Magnetic-field-induced orientational phase transition in a nonuniformly stressed iron borate single crystal

Spatially nonuniform magnetic anisotropy was induced in the basal plane of an iron borate (FeBO₃) single crystal by applying low-symmetry stresses. The effect of nonuniform magnetic anisotropy on the magnetic state of this weak ferromagnet was studied by magnetooptic methods. It is revealed that, when a nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane along a certain direction (depending on the symmetry of the applied stress), a transition from a uniform to a spatially modulated magnetic state occurs, which is not observed in the crystal in the absence of stresses. The modulated magnetic phase of the crystal can be represented as a static spin wave linearly polarized in the basal plane, with the azimuth of the weak-ferromagnetism vector oscillating about the direction of the mean magnetization. The temperature and field dependences of the spatial period of the modulated magnetic structure and the amplitude of oscillations of the ferromagnetism vector are studied, and the temperature dependence of the field range over which the modulated phase exists in the nonuniformly stressed FeBO₃ crystal is found. The results are discussed in terms of the phenomenological theory of magnetic phase transformations. It is shown that the theoretical model used consistently describes all the experimental results of the study of the noncollinear magnetic phase of the nonuniformly stressed FeBO₃ crystal.     

Modulated magnetic structure of a nonuniformly stressed FeBO3 single crystal

The magnetization of a nonuniformly stressed FeBO₃ crystal along any of the two specific directions in the basal plane (the easy plane) at a temperature of T < 140 K in a magnetic field exceeding the threshold value H ₀ is found to lead to a transition of the crystal from the uniform magnetic state to the spatially modulated one. The modulated magnetic phase arising under these conditions exists in a certain temperature-dependent field range H ₀ ≤ H ≤ H _c and is representable in the form of a static spin wave that is linearly polarized in the easy plane of the crystal and has a wave vector k oriented at an angle of ～30° to the magnetization axis. The field, temperature, and orientation dependences of k are investigated. A physical mechanism is proposed to explain the modulation of the magnetic order parameter of the crystal under study. The results obtained are discussed in terms of the magnetic ripple theory.     

Technical magnetization of the nonuniformly stressed iron borate single crystal

Additional spatial nonuniform magnetic anisotropy is induced in the basal plane of a FeBO₃ single crystal using low-symmetry mechanical stresses. The effect of the nonuniform magnetic anisotropy on the magnetic state of this weak easy-plane ferromagnet is studied by a magnetooptic method. When the nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane near a certain preferential direction, the crystal is found to transform from a homogeneous into a spatially modulated magnetic state, which can be represented by a static spin wave in which a local ferromagnetism vector lies in the basal plane and oscillates about the average magnetization direction in the crystal.     

Diffusion within a near-critical nanopore fluid

Results are presented for grand canonical Monte Carlo (GCMC) and both equilibrium and non-equilibrium molecular dynamics simulations (EMD and NEMD) conducted over a range of densities and temperatures that span the two-phase coexistence and supercritical regions for a pure fluid adsorbed within a model crystalline nanopore. The GCMC simulations provided the low temperature coexistence points for the open pore fluid and were used to locate the capillary critical temperature for the system. The equilibrium configurational states obtained from these simulations were then used as input data for the EMD simulations in which the self-diffusion coefficients were computed using the Einstein equation. NEMD colour diffusion simulations were also conducted to validate the use of a system averaged Einstein analysis for this inhomogeneous fluid. In all cases excellent agreement was observed between the equilibrium (linear response theory) predictions for the diffusivities and non-equilibrium colour diffusivities. The simulation results are also compared with a recently published quasi-hydrodynamic theory of Pozhar and Gubbins (Pozhar, L. A., and Gubbins, K. E., 1993, J. Chem. Phys., 99, 8970; 1997, Phys. Rev. E, 56, 5367.). The model fluid and the nature of the fluid wall interactions employed conform to the decomposition of the particle–particle interaction potential explicitly used by Pozhar and Gubbins. The local self-diffusivity was calculated from the local fluid–fluid and fluid wall hard core collision frequencies. While this theory provides reasonable results at moderate pore fluid densities, poor agreement is observed in the low density limit.     

Observation of dislocation line motion in a stressed cholesteric liquid crystal

Summary By means of an optical microscope we observed the instantaneous displacement and motion of dislocation lines in a cholesteric liquid crystal, when the sample was subjected to a mechanical normal strain. It was found that in the linear response regime the equilibrium is reached in a quasi-periodic way. We explain our results just in a phenomenological way. A preliminary account of this work was presented at the IX International Conference on Liquid Crystals, Bangalore (1982).     

Diffusion of directed polymers in a random environment

We consider a system of random walks or directed polymers interacting weakly with an environment which is random in space and time. In spatial dimensionsd>2, we establish that the behavior is diffusive with probability one. The diffusion constant is not renormalized by the interaction.     

应变液晶散射偏光片的试制

介绍了聚合物分散液晶(PDLC)和应变液晶(SLC)以及散射偏光片的概念,实验制备了应变液晶散射偏光片样品,样品由两张胶片夹层聚合物分散液晶膜在紫外光固化过程中施加剪切应力所形成。给出了样品偏光显微镜照片和可见光分光光度计偏光特性光谱分析。实验结果表明,应变液晶散射偏光片样品外观半透明,在正交偏光场中转动样品有颜色变化,通过样品观察液晶显示器屏幕,在不同角度下有透射和散射作用,在偏光显微镜中观察样品有衬垫料被拉动的痕迹,光谱分析显示样品在绿光550nm波长处最大透光率T∥≈50%,最小透光率T⊥≈5%,偏振度P≈82%。实验结果对于研制实用的拉伸液晶散射偏光片具有参考意义。     

Decoherence within a single atom

An “almost diagonal” reduced density matrix (in coordinate representation) is usually a result of environment induced decherence and is considered the sign of classical behavior. We show that the proton of a ground state hydrogen atom can indeed possess such a density matrix. This example demonstrates that the “almost diagonal” structure may be derived from an interaction with a low number of degrees of freedom which play the role of the environment. We also show that decoherence effects in our example can only be observed if the interaction with the measuring device is significantly faster than the interaction with the environment (the electron). In the opposite case, when the interaction with the environment is significant during the measurement process, coherence is maintained. Finally, we propose a neutron scattering experiment on cold He atoms to observe decoherence which shows up as an additional positive contribution to the differential scattering cross section. This contribution is inversely proportional to the bombarding energy.     

Diffusion of directed polymers in a strong random environment

We consider a system of random walks or directed polymers interacting with an environment which is random in space and time. It was shown by Imbrie and Spencer that in spatial dimensions three or above the behavior is diffusive if the directed polymer interacts weakly with the environment and if the random environment follows the Bernoulli distribution. Under the same assumption on the random environment as that of Imbrie and Spencer, we establish that in spatial dimensions four or above the behavior is still diffusive even when the directed polymer interacts strongly with the environment. More generally, we can prove that, if the random environment is bounded and if the supremum of the support of the distribution has a positive mass, then there is an integerd ₀ such that in dimensions higher thand ₀ the behavior of the random polymer is always diffusive.     

Collinear acousto-optic interaction in a single tellurium crystal

The problem of designing acousto-optic imaging filters based on tellurium crystal is discussed. The measurement and calculation results of acousto-optic properties of tellurium are presented. The collinear acousto-optic interaction in tellurium crystal is experimentally implemented for the first time at the light wavelength of 10.6 μm.     

Phase transition in a multiferroic Ni-Mn-Ga single crystal

ABSTRACT

We studied martensitic phase transformation, crystal structure and twinned microstructure of resulting martensite of a Ni-Mn-Ga single crystal as essential conditions for magnetic shape memory effect. Thermal dependence of electric resistivity, magnetic susceptibility and dilatation measurements were measured to characterise kinetics of the transformation. With the help of XRD analysis and optical microscopy we evaluated the hierarchical twinning microstructure in the 10M martensite.     

The crystal magnetic field in a proton-polarized lanthanum magnesium nitrate single crystal

Knowledge of the crystal magnetic fieldH _c is necessary for proton polarization measurements in polarized targets by the internal field method [1, 2]. Because of the importance of lanthanum magnesium nitrate (LMN) single crystals for low-energy polarized proton targets [3],H _c has been calculated for LMN on the basis of new x-ray diffraction data. We find thatH _c=ApP₂(cosθ), wherep is the proton polarization andP ₂ the second Legendre polynomial depending onθ, the angle between crystalc-axis and external magnetic field, andA is a constant characteristic for LMN. Our result \(A = - \left( {0.242 \pm \begin{array}{*{20}c} {0.028} \\ {0.090} \\ \end{array} } \right) Oe\) leads to a field only about half as large as expected according to previous assumptions. The difference is due to the markedly different proton positions, revealed by the x-ray structure analysis.     

Restricted diffusion within a single pore

The time-dependent diffusion of 3Heatoms perpendicular to the axis of a single macroscopically large cylindrical pore is studied using a steady (or constant) gradient-recalled echo sequence. Measurements of the effective 3Hediffusion coefficient extending from the free-diffusion regime to the motionally averaged regime are presented, and are well-described by analytic solutions to the Bloch-Torrey equation based on the gaussian phase approximation. Our data yield the value 0.140(6)m2/s for the self diffusion coefficient of 3Heat a temperature of 296 K and a pressure of 1.00 Torr. Adaptations of these methods should enable the study of complex pore geometries as model systems.     

Instabilities and coarsening of stressed crystal surfaces in aqueous solution

Strong pattern formation occurs on polished miscut surfaces of sodium chlorate (NaClO3) single crystals that are uniaxially stressed perpendicular to the step edge direction and placed in a saturated aqueous solution. The wavelength lambda of the stress-induced surface instability increased continuously in experiments up to 9 days after placed in the solution. There were three successive regimes of coarsening: (i) one-dimensional step bunching with lambda approximately t(1/4) until an undulation transition was reached, (ii) a two-dimensional coarsening mechanism with lambda approximately t(1/2), and a gradual transition to (iii) Ostwald ripening-like coarsening with lambda approximately t(1/3). The coarsening of the surface patterns towards a stable, flat surface implies the spontaneous formation of a stress-free skin on the surface of the stressed solid.     

Polarization bremsstrahlung of a hydrogen-like ion in a single crystal

Polarization bremsstrahlung (PB) that arises when a fast hydrogen-like ion is scattered in a single crystal is investigated theoretically. Four types of the process are analyzed that are caused by virtual excitation of electrons of the target and of the incident particle (IP), as well as by a coherent and incoherent interaction between the IP and the single crystal. The spectral, angular, and velocity (of the IP) characteristics of PB are calculated with regard to the spectral function of a photodetector. Optimal observation conditions and regions in which different types of PB are dominant are determined, and the dependence of these regions on the charge of the IP nucleus and of the target atoms is revealed.     

Vibrational spectra of a GaS single crystal

The Raman and infrared active long wavelength phonons of a GaS single crystal were studied at different temperatures in the 10–600 cm^?1 range. Properly polarized Raman spectra could be obtained with the 4880 Å exciting line and the previous assignment of the E_1g modes controversed recently could be confirmed. Infrared spectra were recorded in the 30–600 cm^?1 region. The vibrational frequencies of the crystal were also calculated using a method developed by Wieting and six new frequencies corresponding to infrared and Raman inactive modes have been proposed.We have observed that the degree of leakage of scattered intensity in unallowed polarizations increases when the wavelength of the exciting line moves off the exciton absorption front. The phonon at 74 cm^?1 was particularly sensitive and the question of the antiresonant behaviour of this compound is raised.     

Nuclear magnetic relaxation in a single crystal of tellurium

The nuclear magnetic relaxation of ¹²⁵Te in pure single crystals of tellurium has been investigated at low temperatures, and a new relaxation mechanism which relates to the electronic structure of tellurium is proposed.