight. Predictions for a point-contact tunneling spectroscopy are made.     

Temperature dependence of linear electrooptic coefficients r 113 and r 333 in lithium niobate

A new method of determination of individual linear electrooptic coefficients is proposed. The technique is based on the dynamic polarimetric measurements and takes into consideration the temperature dependences of ordinary and extraordinary refractive indices. Results obtained for the electrooptic coefficients r₁₁₃ and r₃₃₃ in LiNbO₃ are presented. The coefficients are found to increase significantly within the considered temperature range 25-200°C. The temperature dependences of the intrinsic coefficients m₁₁₃ and m₃₃₃, defined in terms of the induced polarisation, are considered as well.     

Electrooptic properties of lithium niobate crystals for extremely high external electric fields

The electrooptic effect in lithium niobate crystals (LiNbO₃) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10^-3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s₁₃|≤2.3×10^-21 m²/V² is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. Received: 29 October 2002 / Revised version: 14 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: ml@uni-bonn.de     

Perturbation theory of polar hard Gaussian overlap fluid mixtures

A perturbation theory of polar hard Gaussian overlap fluid mixture is discussed. Explicit analytic expressions for the second and third varial coefficients are given. Numerical results are estimated for the thermodynamic properties of quadrupolar hard Gaussian overlap fluid and fluid mixture. It is found that the excess free energy and internal energy depend on concentrationsc ₁,c ₂, molecular diameter ratioR, shape parameterK and the quadrupole momentsQ*₁,Q*₂.     

Power Reflection,Transmission and Absorption Coefficients for a Moving Plasma Slab

Expressions for power reflection (R), transmission (T) and absorption (A) coefficients for p-polarized wave for a warm, collisional, magnetized and moving plasma slab (with sharp boundaries and thickness d₀) are investigated. The effects of plasma slab velocity (β=v/c), electron density (ωp/ω)² and plasma temperature (K_BT) on reflection (R), transmission (T) and absorption (A) coefficients are discussed numerically. It is observed that for the value β=?0.6, reflection coefficient (R) becomes more than unity, whereas absorption coefficient (A) becomes quite negligible while transmission coefficient (T) shows oscillatory behaviour. The variation with plasma frequency (ω_p/ω)² shows that at lower plasma frequency (ω_p/ω)²=0.2 transmission (T) and absorption (A) coefficients are minimum while reflection coefficient (R) is maximum.     

Concerning the two magnetic correlation lengths above T c in an Invar iron-nickel alloy

The magnetic phase transition in the Invar alloy Fe₇₀Ni₃₀ is investigated by means of small-angle neutron scattering over a wide range of momentum transfer. This method was used to measure two magnetic correlation lengths R _c 1 and R _c 2 which coexist in the alloy above the phase transition temperature T _c . The critical correlations with correlation length R _c 1 are described well by an Ornstein-Zernicke expression, and the critical correlations with the second correlation length, an order of magnitude larger than R _c 2, are described well by a squared Ornstein-Zernicke expression. The temperature dependences obtained for the correlation lengths R _c 1 and R _c 2 satisfy the power law R _c ∼((T−T _c )/T _c ) ^−υ with critical exponents υ₁=0.65±0.05 and υ₂=1.3±0.1 for the shorter and longer scales, respectively. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 53–57 (10 July 1997)     

Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle. Fiz. Tverd. Tela (St. Petersburg) 41, 310–318 (February 1999)     

Size effects in the melting and crystallization temperatures of copper chloride nanocrystals in glass

The size dependence of the crystallization temperature T _c and melting temperature T _m of copper chloride nanocrystals in glasses is studied in the size range R _a from 1 to 30 nm. Jumps are found in the temperature dependence of T _c at R _a =13 nm and R _a =1.8 nm, a jump is found in the temperature dependence of T _m at R _a =2.2 nm, and the supercooling of the melt is found to vanish in the case of crystallization at R _a ⩽1.4 nm. It is conjectured that the jump in T _c with large R _a occurs for a nano-crystal radius equal to that of a crystalline nucleus in the melt, and the jumps in T _c and T _m for smaller values of R _a are due to a change in the shape (faceting) of the nanocrystals. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 481–486 (10 October 1997)     

Diffusion and transport of a magnetic field in a turbulent medium with helicity

An analysis is made of the relation between accurate formulas for the coefficients of turbulent diffusion D _T and the alpha effect α _T for a magnetic field in the Lagrange and Euler representations. It is shown that the quadratic term with respect to α _T in the diffusion coefficient derived by Moffatt and Kraichnan is incorrect and should be dropped. First, a numerical solution of the nonlinear equation (DIA equation) for the Green function is presented, describing the transport of a magnetic field for the case of incompressible, uniform, isotropic, steady-state turbulence possessing helicity. These solutions are used to calculate the steady-state coefficients D _T and α _T for various values of the parameters ξ ₀=u ₀ σ ₀/R ₀, a=H ₀/u ₀ ² p ₀, σ ₀/σ ₁, and R ₀/R ₁, where u ₀, σ ₀, and R ₀ are the characteristic velocity, lifetime, and scale of the turbulent pulsations, and H ₀, σ ₁, and R ₁ are similar values describing the helicity of the medium h(1,2)=〈u(1)· (∇×u(2))〉, and the parameter α characterizes the degree of helicity. The DIA values of D _T and α _T and the self-consistent values of these quantities calculated using the Green tensor in the diffusion approximation are in qualitative agreement. It is shown that the coefficient of turbulent diffusion is always positive for all the types of turbulence studied. Nonsteady-state values of D _T(t) and α _T(t) calculated by a self-consistent method are given. Zh. éksp. Teor. Fiz. 112, 1312–1331 (October 1997)     

Stress and dislocations at cross-sectional heterojunctions in a cylindrical nanowire

The limit of crystal lattice coherency of a cross-sectional heteroepitaxial junction in a nanowire is calculated in terms of the critical nanowire radius R _c, based on a general calculation of elastic stresses in a long cylindrical rod. R _c is derived from the kinetics of a possible misfit dislocation which can slip in the heterointerface and rest in an energetic minimum, if it occurs at all, regardless of whether it is of zero total energy as assumed in the literature. A close comparison is made with the known models for the critical radius of a dislocation half-loop and the critical thickness h _c of a heteroepitaxial film, where all models are refined by including the energy of the slip step formed or accidentally annihilated. For a symmetrical, abrupt heterojunction, we obtain R _c as a lower and, therefore, quite safe limit, about five times larger than h _c of a comparable thin film. An even larger R _c is found for junctions of finite transition width instead of abrupt transitions. It is estimated that nucleating the dislocations in a perfect nanowire is difficult, in agrement with experimental reports of dislocation-free nanowires with R well above the theoretical R _c.     

Gravitational Field Equations of Fourth Order and Supersymmetry

We discuss the field equations which stem from a variational principle containing the quadratic terms αR_μνR_μν and βR² besides the Einstein-Hilbert Lagrangian R. Comparison of this theory with a pure theory of fourth order shows that R must necessarily be included if we wish to interpret the field equations as gravitational equations. The Einstein-Bach-Weyl theory (α = ?3β) has the property of being a theory of “supergravitation”. Apart from gravitons without rest-mass, we have here only one additional kind of particles with rest-mass. Their mass may be determined by Planck' slength (hG/c³)^1/2. The occurrence of those particles results from the breakdown of a “supersymmetry”, that is of the conform invariance. The Einstein tensor E_μν ? R_μν ?1/2g_μνR can be regarded as a source of the gravitons without rest-mass.     

Self-broadening, self-shift and self-mixing in the ν2, 2ν2 and ν4 bands of NH3

Using Fourier-transform spectra (Bruker IFS 120 HR, resolution ≈0.004 cm⁻¹) of NH₃ in nine branches of the ν₂, 2ν₂ and ν₄ bands, self-broadening and self-shift as well as self-mixing coefficients have been determined at room temperature (T=295 K) for more than 350 rovibrational lines located in the spectral range 1000–1800 cm⁻¹. A non-linear least-squares multispectrum fitting procedure, including line mixing effects, has been used to retrieve successively the line parameters from 11 experimental spectra recorded at different pressures of pure NH₃. The accuracies of self-broadening coefficients are estimated to be better than 2% for most lines. The mean accuracies of line-mixing and line-shift data are estimated to be about 15% and 25%, respectively. The results are compared with previous measurements and with values calculated using a semiclassical model based upon the Robert–Bonamy formalism that reproduces rather well the systematic experimental J and K quantum number dependencies of the self-broadening coefficients.The results concerning line mixing demonstrate a large amount of coupling between the symmetric and asymmetric components of inversion doublets mainly in the ν₄ band. The line mixing parameters are both positive and negative. More than two thirds of the lines studied here have a positive shift coefficient. However, for most of them the shift coefficients are negative in the 2ν₂ band. They are positive for the R branch of the ν₂ band and for the ^PR and ^RP branches of the ν₄ band. For the other branches they are both positive and negative. Some components of inversion doublets illustrate a correlation between line mixing and shift phenomena demonstrated by a quadratic pressure dependence of line position.     

Measurements and calculations of ethylene line-broadening by argon in the ν7 band at room temperature

Argon broadening coefficients are measured for 32 vibrotational lines in the ν₇ band of ethylene at room temperature using a tunable diode-laser spectrometer. These lines with 3 ≤ J ≤ 19, 0 ≤ K_a ≤ 4, 2 ≤ K_c ≤ 19 in the P, Q and R branches are located in the spectral range 919–1023 cm^?1. The fitting of experimental line shapes with Rautian profile provides collisional widths slightly larger than those derived from the Voigt profile. The independent theoretical estimation of these line widths is performed by the semiclassical approach of Robert-and-Bonamy type with exact isotropic trajectories generalized to asymmetric tops. Even with a rough atom–atom intermolecular potential model the calculated values show good agreement with experimental results.     

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe₇₀Ni₃₀ Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T _c. Small-angle scattering by critical correlations with radius R _c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R _d), where R _d is the correlation length of the second scale. The temperature dependence of the correlation radius R _c was obtained: R _c ∝ ((T−T _c)/Tc)^−ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T _c ^exp , at which the correlation radius becomes constant and equals its maximum value R _c(T _c)=R _c ^max . The maximum correlation radius established (R _c ^max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T _c ^exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T _c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)     

BiFeO_3结构性质与相转变的第一性原理研究

采用基于密度泛函理论的第一性原理计算,系统地研究了BiFeO3的7种不同空间群(R3c,R3m,P4mm,Cm,Pm3m,R3m和R3c)结构及其转变关系.结果表明,铁电相R3c结构是基态,不同结构之间也存在着一定的转变关系,其变化主要包括两种形式,在[111]方向上Bi3+相对FeO6八面体存在一定的位移和FeO6八面体绕[111]极化轴的反铁扭曲旋转.此外,还得出BiFeO3的薄膜结构受到衬底结构的作用会导致其从三方相(R3c)向四方相(P4mm)转变.     

Effect of heat treatment on Ag/Y-Ba-Cu-O contact resistance

The contact resistance (R _c) of the metal/YBCO interface has been studied in pressed indium, painted colloidal silver and thermally-evaporated silver contact pads. Indium contacts always show the highest resistance amongst these three systems. In thermally-evaporated Ag contacts, post-deposition thermal treatments show favourable effects on the reduction ofR _c. Heat treatment in oxygen atmosphere in two steps is essential to reduce theR _c values. Significant improvement in obtaining low resistivity contacts has been attributed to the diffusion of silver atoms to grain boundaries at the surface of YBCO and to the enrichment of oxygen-deficient layer at the interface during thermal treatment.