The elastic stiffness constants of β-NH4LiSO4 around its phase transition at 460 K

The complete tensor of the elastic stiffness constantsC _ij (i,j=1 to 6) of -NH₄LiSO₄ has been measured in the temperature range 290 K to 540 K including the ferroelectric phase transition at 460 K, by the ultrasonic pulse echo overlap method.Some ultrasonic attenuation coefficients were determined.The elastic stiffness constants were calculated using Landau Theory. The elastic stiffness constants are all well described within this theory with the exception ofC ₆₆, which can not be reproduced with coupling terms allowed by group theoretical arguments. This together with double peaks observed in the specific heat and in sound attenuation in some directions leads one to suspect an intermediate phase between the paraelectric and the ferroelectric phases.     

Stable acoustic-emission characteristic of the extent of damage to brittle materials

An experimental study has been made of the dissipation of elastic energy in the course of the inelastic deformation and fracture of brittle materials. The energy characteristics of acoustic emission and the amount of energy expended irreversibly on inelastic deformation were determined simultaneously during uniaxial quasistatic loading. Test samples of grade 600 cement paste, of grade 150 structural ceramic, and of the ceramic ferroelectric materials PKR-1 and TsTS-19 were studied. The results demonstrate the effectiveness of the thermodynamic acoustic-emission damage parameter (). This parameter characterizes the relative contribution of the rate of the buildup of latent energy of fracture to the overall rate of energy dissipation in the course of the inelastic deformation of a solid.North Caucasus Science Center College. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 62–66, April, 1994.     

The effect of laser illumination on the propagation of ultrasonic waves in single crystalline lithium niobate

By means of ultrasonic experiments we have observed in Fe-doped LiNbO₃-crystals elastic inhomogeneities and an ultrasonic absorption which stronglydecrease under laser illumination. The elastic inhomogeneities before illumination lead to a non-exponential decay of the ultrasonic echo pattern. The corresponding variations of the acoustic velocity and absorption across the sound beam have been displayed directly by an ultrasonic topography. — In contrast to the photorefractive behaviour of LiNbO₃ the elastic inhomogeneities and the ultrasonic absorption are not enhanced but disappear as soon as the illumination is switched on: Under illumination the echo pattern becomes well exponential as expected for an ideal crystal. The physical nature of the elastic inhomogeneities and the acoustic absorption inside the sample which can be bleached out by optical radiation is not yet well understood. But there is evidence that Fe⁺⁺-impurities play an important role.Dedicated to Professor Harry Thomas on the occasion of his 60th birthdaySupported by the Science Fund of the Chinese Academy of Sciences     

Photoacoustic X-ray absorption fine-structure spectroscopy of the NiK-edge

The X-ray absorption in a 5 m thick Ni foil for energies from 8.2 keV to 8.8 keV was measured by the photoacoustic method using a gas-microphone-cell. Results are presented for both photoacoustic amplitude and phase as a function of incident photon energy. The absorption fine structure above the NiK-edge is clearly resolved. The data are compared with a simple theory for the signal generation, and discrepancies are explained by discussing the various energy dissipation channels following the X-ray absorption.     

Noise Induced Dissipation in Lebesgue-Measure Preserving Maps on d-Dimensional Torus

We consider dissipative systems resulting from the Gaussian and alpha-stable noise perturbations of measure-preserving maps on the d dimensional torus. We study the dissipation time scale and its physical implications as the noise level vanishes. We show that nonergodic maps give rise to an O(1/) dissipation time whereas ergodic toral automorphisms, including cat maps and their d-dimensional generalizations, have an O(ln(1/)) dissipation time with a constant related to the minimal, dimensionally averaged entropy among the automorphism's irreducible blocks. Our approach reduces the calculation of the dissipation time to a nonlinear, arithmetic optimization problem which is solved asymptotically by means of some fundamental theorems in theories of convexity, Diophantine approximation and arithmetic progression. We show that the same asymptotic can be reproduced by degenerate noises as well as mere coarse-graining. We also discuss the implication of the dissipation time in kinematic dynamo.     

Theory of ultrasonic attenuation in metallic systems containing Rare Earth impurities

A theory of ultrasonic attenuation in metallic systems containing crystalline-field split Rare Earth ions is presented. It is shown that two types of absorption mechanisms can occur. The first one is of the well known Pippard type and describes the energy dissipation of the ultrasonic waves directly into the conduction electron system. However in addition to that mechanism there is another one which is of importance and which describes the energy dissipation into the Rare-Earth ion system.In order to distinguish between both mechanisms the change of the attenuation in an applied magnetic field is investigated. For the purpose of demonstration the theory is applied to a ₇– ₈ crystal field level scheme as it applies for Ce³⁺ or Sm³⁺ in cubic symmetry. An estimation of the relative strength of both absorption mechanisms is given.     

The structure of ‘nearly’ ferroelectric RADP

X-ray scattering techniques have been used to study the structures of two crystals of Rb_1–x (NH₄) _x H₂PO₄ withx0.2, near to the boundary between ferroelectric and structural-glass behavior at low temperature. Below about 83K, both crystals develop shortrange incommensurate correlations with a wavevectorq0.13 a ^* which are presumably related to the glass properties. On cooling below 70 K, the crystal with the slightly larger NH ₄ ⁺ concentrationx, develops the ferroelectric structure in a small fraction of the crystal, while the bulk of the crystal retains the tetragonal structure. The other crystal transforms almost wholly to the ferroelectric phase. The transition to the ferroelectric structure shows considerable hysteresis on heating and cooling, and is spread over about 20 K. The transition is certainly of first order, and the spread in temperature may arise from concentration fluctuations. These results and the structure of the incommensurate modulations are compared with the predictions of a theoretical model for this system.     

High-performance pyroelectric single crystals for uncooled infrared detection applications

Uncooled pyroelectric infrared detectors based on ferroelectric single crystals 0.74Pb(Mg_1/3Nb_2/3)O₃–0.26PbTiO₃ (PMN–0.26PT) were fabricated. The performances of pyroelectric detectors dependence on detector fabrication temperature, absorption layer, and element thickness were compared. The room-temperature voltage responsivity (R_v) of 200 V/W and specific detectivity (D^*) of 10⁸ cm Hz^1/2/W at 12.5 Hz have been achieved. The results reveal that the better pyroelectric response can be expected by controlling temperature below 70 °C during the fabrication of the pyroelectric detectors, selecting absorption layer with high absorption coefficient, and decreasing the thickness of the elements.     

The lifetime of the “ferroelectric” phonon in some perovskite crystals at low temperatures

The lifetime of a low-lying transverse optic phonon with the wave vectork=0 (ferroelectric phonon) in perovskite crystals having the Curie point at low temperatures is investigated. Using a phenomenological Hamiltonian it is shown that mainly the process, in which the ferroelectric phonon and the transverse acoustic phonon are destroyed in order to create another optic phonon from the same branch, determines the lifetime of a ferroelectric phonon. The general result is specialized for SrTiO₃, and the temperature dependence of, exhibiting anomalies in comparison with normal ionic crystals, is discussed.The author thanks Dr. M. Marvan of the Physical Faculty of Charles University in Prague and Dr. P. Glogar and Dr. M. Trlifaj of the Institute of Physics, Czechoslovak Academy of Sciences, in Prague for valuable remarks.     

Anomalies of the refractive index and the optical energy gap of ferroelectric Pb1−x Ge x Te

The refractive index and the optical energy gap of Pb_1–x Ge _x Te (0x0.11) have been determined from transmission- and reflectivity measurements in the temperature range from 4.2 K to 300 K. At the ferroelectric phase transition a change of the temperature coefficient of both quantities is observed. A two bandk·p model calculation demonstrates a correlation of the optical energy gap with the high frequency dielectric constant. For higher values ofx (x=0.09) a splitting of absorption edge and birefringence have been observed.     

Study of relaxation of molecular iodine by the method of saturated absorption spectroscopy

The paper describes detailed experimental studies of the width, shape and shifts of saturated absorption resonances at the X(v=0, j=13)B(v=43, j=12) transition of molecular iodine. Use of the geometry of oppositely travelling and unidirectional waves in the experiment and study of the luminescence decay under pulsed excitation allowed estimation of the cross-sections of elastic and inelastic scattering of I₂ molecules on the upper and lower levels of transition. The observed nonlinear dependence of width and shift of the saturation resonance on iodine pressure is explained by the influence of elastic collisions with no phase mismatch.     

Quantum n-Vector Anharmonic Crystal II: Displacement Fluctuations

Based on the 1/n-expansion derived in a previous paper, the displacement fluctuations are analyzed in a quantum n-vector model of anharmonic crystal in the large n regime. It is shown that in the ferroelectric phase the n limit of the local fluctuation field has faster large-distance correlation decay than its Hartree–Fock approximation. Also, the critical exponent of the global displacement fluctuation is strictly smaller there than the Hartree–Fock exponent. In particular, the displacement fluctuations may be normal in the ferroelectric phase in spite of the Hartree–Fock prediction.     

Wedge-screw dispiration in nonchiral smectic O liquid crystals

Using the approximative expression for the elastic energy density of the smectic O liquid crystal the elastic self-energy of (, b)- and (3, b)-wedge-screw dispirations is estimated. Using the elastic self-energy of (3, b)-wedge-screw dispiration the stability of this defect is discussed.The author expresses his thanks to Professor A. Fukuda, Professor H. Takezoe and Dr. Y. Takanishi for discussions on their observations of (3, b)-wedge-screw dispirations in liquid crystals with antiferroelectric and anticlinic molecular ordering. This work was supported by the grant of Acad. Sci. of the Czech Rep. No. 19062 and also benefited by the grant No. 202/93/155 from the Grant Agency of the Czech Republic.     

Spectral‐Luminescent Properties of Oxazoles and Oxadiazoles in a Gas Phase on Excitation by Electrons

The energyloss spectra of electrons, fluorescence excitation functions, and the fluorescence spectra on excitation of the vapors of a number of oxazoles and oxadiazoles by monokinetic beams of electrons of various energies are determined. In contrast to optical absorption spectra, in the energyloss spectra of a number of studied substances a band associated with the S ₀–T ₁ singlettriplet transition is observed. The –^*type transitions are fixed up to S ₀–S ₅ on excitation of molecules by highenergy electrons, including the region of vacuum ultraviolet. The cross sections of elastic and inelastic collisions of electrons of different energies with POPOP molecules have been measured. The dependences obtained differ substantially from those calculated in the Born approximation. The cross section of elastic scattering is in a rather good correspondence with the geometric section of the molecule.     

Phase transitions in random uniaxial systems with dipolar interactions

The critical behaviour of random uniaxial ferromagnetic (ferroelectric) systems with both short range and long range dipolar interactions is investigated, using the field theoretic renormalization method of Brézin et al. for the free energy above and below the transition pointT _c. The randomness is due to externally introduced fluctuations in the short range interactions (quenched case) or (and) magneto-elastic coupling to the lattice (annealed case). Strong deviations in the critical behaviour with respect to the pure systems are found. In the quenched case e.g. the specific heatC and the coefficientf ₂ (ofM ³ in the equation of state, whereM is the magnetization) change fromC |ln|t^1/3,f ₂ |ln|t^–1 in the pure system toC is the reduced temperature andA _±,C _± are constants) in the random situation. This change should e.g. be experimentally observable by deuterization of the ferroelectric tri-glycine sulfate where the logarithmic behaviour off ₂ has already been detected in the pure case. For nonvanishing magnetoelastic coupling a complex critical behaviour is obtained and discussed. We find the interesting result that if both quenched randomness and a weak magnetoelastic coupling are present the quenched random critical behaviour dominates in the close vicinity ofT _c. Finally the influence of the magnetoelastic coupling on the longitudinal phonons in investigated and it is found that the relative changes in the corresponding elastic constant and structure factor are proportional to the specific heat and the wavevector dependent energy-energy correlation function respectively, suggesting new experiments.     

The relaxation of the structure of RADP in an electric field

X-ray scattering measurements have been made to study the behaviour of the ferroelectric glass of Rb_1–x(NH₄)_xH₂PO₄ withx0.22 in the presence of an electric field. The results showed that the system has very long relaxation times, characteristic of competing interactions. The change in the intensity of the x-ray scattering from the incommensurate modulation and the ferroelectric phase have a roughly linear relationship with time.     

Lattice dynamics of BiFeO<Subscript>3</Subscript> under hydrostatic pressure

The vibrational frequencies of the BiFeO₃ crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm^?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure.     

Anisotropy of infrared optical absorption in layered superconductors

We analyze the problem of infrared optical absorption in a clean layered London superconductor in the vicinity of the gap =2. We conclude that absorption of light with wave vectorqc is enhanced over ordinary Drude absorption (qc) due to resonance absorption (Landau damping). Experimental absorption studies with qc might therefore improve chances to observe a superconducting gap in the high-T _c superconductors.     

Wedge-screw dispiration in smectic liquid crystals with antiferroelectric order

Using the approximative expression for the elastic energy density of the antiferroelectric (AF) liquid crystal the elastic self-energy of (, b)-wedge-screw dispiration is estimated. Then the elastic interaction between (, b)-wedge-screw dispiration and (2)-wedge dischnation in AF liquid crystals is investigated. The attraction of dislocation part of dispiration with (2)-wedge disclination can lead to the formation of (3)-wedge-screw dispiration. When (2)-wedge disclination is trapped by (2d)-screw dislocation the resulting (2, 2d)-dispiration can dissociate into a pair of two (, d)-dispirations.The authors express their thanks to Professors A. Fukuda, H. Takezoe, Dr. Y. Takanishi and to Dr. K. Miyachi for discussions concerning their experimental observations of (3, d)-wedge-screw dispirations and (, d)-twist-edge dispirations in AF structures. This work was supported by the grant No. 19062 from Acad. Sci. Cz. R. and also benefited by the grant No.: 202/93/155 from the Grant Agency of the Czech Republic.