Defect samarium ions and electromotive-force generation in SmS

A model explaining electromotive-force generation in SmS under heating in the absence of external temperature gradients is considered. An analysis of data on the density of SmS single crystals with compositional deviations from stoichiometry in the homogeneity range suggests that excess samarium ions occupy vacancies on the sulfur sublattice. Possible concentrations of defect samarium ions are determined (up to 2.8×10²¹ cm^?3). The temperature interval within which electromotive force appears in various SmS samples (440–640 K) and the critical conduction-electron concentrations at which the generation sets in [(6.0–8.5)×10¹⁹ cm^?3] are calculated. An expression permitting estimation of the magnitude of the observed effect is proosed.     

Radiative and photoelectric properties of CuInS2 single crystals

The photoluminescence and photocurrent spectra of CuInS₂ single crystals grown by the Bridgman method are studied at temperatures of 80 and 300 K. The photosensitivity spectrum is observed in the shortwave photoluminescence band. From the watt-ampere characteristics of photoconductivity, a linear mechanism of recombination of minority charge carriers is established for an illumination level of up to 100 mW/cm² in the temperature range of 80–300K.     

Penetration depth measurement in high quality thin films

The parallel plate resonator method has been used for measuring high quality (YBCO) thin films, which have low temperature residual losses comparable to those previously obtained in single crystals. The surface resistance and the real part of the conductivity show a non-monotonic behaviour with a broad peak around 45 K. The penetration depth and the real part of the conductivity vary linearly at low temperatures. The lowest penetration depth linear fitting has a slope value of to up to 20 K which is lower than previous measurements on YBCO single crystals. An interpretation of this smaller slope in terms of the generally accepted d-wave order parameter symmetry presents difficulties. Received: 22 July 1997 / Revised: 11 March 1998 / Accepted: 23 June 1998     

Recovery during stress relaxation in Cd-Ag alloy single crystals

Stress relaxations in a series of cadmium-silver alloy single crystals containing up to 0·25 at% Ag were carried out at temperatures of 77 K, 145 K and 200 K. The samples were deformed in tension at a strain rate of 1×10^–4 s^–1. When the specimens deformed at 145 K and 200 K were reloaded after stress relaxation, the deformation started with the flow stress which was lower than that before stress relaxation. The observed differences in the stresses could be due to a variation in the internal stress. The activation volume should be estimated from the part of stress relaxation in which the internal stress remains practically constant.     

Specific features in the generation and motion of dislocations in silicon single crystals doped with nitrogen

The specific features in the generation and motion of dislocations are investigated in Si: N single crystals grown by the Czochralski method. The motion of dislocation loops is analyzed by the four-point bending technique in the temperature range 500–800°C. The dislocation loops are preliminarily introduced into the samples with the use of a Knoopp indenter at room temperature. It is found that doping with nitrogen leads to a considerable increase in the critical stress of the onset of dislocation motion from surface sources (indentations) and in the stress of the generation of dislocations from internal sources. The velocity of dislocation motion in Si: N crystals is less than that in undoped crystals (under comparable loads). The hardening effect of nitrogen is explained by the fact that nitrogen promotes the decomposition of a solid solution of oxygen in silicon during postcrystallization cooling.     

High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Sound attenuation in X-cut quartz,a-cut sapphire,and (100)-cut diamond at 70 GHz

The recently developed method of linear frequency modulation of a continuous wave (FM-CW) in soundecho experiments was applied to measure the sound attenuation in single crystals at 70 GHz. The residual sound attenuation at low temperatures and the crystal inhomogeneity were investigated using the method of sound-beam topography in several crystals of X-cut quartz and of a-cut sapphire of different origin. The temperature dependent part of attenuation was also determined and compared with other investigations, especially at 9 GHz and 1 GHz. We also present sound-attenuation measurements in (100)-cut type IIa diamond which are the first ever made in diamond. Echoes were observed up to 230 K at 70 GHz.     

Synthesis,crystal growth and characterization of novel semiorganic nonlinear optical crystal: Dichloro(beta-alanine)cadmium(II)

Semiorganic nonlinear optical material of dichloro(beta-alanine)cadmium(II) (DCBAC) have been synthesized and single crystals were grown by solvent evaporation method at room temperature. The lattice parameters of the grown crystals are determined by single crystal XRD. The modes of vibration of different molecular groups present in the sample were identified by the FTIR spectral analysis. Thermal stability of the crystal was investigated using thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). The dielectric constants of the crystal were studied as a function of frequency and the results are discussed. The grown crystals are subjected to microhardness studies and the variation of the microhardness with the applied load is studied. The optical transmission spectra and second harmonic generation (SHG) were investigated to study its linear and nonlinear optical properties. The nonlinear optical (NLO) property of the crystal was confirmed by powder second harmonic generation (SHG) test. SHG efficiency is comparable to that of KDP.     

Temperature dependence of the yield stress of metallic nano-sized crystals

It is shown that the temperature effect on the variance of local shear stresses is the main factor pre-determining the temperature law of the yield stress of nano-sized crystals. The results of molecular dynamics simulations of uniaxial tension of Mo, α-Fe and W nanowires in three crystallographic directions ([100], [110] and [111]) over the temperature range 100–1000 K are presented. It is found that within this temperature range, the yield stress of nano-sized crystals varies not exponentially, as for bulk single crystals, but is a parabolic function of temperature.     

用单光纤光栅实现扭转与温度的双参量传感测量

采用一种新颖的扭梁设计结构 ,利用单光纤光栅成功地实现了扭转 (扭转角或扭矩 )与温度的双参量同时测量。该方法能够有效地解决扭转角与温度的交叉敏感问题 ,且光纤光栅波长的变化对扭转角、扭矩及扭应力 (力臂一定时 )均呈线性关系。在 - 40°～ +32°范围内 ,扭转角、扭矩和温度的传感灵敏度分别达到 0 .19nm (°)、3.2 9nm Nm和 0 .0 3nm ℃ ,波长线性调谐范围可达 14.2 0nm。     

Dependence of mechanoluminescence in rochelle-salt crystals on the charge-produced during their fracture

The dependence of mechanoluminescence in rochelle-salt crystals on the charge-produced during their fracture is found to be linear up to the applied stress of 344 kg/cm². This fact is discussed on the basis of the instability of cracks in a crystal and it is concluded that the new surfaces created by mobile cracks are responsible for the appearance of mechanoluminescence in rochelle-salt crystals.     

Pseudoelastic deformation and generation of reactive stresses in a Cu-Al-Ni shape-memory alloy in the temperature range 4.2–293 K

Pseudoelastic deformation and the magnitude of reactive stresses in Cu-14.2% Al-4.5% Ni shape-memory alloy single crystals were studied experimentally in the temperature range 4.2–293 K. It is established that pseudoelasticity and the shape-memory effect are observed in this alloy over the entire temperature range indicated above. It is found that, as the constrained samples are heated at a constant rate from liquid-helium temperature, the reactive stresses increase continuously at temperatures of up to 100 K and then remain constant. When the temperature of preliminary deformation is 77 K, the generation of reactive stresses with an increase in temperature occurs by two stages, which agrees with the multistage behavior of the pseudoelastic-deformation curves of this alloy above the liquid-nitrogen boiling temperature. Using the theory of diffuse martensitic transitions, a quantitative calculation is performed of pseudoelastic-deformation curves and reactive-stress curves over the temperature range 4.2–293 K under conditions of two-stage behavior of the martensitic transformation.     

Translational oscillations of a microsphere in superfluid helium

The translational motion of a microsphere (radius 100 μm) in liquid helium is investigated. The sphere is levitating inside a superconducting capacitor and oscillates about its equilibrium position. The velocity amplitude and the resonance frequency are measured as a function of driving force and temperature (0.35 K up to 2.2 K). By increasing the driving force we first find a linear regime (laminar flow) which changes abruptly into a nonlinear one (turbulent flow). For temperatures below 0.7 K the linear drag is given by ballistic roton and phonon scattering whereas for temperatures above 1.1 K the hydrodynamic force on the sphere is described by Stoke's solution. In the turbulent regime, above a temperature independent threshold velocity, we find the drag force to be given by turbulence in the superfluid component plus an essentially laminar drag by the normal component.     

The Great Myths of Polymer Melt Rheology,Part I: Comparison of Experiment and Current Theory

Abstract

Star polymers consisting of poly(?‐caprolactone), (PCL), grafted onto third generation dendrimer, which had hyperbranched and dendron cores, were studied by polarized light microscopy together with reference linear PCL. The degree of polymerization of the PCL arms in the star polymers ranged between 14 and 81. The star polymers exhibited a greater tendency than the linear polymers to form spherulites. It is suggested that the preference of the star polymers for forming spherulites is due to the presence of amorphous material—dendritic cores and PCL cilia—between crystal lamellae that generates the necessary pressure to force the lamellae to diverge at lamellar branch points. The linear growth rate data followed a single crystallization regime. The fold surface free energy was higher for the star polymers than for their linear analogs. It is proposed that the presence of the large and rigid dendritic cores on the fold surfaces of the star polymer crystals increases the fold surface energy.     

Post-yielding dislocation retraction of nano-lamellar TiAl single crystals

The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding. This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses. This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50% of the yielding strength). It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force. The local stress relaxation is due to successive dislocation nucleation in different slip systems, and the interface-mediated image force is caused by the heterogeneous interface. Based on dislocation theory, this study demonstrates that the size effect also plays a vital role in dislocation retraction. Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm. Additionally, the post-yielding dislocation retraction is more pronounced at higher temperatures, making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.     

A study of the structure and scattering mechanisms of subterahertz phonons in lithium fluoride single crystals and optical ceramics

The methods of optical, electron, and atomic force microscopy (AFM) are applied to the study of the real structure of optical lithium fluoride ceramic obtained by hot deformation of single crystals. A comparative analysis is carried out of the scattering mechanisms of weakly nonequilibrium thermal phonons at liquid helium temperatures in LiF single crystals and ceramics. It is demonstrated that the phonon scattering in the original single crystals is determined by the forced vibrations of dislocations in the stress field of an elastic plane wave (a phonon), i.e., by the flutter mechanism. As the degree of deformation of the original material increases, the ceramics exhibit a change in the plastic deformation mechanisms, which leads to a decrease in the average size of grains and to an ordered structure. In this case, the dominant scattering is that by intergrain boundaries. The thickness and the acoustic impedance of these boundaries are evaluated.     

Plastic properties and defect structure of LiF-LiF:Mg layered single crystals at T = 4.2 K

The compressive stress-strain diagrams are obtained for layered single crystals of the LiF-LiF:Mg type with different orientations of the reinforcing layers at T = 4.2 and 300 K. The strength characteristics and specific features of the defect and dislocation structures of the crystals strained in liquid helium are studied and compared with those for the crystals strained at 300 K. It is established that the layered single crystals remain plastic at T = 4.2 K. Under a strain ε > 1%, there arise microcracks and macrocracks responsible for complete fracture of the crystals. It is revealed that long-term (for ten years) storage of the layered single crystals at 300 K substantially affects the magnesium impurity structure in the reinforcing layers and leads to a considerable enhancement of their strength characteristics. This effect is taken into account when analyzing the strength properties of the layered single crystals in the temperature range T = 300–4.2 K. The inference is made that the results obtained in studying the defect microstructure of the model layered single crystals at the initial stage of plastic deformation can be used to predict the strength properties of the composites at T = 4.2 K.     

Generation and relaxation of reactive stresses in a Cu-Al-Ni shape memory alloy upon cyclic temperature variation in the range 293–800 K

The generation and relaxation of reactive stresses in Cu-Al-Ni shape-memory alloy single crystals studied during a single cycle of temperature variation in the range 293–800 K under conditions of the β-phase decomposition (above 600 K) are found to depend on the degree of β-austenite decomposition at the stage of decreasing temperature. The higher this degree, the lower the stresses relaxed and generated upon decreasing temperature and the higher the critical temperatures of the reverse martensitic transitions. Moreover, loading the alloy by reactive stresses during a heating half-cycle causes not only a reversible martensitic shape memory deformation but also an additional austenitic shape memory deformation to occur when the temperature is decreased.     

Magnetic irreversibility lines and critical currents of Bi(2212) single crystals doped by Fe, Ni, Co and Zn

We report measurements of the AC-susceptibility and the irreversible magnetization of Bi(2212) single crystals doped with up to 2 at.% of Fe, Ni, Co and Zn on the Cu position. Low concentrations of the doping elements, typically below 1 at.%, definitely increase the pinning force density and shift the magnetic irreversibility lines towards higher fields. In contrast, higher concentrations strongly suppress the critical current density and the irreversibility fields. A special situation occurs in the regime of low temperatures below about 10 K where we find that even very low concentrations of the doping elements are deteriorative for the critical current density.     

Anisotropic electron spin-wave scattering in terbium

The magnetoresistance in Tb single crystals has been investigated in magnetic fields up to 75 kOe, and in the temperature range 4–36 K. The longitudinal magnetoresistance is negative, in contrast to the results in the transverse field which are positive and show a maximum for T > 20K. The results suggest an anisotropy in scattering of electrons by spin waves, and the ratio of the anisotropic to the total spin wave resistivities are approximately 18 and 12% for a- and b-axis crystals respectively.