Manifestation of phase transformations in optical spectra of Na0.5Bi0.5TiO3 crystals between 25°C and 350°C

The pseudo dielectric function ?ε? (E) = ?ε ₁? (E) + i?ε ₂? (E) of sodium bismuth titanate Na_0.5Bi_0.5TiO₃, has been obtained in the spectral range of electronic excitations (2–9.5 eV) by spectroscopic ellipsometry using the synchrotron radiation source BESSY II. The spectrum contains a broad absorption band at 4.2–4.5 eV. The temperature dependence in the range of 25–350°C is presented by the related susceptibilities ?χ ₁? (T) and ?χ ₂? (T) at 5 eV. A clear peculiarity in the temperature behavior of ?χ ₁? (T) and ?χ ₂? (T) between 180 and 320°C has been revealed and discussed. In this range an extremum-like temperature dependence of intensity of the light reflected from the sample surface, has been revealed and can be explained by the diffraction of light on the grid of the elastic domain structure of the sample.     

Interface-induced polarization and inhibition of ferroelectricity in epitaxial SrTiO₃/Si

We use SrTiO?/Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed (pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.     

The dispersion of BED° in unintentional doped GaN crystals

The polarization and the wavelength of the photoluminescence of BED^° in not intentionaly doped GaN crystal wafers are observed to be dependent on the excitation intensity (I-exc). When I-exc increases from 1 KW/CM² to 1000 KW/CM², they vary at first quadratically, and then, appear saturated. This phenomenon is resulted from dispersion effect of BED^° in GaN crystal wafers, which is proposed earlier (1).     

Study of spin dynamics in NdRh2Si2 crystals byμSR relaxation measurements

We have studied by zero-field and longitudinal fieldSR measurements the uniaxial antiferromagnetic intermetallicsNdRh ₂Si₂. The measurements have been performed on single crystals. Our data show that the magnetic field at the muon site is parallel to thec axis. Below the Néel temperatureT _N the spin-lattice relaxation is driven by a two magnon process. In the paramagnetic region nearT _N we observe, a critical divergence of the damping rate.     

The inhomogeneity of two-wave coupling in photorefractive crystals in 90° geometry

We derived the coupling equations of two-wave coupling in photorefractive crystals in 90° geometry where pump and signal beams enter two mutually orthogonal crystal faces vertically. We solved the equations numerically in the case of non-local response and illustrated the influence of the coupling coefficient, the absorption coefficient and the ratio of intensities of the two incident beams,R, on the inhomogeneity of transmitted pump and signal beams.     

Dislocation dynamics in doped NaCl single crystals determined by pulsed NMR between RT and 300°C

The mean free path of mobile dislocations is measured by determination of the spin-lattice relaxation rate of deforming NaCl single crystals as a function of temperature and of the concentration of Ca⁺⁺ impurities. The latter may influence the magnitude of the mean free path but this depends to a large extent on the point defect configuration. The degree of association and the mobility of the point defects is studied by measuring the spin-lattice relaxation rate without deforming the samples. On the other hand the distribution of dislocations varies also with temperature and this affects the mobility of dislocations too. The work-hardening rate of the crystals is compared with the mean free path as a function of temperature and it is shown that both quantities have extremes under the influence of competitive mechanisms such as an enhanced thermal activation of dislocations at obstacles, an increasing mobility of point defects, and increasing number of intersections of mobile dislocations with dislocation dipoles.     

Thermal stability of Mg2Si epitaxial film formed on Si（111） substrate by solid phase reaction

A single crystalline Mg2 Si film was formed by solid phase reaction(SPR) of a Si(111) substrate with an Mg overlayer capped with an oxide layer(s),which was enhanced by post annealing from room temperature to 100 ℃in a molecular beam epitaxy(MBE) system.The thermal stability of the Mg2 Si film was then systematically investigated by post annealing in an oxygen-radical ambient at 300℃,450℃ and 650 ℃,respectively.The Mg2 Si film stayed stable until the annealing temperature reached 450 ℃ then it transformed into amorphous MgO x attributed to the decomposition of Mg2 Si and the oxidization of dissociated Mg.     

Observations of band structure and reduced group velocity in epitaxial GaN–sapphire 2D photonic crystals

We report experimental results for the band structure of 2-dimensional triangular photonic crystals of air holes in an epitaxial group III–nitride waveguide film. Surface coupling techniques enable the observation of sharp resonance dips in the transmission spectra due to a resonance phenomenon between the incident light and Bloch modes of the photonic crystal. The position of the dips has been measured as a function of angle of incidence and the photonic band structure has been successfully constructed by the measurement. Corresponding Bloch-mode group velocities have also been obtained. Received: 28 May 2001 / Published online: 23 October 2001     

CČerenkov second-harmonic generation by nondiffracting Bessel beams in bulk optical crystals

A new method of optical second-harmonic generation (SHG) in bulk media is proposed, using Bessel beams which realize Cerenkov phase matching. Single-domain crystals are presently considered. For noncollinear emission the scheme has the advantage of achieving conversion efficiencies of typically about double those achieved with unfocused Gaussian beams, over the same temperature bandwidth. The harmonic radiation is analysed using a Hankel transform-based spectral method. Numerical examples are given based on the crystal KNbO3, and possible experimental arrangements are suggested for the practical implementation of this method.     

A contribution to the statics of 90° wedge-shaped domains in BaTiO3 crystals

The existence of 90? domains of wedge and parallelepiped shape was studied in BaTiO₃ crystals of different form. On the basis of a simple model the condition for their origin is discussed.     

Fabrication of two types of one-dimensional Si–C nanostructures by laser ablation

Two types of one-dimensional (1D) nanostructures—amorphous silicon carbide (SiC) nanowires, 5–30 nm thick and 0.5–2 μm long, and carbon nanotubes (CNTs) filled completely with crystalline SiC nanowires, 10–60 nm thick and 2–20 μm long—were synthesized by the laser ablation of carbon-silicon targets in the presence of high-pressure Ar gas up to 0.9 MPa. All the CNTs checked by transmission electron microscopy contained SiC, and no unfilled CNTs were produced. We discuss the growth of the two nanostructures based on the formation of molten Si–C composite particles and their instabilities leading to the precipitation of Si and C.     

Righi-Leduc Effect in InSb between 25°C and 300°C

The Righi-Leduc effect has been measured for n-type InSb crystals of various mobilities and was found to increase with temperature in a manner to indicate that the unipolar contribution is prevalent over the bipolar part of the effect. The experimental values measured over the temperature range between 25°C and 300°C agree well withPutley's theory. The Righi-Leduc coefficient was also determined as a function of the magnetic field and the strong decrease observed is explained in terms of the magnetoresistance.     

The interpretation of X-ray diffraction data from thin epitaxial lead-tin telluride crystals—I

Calculations are made of the expected strength of X-ray diffraction peaks from (100) PbTe and Pb_1−xSn_xTe crystals. The calculations show that in practice the form of the 800 diffraction peak can be related to the epitaxial layer thickness of heterostructures and also indicates the degree of tin diffusion across the heterojunction. The calculations are illustrated in graphical form.The growth system used for the structures examined is shown to produce tin diffusion across a stratum of crystal comparable in thickness to the thickness of the deposited PbTe surface layers.     

A detailed model for the conductivity of magnetite below 119°K

The question of charge transport through the Fe₃O₄ lattice in the ordered state is reexamined. It is shown that only a fairly complicated defect can transfer charge. The implications of this model are compared with the experimental data.     

Precipitation sequences and transformation kinetics in the Al-30 wt.% Zn alloy aged below 160°C

The transformation sequences in the Al-30 wt.% Zn alloy aged below 160 °C have been investigated by X-ray diffraction and transmission electron microscopy. The sequence G.-P. zones _R has been observed between 85 and 161 °C, whereas the direct G.-P. zones to-transformation occurs below 85 °C. The identical isothermal growth kinetics of both the G.-P. zones and the _R -precipitates was found. The log-log plots yielding a set of parallel straight lines from 50 to 150 °C indicate no change in the activation energy of growth of both these precipitates. The change in the precipitation sequence led to the change in the transformation kinetics. The T-T-T curves have been measured for the formation of metastable _R -phase and for the completion of ellipsoidal G.-P. zones or of _R to-transformation. The retardation in reaching equilibrium found below 85 °C was associated with the observed change in the transformation sequence. This was explained by the different accommodation of the misfit at the interfaces between G.-P. zones or _R -precipitates and the-matrix.The authors are much indebted to Doc. Dr. V.Syneek, CSc. for stimulating discussions. Our thanks are also due to Ing. V.íma and Mr. P.Vyhlídka for the preparation and careful chemical analyses of the investigated samples. The assistance of Mr. Z.iký and Mr. V.Petr in X-ray diffraction measurements and the heat-treatment of samples is appreciated.     

Dielectric loss relaxation maxima of undoped and calcium doped NaCl crystals between 20 and 400°C

When studying the dielectric loss dependence of undoped (pure) and CaCl₂ doped NaCl crystals on temperature and frequency, some further maxima were observed apart from the wellknown relaxation maxima of calcium complexes. The temperature of these maxima shifts with rising calcium content to lower temperatures (from 259 to 142°C), whereas the value of the maxima does not change with concentration and remains unchanged even with the purest crystals. It seems probable that the observed maxima are due to the relaxation loss of the crystal lattice itself.     

Acceleration of nickel diffusion by high tensile stress in cold-worked type 316 stainless steel at 450°C

The effect of tensile stress on diffusion was studied by the diffusion couple method. A diffusion couple was prepared by electroless plating a nickel thin layer on the round notch surface of a compact tension-type specimen of 20% cold-worked Type 316 stainless steel. The couple was diffused at 450°C for 4003?h under the maximum tensile stress of 553?MPa in the load direction. A rapid diffusion coefficient of nickel in the Type 316 stainless steel was observed at the high tensile stress zone that was 6.5 times faster than that at the low-stress zone.     

The interpretation of X-ray diffraction data from thin epitaxial lead tin telluride crystals—II

The difusion of tin across heterojunctions formed between thin epitaxial layers of lead telluride, or lead tin telluride grown on each other, has been examined by comparing the experimental X-ray diffraction data with theoretical diffraction envelopes, calculated by convoluting diffraction data with idealized tin diffusion profiles. The results show that, for lead telluride grown on lead tin telluride, the tin diffusion is best described by an error function, and for lead tin telluride grown on lead telluride. an exponential function provides the best fit between experimental and calculated data.