Design and synthesis of plasmonic magnetic nanoparticles

Core–shell nanoparticles containing both iron oxide and gold are proposed for bioseparation applications. The surface plasmon resonance of gold makes it possible to track the positions of individual particles, even when they are smaller than the optical diffraction limit. The synthesis of water-dispersible iron oxide-gold nanoparticles is described. Absorption spectra show the plasmon peaks for Au shells on silica particles, suggesting that thin shells may be sufficient to impart a strong surface plasmon resonance to iron oxide-gold nanoparticles. Dark field optical microscopy illustrates the feasibility of single-particle detection. Calculations of magnetophoretic and drag forces for particles of different sizes reveal design requirements for effective separation of these small particles.     

Structure and melting of dipole clusters

Two-dimensional clusters of particles, repelling due to dipole-dipole interactions and confined by an external parabolic potential, are considered. The model describes different physical systems, particularly electrons in semiconductor structures, or electrons above a drop of He near a metal electrode, a drop of colloid liquid etc. Two kinds of ordering are in competition in the clusters: a triangular lattice and a shell structure. The ground-state configurations corresponding to the local and global minima of the potential energy for clusters with N = 1 – 40 “particles” are calculated. The structure, the potential energy and the radial and angular r.m.s. displacements as functions of temperature are also calculated. Analysing these quantities the melting of clusters is studied. One- or two-stage melting occurs depending on the number of particles in the cluster. In the case of clusters consisting of two shells melting has two stages: at lower temperature reorientation of neighbouring shells (“orientational melting”) arises; at much higher temperatures the radial shell order disappears. In clusters consisting of more than two shells total melting occurs as a first-order one-stage transition (analogously to a dipole crystal). This is connected with the barrier of rotation being less than the barrier of interchange of particles between shells for small microclusters while the barriers are of equal order for clusters with a greater number of particles.     

具有各向异性的梯度颗粒复合材料的有效介电响应

本文研究了具有梯度壳层椭球颗粒复合体系的有效介电响应，在稀释条件下用准静态近似方法, 推导了颗粒壳层具有任意介电梯度形式的椭球颗粒复合体系的有效介电常数和部分共振条件的一般表达式。并以壳层的介电常数为主轴的幂函数形式为例，得出了通过调节壳层的介电梯度形式、颗粒的结构和形状,可以提高该体系的有效介电常数和实现部分共振的结论。     

Hydrodynamic model for particle size segregation in granular media

We present a hydrodynamic theoretical model for “Brazil nut” size segregation in granular materials. We give analytical solutions for the rise velocity of a large intruder particle immersed in a medium of monodisperse fluidized small particles. We propose a new mechanism for this particle size-segregation due to buoyant forces caused by density variations which come from differences in the local “granular temperature”. The mobility of the particles is modified by the energy dissipation due to inelastic collisions and this leads to a different behavior from what one would expect for an elastic system. Using our model we can explain the size ratio dependence of the upward velocity.     

Density functional calculation of the ionization potentials of small metallic particles

A Density Functional Pseudopotential method is used to compute the ionization potential IP of Sodium, Aluminium and Lead microparticles as a function of particle size. The model particles are formed by a central atom surrounded by successive coordination shells of first neighbours, second neighbours, etc. Maxima in IP are obtained for particles with filled coordination shells. The magnitude of IP correlates with the magnitude of the electron density in the region where the electron is extracted.     

Relativistic dynamics of cylindrical shells of counter-rotating particles

Although infinite cylinders are not astrophysical entities, it is possible to learn a great deal about the basic qualitative features of generation of gravitational waves and the behavior of the matter conforming such shells in the limits of very small radius. We study an analytical model of a relativistic cylindrical shell of counter-rotating particles using kinetic theory for the matter and the junction conditions through the shell to obtain its equation of motion. The nature of the static solutions are analyzed, both for a single shell as well as for two coaxial shells. In the latter case, we integrate numerically the time dependent equation of motion of the external shell, when we neglect the wave components of the gravitational field at the shells locations. We obtain solutions that correspond to shells that perform damped oscillations, collapse, or are locally expanding. The collapse ends (numerically) when the external shell hits the interior shell. The numerically work also shows that the radiation becomes important after the bounce of the external shell.     

Cohesive energy of small metallic particles

The cohesive energy of sodium microparticles is calculated using the density functional formalism. The model particles are formed by a central atom surrounded by successive coordination shells of first neighbours, second neighbours, etc. Maxima in the cohesive energy per atom are obtained for particles with filled coordination shells. The sublimation energy is also studied. Insight is gained on the nature of “magic numbers” observed in the experimental size distribution.     

Optical absorption of ultrafine metal spheres with dielectric cores

We compute the optical absorption of a glass containing a small fraction of silver particles with dielectric cores. The results are based on the Maxwell Garnett formalism. The dielectric permeability of the metallic shells is obtained by modifying experimental data for bulk silver to account for size dependent scattering of the conduction electrons. We find an absorption maximum in the visible range at a wavelength which, for sufficiently minute particles, depends rather strongly on the size of the cores and on their dielectric constant. The relevance of these results to recent experimental data by Smithard and by Genzel et al. is pointed out. We argue that their observed absorption maxima can be explained without invoking any quantum effects.     

Two-dimensional small particles coupled by dipolar interactions

We study the effect of the dipolar coupling on the magnetic properties of two small interacting ferromagnetic particles. Each particle is a two-dimensional array of Ising spins with a central spin surrounded by a variable number of shells. The coupling between spins inside each particle is ferromagnetic and the dipolar interaction between the particles is determined as a function of the number of shells, temperature, and distance between their centers. We investigate the system by mean-field approximation and Monte Carlo simulations. The dipolar interaction is calculated in two ways, one assuming effective spins in the centers of the particles, and the other directly computing the interactions among all the pairs of spins, one in each particle. We show that the difference in the corresponding dipolar energies is a power law on the distance with exponent 5. We calculate the magnetization and susceptibility as a function of temperature, number of shells and distance between the particles’ centers. We show that the critical temperature increases with the number of spins in each particle, and it is more noticeable in the mean-field calculations than in the Monte Carlo simulations.     

Dust Coulomb balls: three-dimensional plasma crystals

First experimental investigations of spherical three-dimensional plasma crystals consisting of hundreds or thousands of micrometer-sized polymer particles suspended in a radio-frequency gas discharge are described. These "Coulomb balls" are not subject to the formation of dust-free regions (voids) and have an unusual structure of nested crystalline shells. While small systems are in a solid phase, large systems show melting effects.     

Spatial correlations of pairing collective states

The properties of the low-energy nuclear spectrum are greatly affected by pairing correlations. We study these effects in the nucleus ²¹⁰Pb which has two particles moving outside closed shells. The configuration-mixed wave functions describe the motion of particles which are on the average closer together than they would be if the particles were confined to particular orbitals. Since the energy associated with pairing correlations is much smaller than the Fermi energy the width of the associated probability distribution is determined by the wavelength of single particles moving close to the Fermi surface. Despite the fact that the amplitudes associated with high-lying configurations are small, their net effect is important, typically changing the collectivity of the states by a factor of about two. The results of the microscopic calculations compare well with a semiclassical pairing transition density calculated on the basis of the Thomas-Fermi approximation.     

Role of thermal treatment on the structural behavior of the arc SWCNTs in the purification system process

The role of thermal oxidation before acid treatment and post high-temperature vacuum annealing of single walled carbon nanotube (SWCNTs) was studied by scanning electron microscopy, high-resolution transmission electron microscopy, thermogravimetric analyses and Raman spectroscopy. An efficient procedure is developed for the purification of SWCNTs with minimal damage to the walls and minimal modification in the length of the tubes. SWCNTs are employed in this study, are synthesized by arc-discharge method containing impurity of amorphous carbon, carbon shells, graphitic particles, metallic nanoparticles and metallic nanoparticles encapsulated inside the amorphous carbon or graphite.     

Coulomb clusters: melting and potential barriers

The melting of two-dimensional and three-dimensional Coulomb micro- and macroclusters is studied. Temperature dependences of radial and angular square deviations of particles are investigated. The melting of microclusters has two stages: at lower temperature there is a transition from a frozen phase to a state with a rotatory reorientation of “crystalline” shells relative to each other, different pairs of shells melting at different temperatures. In the case of large N and high triangular symmetry inside the cluster, orientational melting takes place only for external pairs of shells. In this case external shells lose their order. At higher temperature a transition with a loss of radial shell order occurs. The origin of two-stage melting is in the smallness of the barrier energy relative to the rotation of shells in comparison with the barrier corresponding to the radial disordering of shells. It is shown also that the temperatures of orientational and total melting are at 5–15 times lower than the temperatures of disappearance of corresponding potential barriers. The influence of confinement anisotropy on the character of cluster melting is considered. It is found that at some degree of anisotropy the melting becomes one stage. The last is connected with an increase of the ratios of barriers of intershell rotation to barriers of jumps of a particle between the shells.     

一种制备巴基葱的新方法纳米碳化硅高温真空热分解法

实验发现，通过真空热处理碳化硅纳米粉末可以获得巴基葱.透射电子显微镜和高分辨电子显微镜观测表明，当碳化硅完全分解时，形成空心准球状颗粒和葱状石墨颗粒；当碳化硅分解不完全时，形成碳包裹碳化硅结构，该包裹层由准同心石墨壳层构成.由实验结果知，认为平面结构是石墨的最稳定形式的传统观点是值得讨论的. 关键词：     

Structure, melting, and potential barriers in mesoscopic clusters of repulsive particles

This paper discusses two-dimensional mesoscopic clusters of particles that repel according to dipole, Coulomb, and logarithmic laws and are confined by an external parabolic potential. These models describe a number of physical systems, in particular, electrons in semiconductor structures or on a liquid-helium surface allowing for image forces, indirect excitons in coupled semi-conductor dots, and a small number of vortices in an island of a second-order superconductor or in superfluid helium. Two competing forms of ordering are detected in the particles in the mesoscopic clusters-the formation of a triangular lattice or of a shell structure. The temperature dependences of the potential energy, the mean-square radial and angular deviations, the radial and angular distributions of the particles, and the distribution of the particles over the local minima are studied. Melting in mesoscopic clusters occurs in two stages: at lower temperatures, there is orientation melting, from the frozen phase into a phase with rotational reorientation of “crystalline” shells with respect to each other; subsequently, a transition occurs in which the radial order disappears. Melting in dipole macroclusters occurs in a single stage. However, in Coulomb and logarithmic macroclusters, orientation melting occurs only for the outer pairs of shells. Orientation melting is also detected in three-dimensional Coulomb clusters. A connection is established between the character of the melting and the ratio of the energy barriers that describe the breakdown of the orientational and radial structure of a cluster. Zh. éksp. Teor. Fiz. 116, 2012–2037 (December 1999)     

Coupling of ferromagnetic nanoparticles through dipolar interactions

We consider two ferromagnetic nanoparticles coupled via long-range dipolar interactions. We model each particle by a three-dimensional array of classical spin vectors, with a central spin surrounded by a variable number of shells. Within each particle only ferromagnetic coupling between nearest neighbor spins is considered. The interaction between particles is of the dipolar type and the magnetic properties of the system is studied as a function of temperature and distance between the centers of the particles. We perform Monte Carlo simulations for particles with different number of shells, and the magnetic properties are calculated via two routes concerning the dipolar contribution: one assuming a mean-field like coupling between effective magnetic moments at the center of the particles, and other one, where we take into account interactions among all the pairs of spins, one in each particle. We show that the dipolar coupling between the particles enhances the critical temperature of the system relative to the case in which the particles are very far apart. The dipolar energy between the particles is smaller when the assumption of effective magnetic moment of the particles is used in the calculations.     

轻粒子发射时间随测量角的变化

利用小相对角关联方法测量了25MeV/u⁴⁰Ar+⁹³Nb反应中关联出射的轻粒子.通过与三体弹道模型的比较,从关联函数提取了轻粒子的发射时间.结果表明,轻粒子的发射时间随粒子能量的升高而降低,从低能的约600fm/c下降至高能的约50fm/c.将此数据与⁴⁰Ar+¹⁹⁷Au反应中的发射时间进行了比较,发现随着实验室系角度的增加,提取的轻粒子的发射时间变长,并认为其原因在于随发射角度的增加,前平衡发射成份越来越少,平衡发射成份增加.     

窄振动颗粒床中的运动模式

实验研究了竖直振动情况下,窄容器中颗粒的运动模式.发现运动模式与颗粒床厚度及振动加速度有很强的依赖关系.实验表明横向尺寸较小的容器可以抑制对流卷及拱起现象.对于足够厚的颗粒床,即使振动加速度很大,颗粒床下部仍然存在着颗粒聚集态.出现聚集态时,颗粒床对容器底的冲击力是倍周期分岔的.实验表明倍周期分岔点与颗粒床厚度无关.对于较薄的颗粒床,颗粒可以是聚集态或对流卷,视颗粒尺寸而定.如果使用尺寸分布非常窄的球形颗粒,可以观察到颗粒的有序排列.出现同心的圆筒形“壳”结构,每个“壳”上的颗粒是二维六角密排列的. 关键词： 颗粒物质 倍周期分岔 颗粒聚集态 球堆积     

Mesoporous “core-shell” adsorbents and catalysts with controllable morphology

A simple geometrical model is applied to predict the thickness of mesoporous shells over monodisperse spherical particles. As an example, mesoporous Ti-silicate nearly monodisperse particles with the “core-shell” structure, synthesized via the one-pot procedure are considered. The unique features of the materials are orientation of mesopores perpendicularly to the surface of non-porous cores and uniformity of mesoporous shells structure and thickness. This allows considering these materials as interesting catalysts for partial oxidation of bulk organic molecules with hydrogen peroxide.     

Comparative study for nucleus-nucleus interactions and modified cascade evaporation model at (4.1 – 4.5) AGeV/c

The characteristics of the nucleus-nucleus collisions at high energy for the interactions of ¹H, ⁴He, ¹²C, ¹⁶O, ²²Ne, ²⁸Si and ³²S with emulsion at momentum (4.1 – 4.5) AGeV/c have been investigated. It has been found that the multiplicity distributions of the different emitted particles and their average values can be described by the modified cascade evaporation model. The model reproduces satisfactorily the multiplicity distributions of the shower, grey and black particles and the correlations between their multiplicities. It has been seen that the number of the produced shower particles increases with the increase of the projectile mass number. From the correlation between the average multiplicity of the evaporation particles and the number of the produced particles, it was found that a phase transition in the target system may occur. The calculated pseudo-rapidity distributions of the produced shower particles are typically Gaussian shaped in the mid-rapidity region and agree well with the experimental data. Also, the angular distributions of the grey and black particles have been investigated. The angular distributions of the grey particles show a universal shape independent of the type of projectile. The angular distributions of the black particles are nearly isotropic with a small asymmetry in the forward direction. The modified cascade evaporation model, reproduces the general characteristics of the nucleus-nucleus interactions and gives an explanation for the multiparticle production process.