Influence of disorder on the interface sharpness of an Ising ferromagnetic system

We study the thermodynamic behavior of a two-component random ferromagnetic Ising system in the presence of given boundary conditions. The system consists of two species A and B occupying the sites of a Bethe lattice which terminates on a surface layer where the spins are kept fixed. We study the interface of the system when the spins on half of the surface of the lattice are fixed opposite to the spins on the other half. More specifically, we study the influence of disorder on the interface width. We find that disorder clearly increases the interface width at temperatures well belowT _c, indicating that the interface roughening of disordered Ising systems in 3D real lattices should occur at temperatures significantly below those of the corresponding ordered ones.     

Electric-field induced capillary interaction of charged particles at a polar interface

We study the electric-field induced capillary interaction of charged particles at a polar interface. The algebraic tail of the electrostatic pressure of each charge results in a deformation of the interface u approximately r(-4), where r is the lateral distance. The capillary interaction of nearby particles is repulsive and varies as rho(-6) with their distance rho. As a consequence, electric-field induced capillary forces cannot be at the origin of the secondary minimum observed recently for charged poly(methyl methacrylate) particles at an oil-water interface.     

柔性管内油水环状流流动结构数值模拟

海洋中柔性管道受到外界猛烈撞击,撞击物的不同会导致管道内流道结构发生不同的改变.为探寻流道结构变化对环状流动结构的影响规律,采用N-S方程和湍流模型描述流体运动,用VoF模型与CSF模型追踪油水界面,建立数值方程与分析模型,分析流道不同形式的结构变形、不同程度变形和不同流速下油水环状流在管内的湍动能和漩涡结构演变,及环状流流型的演变和压力场的演变.分析结果表明:凹陷变形程度越大,漩涡的强度及面积越大,V形变形影响远大于○形变形影响,环状流的流型发生改变时仍然保持着水相包裹油相的稳定状态,最大压缩区域的油水界面压强也随之增大,且增速变大.所得结论对研究油水环状流在柔性管道运输中的稳定性具有一定指导意义.      

Comparisons of interface shear stress degradation rate between C/SiC and SiC/SiC ceramic-matrix composites under cyclic fatigue loading at room and elevated temperatures

The interface shear stress in C/SiC and SiC/SiC ceramic-matrix composites with different fiber preforms, i.e. unidirectional, cross-ply, 2D woven, 2.5D woven, and 3D braided, under cyclic fatigue loading at room and elevated temperatures have been estimated. An effective coefficient of the fiber volume fraction along the loading direction was introduced to describe the fiber preforms. Based on fiber slipping mechanisms, the hysteresis loops models considering different interface slip cases have been developed. Using the experimental fatigue hysteresis dissipated energy, the interface shear stress degradation rates of C/SiC and SiC/SiC composites with different fiber preforms at room and elevated temperatures have been obtained and compared. It was found that the interface shear stress degradation rate is the highest for 3D braided SiC/SiC at 1300 °C in air, and the lowest for 2D woven C/SiC at room temperature under cyclic fatigue loading.     

Stability of cellulose lyotropic liquid crystal emulsions

We studied a new kind of W/O emulsions based on a lyotropic liquid crystal as the aqueous droplet phase. The cholesteric phase, a solution hydroxypropyl cellulose in water was dispersed in the continuous oil matrix, paraffin oil or heptane. We made a specific choice of surfactant in order to impose director anchoring conditions at the oil-water interface and orient the liquid crystal inside the droplet. The strong anchoring conditions resulted in a topological defect inside the droplets of size above the critical value R^*. The defect elastic energy creates a barrier against droplet coalescence, the effect of topological size selection. We have studied the orientation of the director inside the droplets and their size distribution.     

Well-width dependence of interface roughness scattering in GaAs/Ga1 − xAlxAs quantum wells

Well-width dependence of quantum and transport mobilities of electrons in GaAs/GaAlAs multiple quantum wells is studied for wells with widths ranging between 50 Å and 145 Å Experimental results are obtained from the amplitude analysis of the Shubnikov–de Haas (SdH) oscillations and from conventional Hall measurements at temperatures betweenT = 15 K and 4.2 K. A novel technique is employed to estimate, theoretically, the interface roughness parameters from electron quantum and transport lifetimes. The modelling is carried out for a range of layer fluctuations, width (Δ) and lateral size (Λ), as to obtain the best fit to the experimental results for individual samples. Our results indicate that the interface roughness scattering limits equal both quantum and transport mobilities at low temperatures, and that the nature of scattering by interface roughness (small or large angle) depends not only on the size and the width of the fluctuations but also on the distribution of these fluctuations within the samples. Therefore, unlike the predictions of the existing theoretical models, which assume constant values of Δ and Λ for all well widths, the well-width dependence of interface roughness scattering cannot be verified experimentally.     

Interface profile of the Ising ferromagnet in two dimensions

The interface profile of the two-dimensional Ising ferromagnet is obtained for all temperatures in the thermodynamic limit. The width of the interface depends on its length as (length)^1/2.On leave from Oxford University. Partially supported by NSF grant no. MPS 75 20638.     

Generalized Ginzburg-Landau functional calculations of the structure and energy of the coherent interphase boundaries between austenite and martensite in iron

The earlier-developed methods and experimental data on the phonon spectra and thermodynamics of fcc and bcc iron are used to construct a generalized Ginzburg-Landau functional for microscopic investigations of the austenite-martensite phase transformations in iron. This functional is used to calculate the structure and properties of a plane austenite-martensite interface for arbitrary orientations of this interface at various temperatures. The transformation parameter profiles in the interface region are calculated. The interface width is found to substantially exceed the interatomic distances. The interfacial surface energy is 500?C800 erg/cm², depends strongly on the orientation, and has a sharp maximum when the interface is parallel to a close-packed plane.     

On the local structure of the phase separation line in the two-dimensional Ising system

We investigate the structure of the phase separation line between the pure phases in the two-dimensional Ising model, the liquid and vapor phase in lattice gas language, at low temperatures. The fluctuations in the location of this line are known to diverge in the thermodynamic limit, something which is also believed to happen to the continuum liquid-vapor interface in three dimensions (in the absence of the gravitational field). We show that despite this global divergence it is possible to define precisely the local structure of the phase separation line. This has a finite, exponentially small, width at low temperatures which is related by a central limit theorem⁽¹⁾ to the width of the global fluctuations on the appropriate (divergent) length scale. The latter has been computed explicitly⁽²⁾ for all temperatures below the critical temperatureT _c, where it diverges as (T _c –T)^–1/2. We also prove a Gibbs formula for the surface tension at low temperature, which relates it to the local structure of the phase separation line.Supported in part by NSF grant No. M^rPHY 78-15920 and MCS78-01885.On leave from: Departement de Physique Théorique, Université de Louvain, Belgium.     

场助InP／InGaAsP／InP半导体光电阴极异质结能带的计算

采用双曲型的渐变函数,同时考虑加偏压时引起的阴极表面空间电荷区的变化,对场助InP/TnGaAsP/InP半导体光电阴极异质结的能带结构进行了详细的分析和计算,得到了在不同材料参数时,异质结能带结构的分布曲线.计算结果指出了达到理想的异质结传输效率时,发射层的厚度和掺杂浓度、吸收层的掺杂浓度、异质结界面处渐变区宽度以及场助偏压应满足的条件.它有助于场助半导体光电阴极的结构设计和材料参数的选择.     

Monte Carlo simulation of the compensation and critical behaviors of a ferrimagnetic core/shell nanoparticle Ising model

A Monte Carlo simulation has been used to study the magnetic properties and the critical behaviors of a single spherical nanoparticle, consisting of a ferromagnetic core of spins surrounded by a ferromagnetic shell of S=±1, 0 or , spins with antiferromagnetic interface coupling, located on a simple cubic lattice. A number of characteristic phenomena has been found. In particular, the effects of the shell coupling and the interface coupling on both the critical and compensation temperatures are investigated. We have found that, for appropriate values of the system parameters, two compensation temperatures may occur in the present system.     

Heteroepitaxial growth of high-temperature superconductors and doped manganites in ramp type geometry

We have studied heteroepitaxially grown ramp type structures consisting of high-temperature superconductors (HTS) and doped manganites in various configurations. Firstly, the coupling of two HTS electrodes via doped manganite barriers was analyzed and secondly the interface properties between HTS and doped manganites as well as between doped manganites with different Curie temperatures have been investigated. As a common feature of the interfaces involving doped manganites an increase in resistance at low temperatures was found which appears together with non-linear current–voltage characteristics. On applying external magnetic fields up to 8 T, the low temperature interface resistance is suppressed considerably. These observations can be understood in terms of a spin glass behavior of the doped manganites just at the interface caused by strain due to lattice misfit and disorder. The differential conductance spectra of YBa₂Cu₃O_7−δ–La_2/3Ca_1/3MnO₃ junctions with low transparency show a pronounced gap feature but no zero-bias conductance peak.     

基于量子修正的石墨烯纳米带热导率分子动力学表征方法

本文提出了基于量子修正的非平衡态分子动力学模型,可用于石墨烯纳米带热导率的表征.利用该模型对不同温度下,不同手性及宽度的石墨烯纳米带热导率进行了研究,结果发现:相较于经典分子动力学模型给出的热导率随温度升高而单调下降的结论,在低于Debye温度的情况下,量子修正模型的计算结果出现了反常现象.本文研究还发现,石墨烯纳米带的热导率呈现出明显的边缘效应及尺度效应:锯齿型石墨烯纳米带的热导率明显高于扶手椅型石墨烯纳米带;全温段的热导率及热导率在低温段随温度变化的斜率均随宽度的增加而增大.最后,文章用Boltzmann声子散射理论对低温段的温度效应及尺度效应进行了阐释,其理论分析结果说明文章所建模型适合在全温段范围内对不同宽度和不同手性的热导率进行精确计算,可为石墨烯纳米带在传热散热领域的应用提供理论计算和分析依据.     

Determination of the LO phonon energy by using electronic and optical methods in AlGaN/GaN

The longitudinal optical (LO) phonon energy in AlGaN/GaN heterostructures is determined from temperature-dependent Hall effect measurements and also from Infrared (IR) spectroscopy and Raman spectroscopy. The Hall effect measurements on AlGaN/GaN heterostructures grown by MOCVD have been carried out as a function of temperature in the range 1.8-275 K at a fixed magnetic field. The IR and Raman spectroscopy measurements have been carried out at room temperature. The experimental data for the temperature dependence of the Hall mobility were compared with the calculated electron mobility. In the calculations of electron mobility, polar optical phonon scattering, ionized impurity scattering, background impurity scattering, interface roughness, piezoelectric scattering, acoustic phonon scattering and dislocation scattering were taken into account at all temperatures. The result is that at low temperatures interface roughness scattering is the dominant scattering mechanism and at high temperatures polar optical phonon scattering is dominant.     

Measurement of long-range repulsive forces between charged particles at an oil-water interface

Using a laser tweezers method, we have determined the long-range repulsive force as a function of separation between two charged, spherical polystyrene particles (2.7 microm diameter) present at a nonpolar oil-water interface. At large separations (6 to 12 microm between particle centers) the force is found to decay with distance to the power -4 and is insensitive to the ionic strength of the aqueous phase. The results are consistent with a model in which the repulsion arises primarily from the presence of a very small residual electric charge at the particle-oil interface. This charge corresponds to a fractional dissociation of the total ionizable (sulfate) groups present at the particle-oil surface of approximately 3 x 10(-4).     

Novel surface tension isotherm for surfactants based on local density functional theory

In this Letter we report a new general method for calculating of surface tension isotherms in the presence of surfactants, based on a local density functional. We illustrate this method by deriving the interfacial tension isotherm for nonionic surfactants at an air-water or oil-water interface by using the self-consistent field theory of polymer brushes. We consider a particular case of local density functional to calculate explicitly how the interfacial tension and the surfactant adsorption depend on the surfactant bulk concentration. Experimental data for the surface tension and the surfactant adsorption isotherm for nonionic surfactants were interpreted with the help of the new isotherm. Very good agreement between the adsorption of n-dodecyl pentaoxyethylene glycol ether (C12E5) at an air-water interface, calculated from the surface tension isotherm and small-angle neutron-scattering is obtained.     

Chemically driven nonlinear waves and oscillations at an oil-water interface

An experimental investigation is presented of chemically driven dynamic instability of an oil-water interface in a cylindrical and annular glass container. The immiscible liquids are water containing a surfactant (TSAC) and nitrobenzene containing iodine. The reaction at the interface leads to complex deformation patterns including rotating solitary waves, multiple wave trains, periodic and nonperiodic oscillations, source-to-sink propagation, and intermittent behavior. A phase diagram is established for the two solute concentrations ranging from 10^-4 to 10^-1 M. It shows five distinct regions of different dynamic regimes, determined by interaction of at least three mechanisms: dynamic wetting of the container wall, capillary effect, and Marangoni instability of the liquid-liquid interface. The influences of aspect ratio, concentration product, and temperature are investigated and local time traces are derived from electropotential differences. Their spectral analysis reveals details of periodic or irregular motion. Transitions between the dynamic modes of the system during its temporal evolution are recorded. Quantitatively determined profiles of regular waves are analyzed by exponential functions and a simple model for this is proposed.     

Interface states in two-dimensional electron systems with spin-orbital interaction

Interface states at a boundary between regions with different spin-orbit interactions (SOIs) in two-dimensional (2D) electron systems are investigated within the one-band effective mass method with generalized boundary conditions for envelope functions. We have found that the interface states unexpectedly exist even if the effective interface potential equals zero. Depending on the system parameters, the energy of these states can lie in either or both forbidden and conduction bands of bulk states. The interface states have chiral spin texture similar to that of the edge states in 2D topological insulators. However, their energy spectrum is more sensitive to the interfacial potential, the largest effect being produced by the spin-dependent component of the interfacial potential. We have also studied the size quantization of the interface states in a strip of 2D electron gas with SOI and found an unusual (non-monotonic) dependence of the quantization energy on the strip width.     

The effect of interface roughness scattering on low field mobility of 2D electron gas in GaN/AlGaN heterostructure

We report the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of about at 3.8 K remains almost constant up to lattice temperature , it then decreases rapidly down to about at . The results are discussed using a theoretical model that takes into account the most important scattering mechanisms contributing to determine the mobility of electrons in 2DEG. We show that the polar optical phonon scattering is the dominant mechanism at high temperatures and the acoustic deformation potential and piezoelectric scatterings are dominant at the intermediate temperatures. At low temperatures, the Hall mobility is confined by both the interface roughness (IFR) and ionised impurity scattering. The correlation length (Λ) and lateral size (Δ) of roughness at the GaN/AlGaN heterointerface have been determined by fitting best to our low-temperature experimental data.     

Adhesion and bonding of the Al/TiC interface

The electronic structure and adhesion of Al/TiC(0 0 1) interface are examined by density functional theory. Our results show the preferred configuration is the Al atom above the ceramic’s metalloid atom. The calculated adhesion explains the conflicting experimental results of the W_ad from the aspect of the establishing different chemical equilibrium bonds at the different temperatures. By applying several analysis methods we have thoroughly characterized the interfacial electronic structure. For the Ti-site the interfacial Al and Ti atoms form the metal/covalent bond, while for the C-site the interfacial Al and C atoms form the polar covalent interaction. In addition, we examine the effects of Mg and Si alloying elements at the interface, and find that Mg greatly deteriorates the interface and Si slightly improves the interface. The cleavage may take place preferentially at the interface with the help of interface strain energy, especially with the addition of Mg. This is in good agreement with the experimental result.