Showing papers in "Czechoslovak Journal of Physics in 1973"

Domain structures in orthorhombic KNbO3 and characterisation of single domain crystals

Abstract: Domain configurations in large KNbO3 single crystals grown by a top seeded flux growth technique are studied by optical methods and etching techniques. The orientation of observed domain walls is consistent with the principles of mechanical compatibility and electrical neutrality. Experimental evidence for the existence of a new type of ferroelectric domain wall (S-wall) is presented. Miller indices of such walls are irrational and depend on spontaneous strain tensor coefficients. The preparation of large single domain crystals (up to 1/3 cm3) is described. Assessment of crystal perfection by x-ray topographic and interference methods reveals a very small mosaic spread (one minute of arc) and a good optical homogeneity.

Glass-forming tendency in the GeSx system

Abstract: Method of preparation of bulk glassy samples in the GeS (x=1–2) system by cooling (quenching) from the melt in water (with the rate of cooling of the order of 102 °C/sec) is described. Initial substances were of 6–7N purity. Characteristic temperatures of phase transitionsTg (glass transition),Tk (crystallization) andTm (melting) were determined using DTA. Glass-forming tendency was evaluated by means of the relationTk —Tg/Tm —Tk that was previously proposed by the author. It is found that glass can formed for compositions from GeS1.1 upwards, the minimum in the glass-forming tendency being in the vicinity of GeS1.0. Glasses in the whole range of compositions investigated consist of several phases, in accordance with the equilibrium diagram of Ge-S alloys. It is suggested to divide these glasses into three groups: 1. GeS-GeS1.12, which are formed hardly, are composed prevalently from GeS and small quantities of GeS2 and G, 2. GeS1.12-GeS2, which are formed more easily, are composed from GeS and GeS2 (increasing GeS2 content), 3. from GeS2 upwards, which are formed very easily, are composed from GeS2 and sulphur.

The demagnetization tensor of a cylinder

Abstract: All independent components of the demagnetization tensor of a cylinder are expressed by means of tabulated functions. That allows to determine the demagnetizing field of a cylinder homogeneously magnetized along an arbitrary direction. As an example the demagnetizing field is computed over the endface of a cylinder homogeneously magnetized perpendicular to its axis of rotation.

On solid solution hardening in crystals with randomly distributed solute atoms

Abstract: The temperature dependence of the critical resolved shear stress (CRSS) of solid solution single crystals is explained on the basis of random distribution of solute atoms in the alloy. The calculated temperature dependence of CRSS in a crystal with both isolated solute atoms and their pairs is very similar to the experimentally observed one, i.e., the pronounced temperature independent region, so called “plateau”, appears.

Structured discharges in high frequency plasmas

Abstract: Experiments are described in which structured discharges, including stationary and moving striations, were observed in 4 MHz rf plasmas generated using two solenoidal coil configurations The structured plasmas which formed in the vicinity of the solenoidal coils were not quenched when elaborate precautions were taken to shield the capacitive electric fields Measurements of the rf power input to the plasma and the resistance reflected by the plasma into the drive circuit showed no discontinuous change during the formation or disappearance of the structured discharges surrounding the solenoidal coils Observations of striated rf discharges reported by other investigators are reviewed Arguments are given to support the conclusion that the striation-like structures observed in rf plasmas and the moving striations observed in dc plasmas are part of the same general family

Single crystals of triglycine sulphate containing palladium

Abstract: Single crystals of TGS doped with Pd2+ have been grown from aqueous solutions using the method of temperature decrease. It was found that the relative growth rate along thec-axis is not constant through the growth process being larger by more than an order of magnitude (in medium size crystals, ca. 30 g) than in pure TGS crystals. Investigation of the domain structure revealed that in the growth pyramids ¦110¦, the crystals are poled in one direction. The internal bias field in these growth pyramids was experimentally found to be of the order of 1 kV/cm. The high internal field excludes the possibility of spontaneous depoling and makes the Pd2+-doped TGS crystals suitable for pyroelectric applications.

Creep of cadmium single crystals

Abstract: The creep of cadmium single crystals has been investigated in the temperature range of 78 to 297 K. It has been shown that the transient creep of Cd single crystals is described by a time dependence of the creep strain ratea ∼ t−m wherem is a function of the resolved shear stress and temperature. The activation areaA* depends on the resolved shear stress,τ, A* ∼ τ−r wherer=1 for room temperature andr ≈ 1·4 for 204 and 78 K. The activation enthalpy does not practically depend on the resolved shear stress in the whole region of the resolved shear stress measured and with increasing temperature it increases roughly withT2. For temperatures above 200 K the nonconservative jog motion is proposed to be the dominant thermally activated process.

The effect of the strain amplitude changes on the stress amplitude and resistivity of torsionally fatigued copper

Abstract: Polycrystalline copper wires were cyclically strained in torsion in liquid nitrogen and the effect of the strain amplitude changes on the stress amplitude and resistivity was followed. It was found that both stress amplitude and resistivity are able to increase or decrease in dependence on the applied plastic strain amplitude. When the strain amplitude was decreased, the reversibility was not complete. By means of intermediate annealing it was found that both the dislocation density and the point-defect concentration follow the changes of the plastic strain amplitude.

Note on the description of an unstable system

Abstract: The paper collects some general properties of the description of an unstable particle considered byHorwitz andMarchand. It is proved generally, that the initial decay rate of finite energy states equals to zero.

The efficient inject of high microwave powers into the overdense magnetoactive plasma in the waveguide

Abstract: On the boundary vacuum — a dense magnetoactive plasma, an incident RHCP (right hand circularly polarized) wave is strongly reflected ifω 0 2 /ω 2 ≫ 1 andω ce/ω > 1;ω, ω 0,ω ce are the ncident wave frequency, the plasma frequency and the electron cyclotron frequency, respectively. This reflection strongly lowers the efficiency of the plasma heating by microwaves. Our experimental study shows that a suitable matching system consisting of dielectric plates, placed in front of the plasma, can ensure practically complete transfer of the RHCP wave energy into a dense plasma. The experiment was carried out in the S band waveguide (λ=12·7 cm) with the power of the order of l kW.

The role of surface and volume diffusion in the growth of TGS single crystals

Abstract: Msasurements of the growth rates of {110} and {001} faces of triglycinsulphate (TGS) as functions of supersaturation of the solution are analyzed on the basis of the surface-diffusion model of Burton, Cabrera and Frank. Approximate values of the free activation energies of dehydratation, surface diffusion and incorporation of TGS molecules into the crystal lattice are determined. It is shown that surface diffusion is responsible for the low growth rate of {001} faces; in the case of {110} faces, this mechanism is less important at higher values of supersaturation than volume diffusion. This fact is discussed on the basis of results obtained from measurements of the influence of hydrodynamical conditions on the growth of the prismatic {110} faces of TGS crystals.

Evolution of the photon statistics of light propagating through a random medium

Abstract: The descriptions of the photon statistics of light propagating through a random medium recently proposed byDiament andTeich and byTatarski are derived as useful approximations of the exact solution of propagation of light through a random medium developed in the preceding paper; they are shown to be valid for small fluctuations of the permittivity of the random medium. Moreover, it is shown that these approximate descriptions neglect the self-radiation of the random medium. The exact photodetection equation in the presence of a random medium is obtained, valid for arbitrary fluctuations of the permittivity including the contribution of the self-radiation of the random medium; particularly the photodetection equation for small fluctuations is obtained. It is also shown that the problem of propagation of light through a random medium is described by the model of the superposition of signal and noise under very general assumptions, and that in the strong-field limit the Diament-Teich, Tatarski and exact descriptions coincide.

The demagnetizing field on the surface of disc-shaped ferromagnetic samples

Abstract: The local demagnetizing fields on the surface of flat cylindrical samples (disc) are measured by means of the free radical electron paramagnetic resonance for several thickness-to-diameter ratios from 0·01 to 0·15 and at two values of the external magnetic field (around 4 and 14 kOe). An empirical expression for the demagnetizing field intensity is found and anisotropy of the demagnetizing field is detected in the sample plane. The experimental results are compared with the conclusions of the theory of Joseph and Schlomann.

Influence of normal stress on yielding behaviour in Fe -3 ·2% Si alloy single crystals

Abstract: Specimens of Fe-3·2 wt% Si alloy single crystals of various orientations, both with 18 ppm C and decarburized, have been deformed in compression (\(\dot \varepsilon \) ∼ 10−4 s−1) at different temperatures between 125 K and 293 K. It has been found that the magnitude of CRSS, the choice of the slip planes and the shape of the stress-strain curves depend on the angle between the compression axis and the Burgers vector (angleξ). The stress normal to the maximum resolved shear stress plane is strongly altered on changing the angleξ. The discussion of the obtained experimental results seems to indicate that the normal stress influences the structure of screw dislocation core and subsequently the dislocation mobility.

Two types of magnetoresistance in amorphous germanium

Abstract: The magnetoresistance ot sputtered films of a-germanium has been measured in the temperature region 78–450 K in magnetic fields up to 25 kG. Two types of dependences of the magnetoresistance on the magnetic field and the temperature have been found. Differences between these two types are ascribed to the influence of deep impurity levels on the electrical conductivity in amorphous Ge.

On the role of the image potential in electron surface studies

Abstract: Exchange and correlation contributions to the vacuum part of the effective electron potential of a semiinfinite crystal are modelled by a simple shifted image potential. Modifications, introduced by this potential, are studied for the cases of electron surface states and of the diffraction of low-energy electrons on a simple one-dimensional model.

Thermally stimulated depolarization of a phosphate glass

Abstract: Thermally stimulated depolarization (TSD) of phosphate glass of the composition 72·9% P2O5, 8·5% Al2O3, 18·6% CaO, 0·23% MnO has confirmed the existence of two types of “slow” relaxation. Migration polarization leads to TSD maximum near 0 °C, while the high temperature TSD maximum at 113 °C probably corresponds to space charge polarization. An analysis of the high temperature maximum shows that it is formed by a spectrum of relaxations with a single activation energy 1·0 eV, distributed over the dipole frequency factors with the most probable value α0 ∼ 1010 sec−1. Strong dispersion of permittivity points to the space charge polarization, though it is found that the effect is linear and independent of the sample geometry.

Magnetic properties of electrodeposited iron-nickel alloys

Abstract: The basic magnetic properties, viz., magnetic saturation and coercive force, have been established by measurement of about 150 specimens for electrodeposited iron-nickel alloys over the entire range of chemical compositions. The magnetization data were related ot the physicochemical aspects of the electrodeposition process.

Preparation and electrical properties of copper-doped cadmium telluride single crystals

Abstract: Preparation and the electrical properties of copper-doped cadmium telluride single crystals prepared by directional freezing and by diffusion of copper from a layer of Cu2 Te are described in this paper. The dependence of electrical properties on the preparation mode is followed. In the enclosed infra-red spectra it is proved that samples suitable for optical measurements can be prepared by both described techniques.

Growing and morphology of crystals of adenine sulphate

Abstract: Single crystals of adenine sulphate (C5H5N5.H2SO4, molecule weight 233.19) of size 10 to 16 mm have been grown using the method of cooling the solution. Both the apparatus and technology are described. The chemical composition of the crystals and the crystal class of the orthorhombic system (2/m 2/m 2/m), by means of an optical double-cirle goniometer have been determined.

Emission of electrons from mim systems: Energy distributions

Abstract: In this part of our study we measured energy distributionsN(E) andN(E X ) of electrons emitted from sandwich cathodes, and their basic temperature (at 300, 200 and 80 K) and voltage dependences. From the distributionN(E) at 80 K we calculated the mean free path of electrons in cathode and the effective temperature of distribution. The anomalous emission (beginning at small leakage voltages) was observed and its energy distribution measured, too. From the graphs it can be seen that some electrons acquire energy 2–5 eV during their transition through the sandwich cathode.

Emission of electrons from mim systems: Discussion of processes in the cathode

Abstract: Experimentally derived emission characteristics — the transfer ratio, angular distribution, energy distributionsN(E) andN(Ex) and energy-angular distribution of emitted electrons — are discussed on the basis of our theoretical model of the sandwich cathode. It was found that electrons during their transfer through the cathode are scattered in the dielectric layer mostly by interactions with optical phonons and traps and in the top metal electrode by collisions with conduction electrons. Comparison of our model with experimental results gave us approximate values of mean free paths and further cathode parameters.

Energy levels of 167 Er populated by the (d,d') reaction

Abstract: Data on the inelastic deuteron scattering on low lying energy levels in167Er are presented as a supplement to an investigation of this nucleus by the (d,p) and (d,t) reactions.

Zero-mass scalar and electrostatic fields with the central symmetry in the general relativity

Abstract: All asymptotically flat space solutions of Einstein equations with energy-momentum tensor of electrostatic and zero-mass scalar static central symmetric fields as a source were found. There are five branches of general solution; only two of them are contained in previous Penney's solution. In a limit of pure electrostatic field and pure scalar field our solutions become identical with corresponding solutions known previously.

The angular dependence of Nb 3 Sn critical currents in transverse magnetic fields

Abstract: The dependence of critical currents of vapour-deposited and diffusion-processed Nb3Sn tapes on the magnitude and direction of magnetic field perpendicular to the current is experimentally investigated. By comparing the experimental results with theoretical considerations we conclude that the form of grains in the vapour-deposited samples is almost sinusoidal, whereas for the diffusion-processed ones it is more ellipsoidal. For both kinds of tapes, detailed structural microanalysis (electron microscopy, scanning electron microscopy, optical microscopy) of the substrate and the Nb3Sn tapes has been performed in order to explain the existence and the character of the above mentioned dependences from the processes of the grain formation.

Internal conversion of the 84·23 keV transition in170Yb

Abstract: The conversion coefficientαK=1·48±0·07 and the total conversion coefficient α= =8·05±0·2 have been determined by means of the 4π scintillation counter.

Diffusion-elastic phenomena I. Theory of diffusion-induced elastic bending

Abstract: A special case of diffusion-elastic phenomena for interstitials is treated and an analytical solution is presented expressing the elastic bending during the diffusion process as function of physical (diffusivities, Young's moduli, constants in the second Vegard's law) and geometrical characteristics of the diffusion system. To enable the solution of the problem in principle, the second law of Vegard for interstitials was formulated analogously to the original version valid only for substitutional atoms. A series of numerical results in form of graphs indicate the properties of the solution of the problem. A description of verifying experiments will follow in the next paper.

A computational method of determination of constants of spin Hamiltonian of fine structure ESR spectra

Abstract: In the case of strong electric crystal fields one cannot use the well-known method of determination of constants of the spin Hamiltonian based on the perturbation theory. This method results in less accurate values of determined constants. In the paper is presented a new method which uses exact numerical diagonalization of the matrix of spin Hamiltonian for determination of constants. The constants determined by this method satisfy the exact solution of the equations for the resonance transitions. This method causes practically no error in the determined constants.