声子晶体中的半狄拉克点研究

在本文中, 构建了一种易于实现的二维声子晶体: 截面为正方形的铁柱以三角晶格形式排列在水中. 研究发现, 在此声子晶体的布里渊区中心Γ点有半狄拉克点出现: 其带结构沿ΓY方向是线性的, 但沿着ΓX方向却是二次型的. 若散射体绕中心轴旋转角度θ = 45°, 则半狄拉克点的二次型带结构则会转至ΓY方向, 与ΓX相互垂直. 接着, 本文采用k· p 微扰法系统研究了在不同旋转角θ 值下, 简并点附近的带结构特点, 并在此基础上分析了半狄拉克点的出现原因. 在半狄拉克点附近, 以布洛赫简并态为基矢, 文中构造了一个有效哈密顿量, 根据它能准确计算贝利相位, 并发现其值为零. 此外, 通过有限元仿真, 还研究了在半狄拉克点频率附近声波沿着不同方向穿过该声子晶体的透射现象. 本文可以为经典体系中半狄拉克点色散关系的起源、有关传播性质的研究以及其在声子晶体的应用提供理论参考.     

超声悬浮过程中圆柱体的旋转运动机理研究

研究了圆柱体在超声悬浮过程中的旋转运动机理.实验发现:悬浮圆柱体的密度和长径比越小,转动惯量越小,其稳态旋转的转速越大;反射端在水平方向的偏移会产生回复力矩,使圆柱体停止旋转,且圆柱体静止时的轴线方向与反射端偏移方向垂直;在圆柱体两端加入适当的外界干扰可以主动抑制其旋转.计算表明,悬浮圆柱体的旋转起源于其质心偏移产生的力矩,而反射端位置的偏移以及发射端的倾斜均会抑制圆柱体的旋转.     

Strain-induced matrix and droplets anisotropic deformation in liquid crystalline cellulose dispersed liquid crystal films

The present work deals with the preparation of new liquid crystalline cellulose dispersed liquid crystal films (100 μm) using a shearing casting technique. The matrix of the films presents the so-called band texture perpendicular to the shear direction. The nematic low molecular weight liquid crystal is encapsulated in micron and submicron size ellipsoids. The ratio between the lengths of the main axis and the short axis is around 1.23. The main axis is oriented, on average, 28° away from the shear direction. The evolution of the band texture and of the ellipsoidal liquid crystal droplets is investigated by polarizing optical microscopy and light scattering techniques as a function of the strain imposed along and perpendicular to the shear direction. Stretch along shear with strain equal to 0.8 seems to have no effect on the banded structure of the matrix, while the nematic liquid crystal ellipsoids slightly orient the main axis to the stress direction and their shape anisotropy increases by a factor of 2. Deformation in the direction transverse to the shear direction induces a deep change of the polymeric matrix and, at the end, a fibrillar structure is found. The nematic ellipsoids rotate their main axis and align along the stretch direction. Their shape anisotropy evolves from a value of 1.23 to 1 for intermediate deformations with strain equal to 0.5, to a final value of 5 with strain equal to 0.8, in the perpendicular direction.     

Temperature-induced magnetic phase transitions in crystals with competing single-ion and interionic magnetic anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.     

A simple induction anisometer

Magnetic anisotropy of rotationally symmetric samples is measured by a simple induction method based on transverse susceptibility measurements. The sample is magnetically saturated by a d.c. field perpendicular to the axis of rotation. A small a.c. field is applied in the direction perpendicular to both the d.c. field and the axis of rotation. The transverse a.c. susceptibility is measured by a Hartshorn bridge of mutual inductance. From the transverse susceptibilities along different directions the anisotropy constants as well as the saturation magnetization can be determined.     

The effect of rotation on the steady state response of a spring-mass system under harmonic excitation

A two degree-of-freedom system, consisting of a point mass which is constrained to move in one plane, is considered. The motion is controlled by linear springs and viscous damping. A constant amplitude harmonic force is applied along one axis in the plane, which is rotating at a constant angular velocity about an axis perpendicular to the plane. Due to the rotation, oscillation takes place in the direction perpendicular to, as well as along, the axis of excitation.The amplitude and phase of the steady state vibrations are derived as a function of the excitation frequency and the rate of turn. For rates of turn very much less than the system natural frequencies, this theory covers the principles of vibratory rate sensors such as the tuning fork; however, the emphasis here is on the performance of the system when the angular velocity is of the same order as the natural frequencies of the system.     

Experimental and theoretical evidence for the existence of absolute acoustic band gaps in two-dimensional solid phononic crystals

Experimental measurements of acoustic transmission through a solid-solid two-dimensional binary-composite medium constituted of a triangular array of parallel circular steel cylinders in an epoxy matrix are reported. Attention is restricted to propagation of elastic waves perpendicular to the cylinders. Measured transmitted spectra demonstrate the existence of absolute stop bands, i.e., band gaps independent of the direction of propagation in the plane perpendicular to the cylinders. Theoretical calculations of the band structure and transmission spectra using the plane wave expansion and the finite difference time domain methods support unambiguously the absolute nature of the observed band gaps.     

Force among magnetic nanocylinders trapped in triangular arrays

We study the effect of the forces among magnetic nanocylinders trapped in a membrane such as those used to produce them. The interaction force between two parallel and identical magnetic nanocylinders is revisited to obtain expressions that can later be used to add them both in a discrete and a continuum approximation. At this point a particular geometry has to be assumed and we use a particular configuration reported in the literature, namely, a bundle of parallel magnetic cylinders trapped in a circular membrane. When a strong enough external magnetic field is imposed along the axis of the membrane (also along the axes of the cylinders) all magnetizations point along this direction and cylinders repel among themselves. In a first approximation we will consider a soft enough membrane so energy is mostly relaxed through a deformation of the membrane leaving the magnetization of the cylinders basically as it was in the absence of external field. Then we obtain the forces among these cylinders by two methods: one summing the contributions of a discrete number of objects and another one in which we consider a continuum distribution of them to reach larger systems. Numerical evaluation of these forces can reach 50 mdyn approximately. Such forces will act on the membrane at the positions of the magnetic cylinders; in the case of a circular silicon membrane of radius 1.0 mm the radial expansion of the membrane can be of the order of 1 nm. This effect could be larger for softer membranes. A discussion of experimental techniques to detect this phenomenon is also done followed by the proposal of a possible application.     

Ultrasonic studies on bismuth single crystals

Studies of ultrasonic velocities and attenuation are carried out in bismuth single crystals along the trigonal axis and perpendicular to the trigonal axis and the results are interpreted in terms of the changes in carrier concentration with change of direction. Temperature dependence of ultrasonic attenuation is also reported.     

An electron microscopic study of the archaeal feast/famine regulatory protein: 2. Crystal formation by a full length FFRP,pot0434017 (FL11)

In the crystal the archaeal feast/famine regulatory protein pot0434017 (FL11) forms helical cylinders, each extending along the c axis and densely packed with a hexagonal symmetry in the a/b plane. By applying cryo-electron microscopy to protein solutions yielding crystals, with selectively focussing zero-loss electrons accelerated at 200 KV, two types of regular objects were observed, hexagonal (i.e. projections of crystals to their a/b planes) or rectangular (i.e. projections onto planes perpendicular to a/b). The two types of images are different in the ranges of sizes, suggesting that the crystallization might initiate by forming a hexagonal sheet on the a/b plane, subsequently extending along the c axis. Some other images obtained were intermediate between regular and amorphous, suggesting that some crystals were growing inside amorphous precipitates by rearranging the protein molecules, and that some larger crystals were growing by absorbing smaller amorphous precipitates. Tubes running parallel to each other were also observed in pieces of thin films. These tubes have hollows in their centers, and their lateral arrangement with a periodicity of ~150 Å and the presence of a helical component ~50 Å suggest that they are projections of the helical cylinders, forming mono-layers on the a/c or b/c planes.     

Analytical solution for spatially axisymmetric problem of thick-walled cylinder subjected to different linearly varying pressures along the axis and uniform pressures at two ends

To our best knowledge, in the open literature, there is no analytical solution of thick-walled cylinder subjected to uniform pressures at two ends and different inner-and outer-surface pressures that are constant circumferentially but vary linearly at different rates along the axis. We now present such a solution. After repeated trials, we have finally succeeded in finding a necessary new displacement function. Based on A. E. H. Love method, the stress, displacement and volume strain formulas are derived by using the new displacement function. The present results include the Lamé’s formulas as special cases. Furthermore, the results obtained here can be applied to not only the thick-walled cylinders subjected to uniform pressures on the inner and outer surface of the thick-walled cylinder, respectively, but also the cylinders subjected to uniform pressures at two ends and different inner-and outer-surface pressures that are constant circumferentially but vary linearly at different rates along the axis, respectively. Finally we give a numerical example to compare our exact method with the approximate method.     

Changes in ADC caused by tensile loading of rabbit achilles tendon: evidence for water transport

Water diffusion measurements were performed on rabbit Achilles tendons during static tensile loading and tendons in an unloaded state. The apparent diffusion coefficient (ADC) was measured along two directions: parallel and perpendicular to the long axis of the tendon. Tendons were studied after being prepared in two ways: (a) after being stored frozen in phosphate-buffered saline (PBS) and (b) freshly isolated. Statistically significant directional anisotropy was observed in the ADC in all tendons. The ADC was significantly greater in the direction parallel to the long axis of the tendon than in the perpendicular direction. The anisotropy is attributed to the greater restrictions seen by the water molecules in the perpendicular direction and is consistent with the known geometry of the tendon. Storage in PBS caused tendons to swell. This increased the ADC measured along both directions and reduced the anisotropy. The existence of anisotropy in the ADC was not related to the orientation of the specimen in the magnet. The ADC increased along both directions following the application of a 5-N tensile load; the increase was greatest along the perpendicular axis of the tendon. In order to determine whether load-related changes in the ADC reflected changes in interfibrilar spacing, we used electron microscopy to measure load-related changes in fibril spacing. Load-related changes in fiber spacing could not account for the observed changes in the ADC. The increase in ADC caused by loading was attributed to the extrusion of tendon water into a bulk phase along the outside surface of the tendon. In PBS-stored samples, enough fluid was extruded that it could be visualized. The transient response of the ADC to a 5-N tensile load was also studied. The absolute ADC in both directions increased with loading and recovered to baseline upon unloading. The transient changes in ADC, for both loading and unloading, had a mean time constant of approximately 15 min. The magnitude of the load-induced transient ADC changes was comparable to that seen in the static-loading experiments.     

Silicon diffusion in molybdenum disilicide: correlation effects

Correlation factors for silicon diffusion by a vacancy mechanism in the silicon sublattice of the tetragonal MoSi₂ structure have been calculated by combining an analytical and a Monte Carlo approach. The ratio of the silicon diffusivity perpendicular to the tetragonal axis to that parallel to the tetragonal axis is also deduced. An effect of forward correlation of tracer atom jumps in the silicon sublattice with the corresponding partial correlation factor of 1.5 appears at small frequencies of silicon atom jumps along the tetragonal axis with respect to the jump frequencies in the silicon layer perpendicular to the tetragonal axis of the MoSi₂ structure. The anisotropy of silicon diffusion in MoSi₂ measured by Salamon et al. is explained in terms of correlation effects of silicon diffusion on its own sublattice.     

Radius Measurement of a Concave Mirror by a Zone Plate Interferometer

A new technique to measure the radius of a concave mirror is proposed, in which the radius is calculated from the difference of the two interferograms obtained before and after shifting the sample perpendicular to or along the optical axis. By comparison of these two shifting directions, it is shown that since the amount of shift can be measured accurately and the result is insensitive to the tilt during the shifting, better accuracy can be obtained by shifting the sample along the optical axis.Presented at the International Commission of Optics Topical Meeting, Kyoto, 1994.     

Exercising control by changing the modulus of the ferromagnetic order parameter of twisting deformation

The distribution of the ferromagnetic order parameter (OP) is studied for an easy-axis type of ferromagnet under the effect of intense flow twisting applied along and perpendicular to the easy axis of a sample. The periodic changes in the OP modulus and the emergence of a perpendicular component of the magnetic OP are described. It is established that the law of the distribution of the magnetic moment depends on the temperature.     

Electromagnetic Waves in a Medium with Screw Dislocations

It is shown that in a medium with screw dislocations oriented predominantly along one axis the rotational velocity of the plane of polarization of an electromagnetic wave is much greater when it propagates in the direction perpendicular to this axis than in the parallel direction. For a given dislocation density tensor, the conditions under which the rotational velocity of the plane of polarization of the electromagnetic wave reaches its maximum are found.     

“Galileo Galilei-GG”: design,requirements, error budget and significance of the ground prototype

“Galileo Galilei-GG” is a proposed experiment in low orbit around the Earth aiming to test the equivalence principle to the level of 1 part in 10¹⁷ at room temperature. A unique feature of GG, which is pivotal to achieve high accuracy at room temperature, is fast rotation in supercritical regime around the symmetry axis of the test cylinders, with very weak coupling in the plane perpendicular to it. Another unique feature of GG is the possibility to fly 2 concentric pairs of test cylinders, the outer pair being made of the same material for detection of spurious effects. GG was originally designed for an equatorial orbit. The much lower launching cost for higher inclinations has made it worth redesigning the experiment for a sun-synchronous orbit. We report the main conclusions of this study, which confirms the feasibility of the original goal of the mission also at high inclination, and conclude by stressing the significance of the ground based prototype of the apparatus proposed for space.     

Radiation scattering by finite dielectric cyclinders

The volumetric integral equation formalism (VIEF) is used to determine characteristics of scattering of radiation falling along the axis of dielectric cylinders that scatter the radiation. The spectral dependence of the extinction efficiency factor Q is studied with changes in the length l of the cylinder (300–2200 nm), its width d (100–300 nm), and the refraction index m (1.33–1.65). In the range of angles θ=0–180°, for a cylinder with l=700 nm, d=100 nm, and m=1.33, angular intensity distribution functions i_i and i₂ are calculated for the components of the scattered radiation that are polarized perpendicular and parallel to the plane of observation, respectively. No effect of scattered-radiation depolarization is found. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 256–260, March–April, 1998.     

沿轴线的中垂线方向匀速直线运动的电偶极子的电场

本文计算了沿轴线的中垂线方向匀速直线运动的电偶极子的电场.     

Temperature measurement of air convection using a Schlieren system

In this work, we have developed a procedure for full-field measurement of temperature of a fluid flow by using the schlieren technique. The basic idea is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. The method is applied in the measurement of temperature fields of the air convection caused by a heated rectangular metal plate (7.3 cm×12 cm). Our tests are carried out at plate temperatures of 50 °C and 80 °C. To validate the proposed method, the schlieren temperature results are compared to those obtained by a thermocouple. Thermocouple data are obtained along two mutually perpendicular directions (one direction along the optical axis, z-direction, and other direction along the x-axis, which is perpendicular to the optical axis) at points located on a 9×9 grid with a variable spacing. The thermocouple measurements were integrated along the z-axis in order to be compared with the measurements obtained by the schlieren technique. The results from the two methods show good agreement between them.