Statics and dynamics of 2D colloidal crystals in a random pinning potential

We report the first experimental study of a model system of a two-dimensional colloidal crystal in a random pinning potential. The colloidal crystal consists of monodispersed charged polystyrene microspheres suspended in deionized aqueous media and confined near a rough charged surface. It is found that the static orientational correlation function g6(r) decays exponentially for intermediate and strong pinning, in agreement with theories. The driven depinning is dominated by thermally activated creep motion along 1D-like channels between regions with short-range order. A coexistence model is proposed for describing the observations.     

Interface states on semiconductor/insulator surfaces

It is well known that impurities and defects in semiconductors are associated with energy levels in the forbidden gap. Similar states occur at the surface of a semiconductor where the crystal lattice and the symmetry are strongly disturbed. These states are called surface states. Owing to the two-dimensional nature of the surface, their density is measured per unit area, in contrast to bulk states, which are measured per unit volume. A third type of states, similar to surface states, occurs at the interface between two adjacent materials. These states are called interface states. Very often they are also simply called surface states.     

Spontaneous disintegration of a noncylindrical jet emitted from the liquid surface unstable against self-charge

A dispersion relation for waves on the surface of a charged viscous incompressible conducting liquid jet with an arbitrary azimuthal number is derived. It is shown that the influence of deformation on the growth rate and wavenumber of the most unstable mode varies according to the sign of local deformation relative to the cylindrical jet (the locality is specified by the wavelength), azimuthal number, and electric charge per unit length of the jet. This circumstance should be taken into account to comprehend conditions of liquid spontaneous electrodispersion.     

Irradiance dependence of photofield emission from tungsten

Measurements of the irradiance dependence of photofield emission from tungsten have been extended to 1.0 GW m⁻². The photoyield (the photocurrent per unit irradiance) from the (111) crystal plane is found to be almost independent of irradiance. The (211) and (310) crystal planes are found to exhibit small nonlinearities which are attributed to changes in the work function of the emitting surface caused by laser heating. Even at the highest irradiance, no measurable current of photoelectrons arising from multiphoton excitation is observed.     

A study of growth mechanism of KDP and ADP crystals by means of quantum chemistry

Impurity adsorption, crystal growth by adsorption of growth unit and step-pinning mechanism of metal ion adsorption were investigated for potassium dihydrogen phosphate (KDP; KH₂PO₄) and ammonium dihydrogen phosphate (ADP; NH₄H₂PO₄) by quantum chemistry. In this study, the ideal crystal morphologies, the growth unit and the crystal surface with and without metal ions were calculated and analyzed by using electrostatic property. It is found that the computational results based on electrostatic potential distribution can account for the observed behaviours on KDP and ADP crystal growth.     

Numerical analysis of a photonic crystal fiber based on two polarized modes for biosensing applications

This paper presents a theoretical study on a photonic crystal fiber plasmonic refractive index biosensor. The proposed photonic crystal fiber sensor introduces the concept of simultaneous detection with the linearly polarized and radially polarized modes because the sensing performance of the sensor based on both modes is relatively high, which will be useful for selecting the modes to make the detection accurately. The sharp single resonant peaks of the linearly polarized mode and radially polarized mode, are stronger and more sensitive to the variation of analyte refractive index than that of any other polarized mode in this kind of photonic crystal fiber. For linearly polarized mode and radially polarized mode, the maximum sensitivities of 10448.5nm per refractive index unit and 8230.7nm per refractive index unit can be obtained, as well as 949.8 and 791.4 for figure of merits in the sensing range of 1.33-1.45, respectively. Compared with the conventional Au-metalized surface plasmon resonance sensors, our device is better and can be applied as a biosensor.     

Microscopic Origin of Universality in Casimir Forces

The microscopic mechanisms for universality of Casimir forces between macroscopic conductors are displayed in a model of classical charged fluids. The model consists of two slabs in empty space at distance d containing classical charged particles in thermal equilibrium (plasma, electrolyte). A direct computation of the average force per unit surface yields, at large distance, the usual form of the Casimir force in the classical limit (up to a factor 2 due to the fact that the model does not incorporate the magnetic part of the force). Universality originates from perfect screening sum rules obeyed by the microscopic charge correlations in conductors. If one of the slabs is replaced by a macroscopic dielectric medium, the result of Lifshitz theory for the force is retrieved. The techniques used are Mayer expansions and integral equations for charged fluids.     

Kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure

The kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure was studied as the first stage of a first-order phase transition in a deformable media. The shape and the critical dimensions of microvoids depending on the critical shear stress were found. The number of critical microvoids per unit volume arising per unit time on the surface of a slip plane was determined based on the kinetics of formation of pairs of charged vacancies. The volume fraction of pores at the initial stage of plastic deformation in CsBr and CsI crystals was estimated.     

Hardness of covalent crystals

Based on the idea that the hardness of covalent crystal is intrinsic and equivalent to the sum of the resistance to the indenter of each bond per unit area, a semiempirical method for the evaluation of hardness of multicomponent crystals is presented. Applied to beta-BC2N crystal, the predicted value of hardness is in good agreement with the experimental value. It is found that bond density or electronic density, bond length, and degree of covalent bonding are three determinative factors for the hardness of a polar covalent crystal. Our method offers the advantage of applicability to a broad class of materials and initializes a link between macroscopic property and electronic structure from first principles calculation.     

Optical phonons in ionic crystals with a diatomic unit cell

Optical phonons in an ionic crystal with two atoms per unit cell have been considered. The equation of relative motion of oppositely charged sublattices has been derived with the use of the concept of the self-consistent electromagnetic field. The frequency corresponding to transverse optical phonons and the expression for the frequency of the eigenmodes of the ionic crystal in terms of the physical characteristics of the crystal have been obtained from the condition of electrostatic equilibrium. The calculated values of the latter frequency agree with the experimental data.     

First-principles study of bulk and surface oxygen vacancies in SrTiO<Subscript>3</Subscript> crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F⁺ centres) in the bulk and on the (001) surfaces of SrTiO₃ crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO₂ surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO₂ surface. The charged F⁺ centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.     

Choice of the parameters of a magnetic undulator

The total output power of undulator radiation is investigated as a function of the constructional parameters of the undulator and its supply current. Calculations are presented of the magnetic field in the undulator, and the optimum undulator dimensions for maximum magnetic field intensity per unit length of the system are calculated. It is shown that the total radiated power of the charged particles depends on the undulator periodicity.     

正弦平方势与小振幅近似下的弯晶沟道辐射

在理想情况下和经典力学框架内,引入正弦平方势,把粒子在弯晶中的运动方程化为具有外力矩的摆方程。并对系统的相平面特征进行了数值分析。在小振幅近似下,把粒子运动方程化为具有硬特性的弹簧-振子系统,用Jacobian椭圆函数和椭圆积分解析地给出系统的解和粒子运动周期。讨论了弯晶沟道辐射频率、无量纲偏转角和辐射谱的一般特征。指出利用沟道辐射作为激光的可能性。以正电子在碳单晶中沟道辐射为例进行了具体计算,得到了与其他工作基本一致的结果。     

二维云母界面对晶体定向生长的诱导作用研究

利用原子力显微镜(AFM)研究了云母衬底对氯化钠晶体定向生长的诱导作用,发现云母表面的氯化钠按照夹角60°左右的方向或者相互平行、相互垂直的方向生长,此角度分布规律与文献中生物分子在云母表面诱导下组装的纳米结构的夹角相互一致。说明云母表面不但可以用于诱导生物分子按照一定的方向进行自组装,还可以用于诱导无机晶体按照特定的方向进行组装生长。     

Improving electret properties of PP filaments with barium titanate

Barium titanate (BaTiO₃) containing polypropylene (PP) composite filaments were melt spun to modify polymer electrostatic charging characteristics. Sample filaments were charged with a corona instrument and their surface potentials were measured. Initial surface potential as well as potential stability was monitored through an accelerated decay procedure. It was found that both BaTiO₃ concentration and charging temperature influence the charging characteristics of the fibers. When BaTiO₃/PP composite filaments were charged at 130 °C, significant enhancements were observed when compared to samples charged at room temperature. The distribution of BaTiO₃ particles within the filaments and changes in the crystal structure were also examined.     

铝、铜、镍三元合金系中τ相的晶体结构变迁

A thorough investigation by means of X-rays has been carried out with the purpose to determine the nature of the ternary phase τ in Al-Cu-Ni alloys. In contrast with the conventional concept of alloy phase which is characterized by a definite type of crystal structure, systematic structure changes are found in the single phase field of τ which occupies quite an extensive area in the isothermal section of the phase diagram at room temperature. There are eight types of structures altogether, all derived from a basic rhombohedron with corners occupied by Al atoms and centres either occupied by the heavy atoms or remaining vacant. The basic rhombohedron is the building stone in the crystal architecture. By transforming the basic rhombohedron into a hexagonal prism in the usual way, all structures may be considered to be built up by stacking together a number of these hexagonal prisms along the triad. The transformation of one structure into another is quite systematic in the way that the number of the stacking stories in the unit cell increases according to the order 10, 11, 12, 13, 14, 15, 16, 17. The atomic arrangements in the different structures are closely related too, in the respect that they are all superstructures due to the presence of ordered vacancies in the rhombohedral centres.The principal factor controlling the formation of these structures has been fully considered. In view of the fact that the change of structure types follows closely with the content of Ni or Cu for alloys of constant Al content, the atomic size factor appears to be unimportant in the formation of these alloys. It has been shown that for alloy phases of the defect lattice type as the r-phase, the most fundamental factor is the average number of valency electrons per structural unit which is the basic rhombohedron in the present case. By assuming Hume-Rothery's valencies, the average number of valency electrons remains remarkably constant throughout the entire phase field, while the electron concentration varies with compositions. It has also been pointed out that for alloy phases where there is no unit cell change, the average number of electrons per structural unit is equivalent to the number of electrons per unit cell, and for alloy phase where there is no defect, this is in effect equivalent to the electron concentration.     

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensed-phase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investigations carried out earlier in [1] with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investigated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial localization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the ice-forming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mechanism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The leading factor that determines the ice-forming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.     

Effect of surface charge on laser-induced neutral atom desorption

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges (Barth and Henry in J. Phys. Chem. C 113:247, 2009). In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures (Stoneham et al. in Appl. Phys. A 69:S81, 1999). We report here that such surface charging strongly impacts neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single-crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution.     

Boundary-driven colloidal crystallization in simple shear flow

Using confocal microscopy, we directly observe that simple shear flow induces transient crystallization of colloids by wall-normal propagation of crystallization fronts from each shearing surface. The initial rate of the front propagation was 1.75±0.07 colloidal layers per unit of applied strain. The rate slowed to 0.29±0.04 colloidal layers per unit of applied strain as the two fronts approached each other at the midplane. The retardation of the front propagation is caused by self-concentration of shear strain in the growing bands of the lower-viscosity crystal, an effect that leads to a progressive reduction of the shear rate in the remaining amorphous material. These findings differ significantly from previous hypotheses for flow-induced colloidal crystallization by homogeneous mechanisms.     

Thermodynamic limit in a charged plasma in equilibrium

We study the existence conditions for the thermodynamic limit in the chain of BBGKY equations for the equilibrium correlation functions of a charged plasma. It is shown that in order for the thermodynamic limit to exist the charge of the plasma cannot increase faster than the surface area of the plasma. When this condition is satisfied the equilibrium correlation functions of the charged plasma are asymptotically identical to the correlation functions of a neutral plasma in a self-consistent electrostatic field, which depends only on the one-particle correlation functions. For a plasma which is uniform everywhere except in a thin surface region, this field is found in explicit form. For a plasma occupying an infinite half-space, the problem is equivalent to a neutral plasma near a charged wall.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, 11–16, February, 1987.