Size-effects in clusters and free nanoparticles probed by soft X-rays

Recent progress from experiments on clusters and free nanoparticles is reported, where emphasis is put on studies in the soft X-ray regime. We review selected examples on the characterization of free molecular clusters and nanoparticles. Specifically, we discuss recent progress in changes of the local structure in free clusters. Photoemission and particle charging as well as elastic light scattering from free nanoparticles are reported. These approaches permit the characterization of the intrinsic properties of nanoscopic systems as building blocks of nanoscopic matter and possible nanomaterials.     

Gaussian random field p-spin-interaction ising model in a transverse field

The Gaussian random field Ising model with p-spin interactions in the presence of a transverse field is studied by combining the Suzuki-Trotter approach and the thermodynamic perturbation theory. The first-order phase transitions are found in the limit p → ∞, in contrast to the cases with p=2.     

Size classification of Si nanoparticles formed by pulsed laser ablation in helium background gas

We have developed an integrated process system for the formation of nanoparticles by pulsed laser ablation (PLA) in helium background gas, size classification using a differential mobility analyzer (DMA), and deposition on a substrate. The DMA has been improved to operate at pressures of less than 10 Torr. The classification resolution of the low-pressure operating DMA (LP-DMA), transporting properties of nanoparticles under low pressure, have been investigated theoretically in order to evaluate the performance of the size classification for the integrated system. By operating the integrated system at less than 10 Torr, we have measured the size distribution of Si nanoparticles in the gas phase formation field by sweeping the applied voltage to the LP-DMA and counting the charged nanoparticle concentration with an electrometer. Moreover, we successfully deposited the classified Si nanoparticles on a substrate by fixing the voltage. We have verified that the integrated system can be applied to the clean physical vapor deposition process for accurately size-controlled nanoparticles.     

Percolation in strongly correlated systems: The massless Gaussian field

We derive a number of new results for correlated nearest neighbor site percolation onZ ^d. We show in particular that in three dimensions the strongly correlated massless harmonic crystal, i.e., the Gaussian random field with mean zero and covariance –, has a nontrivial percolation behavior: sites on whichS _x h percolate if and only ifh _c . with0 _c < . This provides the first rigorous example of a percolation transition in a system with infinite susceptibility.     

高斯相关随机表面光散射散斑场相位奇异及其特性的理论研究

通过对二维高斯相关随机表面在远场平面上产生的散斑场及其相位的计算模拟, 发现在某一平面上除了实部零值线与虚部零值线有传统相交之外, 还有相切和重合的情况.切点和重合线也可以形成相位奇异, 并且其周围相位分布与传统的实部零值线与虚部零值线相交形成的奇点周围相位的螺旋变化不同, 呈现出对称性和不连续性的特征. 随着光波的传播, 在不同的观察面上散斑场复振幅的实部零值线和虚部零值线的相对位置经历了由相切到重合再到相交的演变过程, 相位奇异现象也随之发生变化. 关键词： 相位奇异 相位跃变 散斑场     

Raman spectroscopy of small para-H2 clusters formed in cryogenic free jets

Small para-H2 clusters (pH2)N with N=2,...,8 have been identified by Raman spectroscopy in cryogenic free jets of the pure gas, near the Q(0) Raman line of the H2 monomer. The high resolution in space, time, and number size makes it possible to follow their growth kinetics with distance from the orifice. At lower source temperatures liquid clusters appear early in the expansion and then undergo a gradual phase transition to the solid state. The technique is very promising for exploring superfluidity in pure (pH2)N clusters.     

Anomalous magnetism of small metallic clusters in a weak magnetic field

Anomalous diamagnetism in small magnetic fields may be observed in systems consisting of size-quantized metallic clusters. With the increase of magnetic field the diamagnetic susceptibility decreases until ordinary values χ₀ ～ 10^-5 -10^-6. The conditions for the existence of anomalous paramagnetism and orbital ferromagnetism in such systems are also discussed. An increase of the diamagnetic susceptibility in anisotropic semiconductors due to impurities is predicted.     

Generalization of the Zhukovsky function to the electric field of gratings formed by tilted metallic strips

Analytic expressions describing the electrostatic field of a shutter grating are obtained by conformal mapping of a domain containing the shutter grating onto a domain containing a grating of cylindrical rods. These expressions are applicable for optimizing devices used for determining the parameters of charged particle flows and for analyzing the effects associated with focusing of particles to some thermonuclear targets and with the grid confinement of the plasma.     

Spectral-luminescent properties of silver molecular clusters and nanoparticles formed by ion exchange in antimony-doped photo-thermo-refractive glasses

Optics and Spectroscopy - The formation of silver molecular clusters and nanoparticles in photo-thermo-refractive (PTR) glasses with different antimony contents has been investigated using ion...     

离轴平顶高斯涡旋光束形成的合成光涡旋及其在自由空间中的传输

对两束拓扑电荷m_1,2=±1的平行、离轴平顶高斯涡旋光束在束腰面叠加形成的合成光涡旋及其在自由空间的传输做了研究.详细的数值计算和分析表明,合光涡旋的位置、数目和净拓扑电荷与光束的控制参数,包括相对位相,振幅比,束腰宽度比,相对离轴参数,光束阶数,以及传输距离有关,但拓扑电荷不总是守恒. 关键词： 奇点光学 合成光涡旋 平顶高斯涡旋光束 拓扑电荷     

On the decay of hot metallic clusters by evaporation

Starting from the Weisskopf theory decay rates for the evaporation of cluster atoms from hot liquid alkali metal clusters are derived. The crucial input quantity is the level density which is determined from empirical properties of the bulk, namely from the specific heat and the thermal expansion coefficient. The resulting rate expression is compared with decay rate formulas given by Engelking, Klots and Gspann. Furthermore, critical (appearance) sizes of multiply charged clusters are calculated by equating the rates for neutral monomer and light charged particle emission. Also shrinking and cooling rates of large hot clusters are determined by treating multiple emission of cluster atoms, thus establishing a time scale for the decay of clusters theoretically.     

Random field effects on the anisotropic quantum Heisenberg model with Gaussian random magnetic field distribution

The effect of a zero-centered Gaussian random magnetic field distribution on the phase transition properties of the anisotropic quantum Heisenberg model has been investigated on a honeycomb lattice within the framework of effective field theory (EFT) for a two-spin cluster (which is abbreviated as EFT-2). Particular attention has been devoted to investigation of the effect of the anisotropy in the exchange interaction on a system with Gaussian random magnetic field distribution. The variation of the critical temperature with the randomness parameter (i.e., the width of the distribution) has been obtained for several anisotropy parameters. Critical Gaussian distribution width values, which make the critical temperature zero, have been obtained. Moreover, it has been concluded that all critical temperatures are of second order, and that reentrant behavior does not exist in the phase diagrams.     

Analysis of the ferromagnetic resonance spectra of aggregates of magnetite nanoparticles formed by a magnetic field

The ferromagnetic resonance (FMR) spectra of magnetite nanoparticles in aqueous solutions and solid polymer films were analyzed at different particle concentrations, matrix rigidities, temperatures, external magnetic effects, and positions of flat samples in the field of the spectrometer. The formation of linear aggregates of nanoparticles under the influence of magnetic fields is the major factor that changes the FMR spectrum shape and position. The results were analyzed in terms of phenomenological theory of FMR. The applicability of the equations of phenomenological theory was verified, and the fraction of nanoparticles in linear aggregates was evaluated.     

Network formed by traces of random walks

We propose a model of time evolving networks in which a kind of transport between vertices generates new edges in the graph. We call the model “Network formed by traces of random walks”, because the transports are represented abstractly by random walks. Our numerical calculations yield several important properties observed commonly in complex networks, although the graph at initial time is only a one-dimensional lattice. For example, the distribution of vertex degree exhibits various behaviors such as exponential, power law like, and bi-modal distribution according to change of probability of extinction of edges. Another property such as strong clustering structure and small mean vertex–vertex distance can also be found. The transports represented by random walks in a framework of strong links between regular lattice is a new mechanisms which yields biased acquisition of links for vertices.     

Uniform growth of clusters of magnetic nanoparticles in a rotating magnetic field

The suitability of silicon nanoparticles of 1?nm in diameter for fluorescent sensing was investigated. Silicon nanoparticles were produced in a cluster beam and co-deposited with a beam of vapourised liquids (water, ethanol, isopropanol) onto a cold substrate. Melting of the frozen cluster-ice mixture yielded an aqueous suspension which emitted strong fluorescence in the deep blue spectral range when exposed to UV light. The fluorescence wavelength of the strongest peak was found to correlate with the dipole moment of the solvent molecules which allowed us to derive the transition energy for an isolated nanoparticle. The strong solvent sensitivity showed that the fluorescence originated from a surface state. A second fluorescence peak showed almost no sensitivity to different solvents, hence the peak was attributed to a transition within the bulk?Cvolume of the nanoparticles. Our findings establish that silicon nanoparticles may serve as highly specific bio-sensors in living organism.     

An approximation for solving the non-LTE line transfer in a spatially correlated random velocity field

We solve the non-LTE line transfer problem in the presence of random, non-thermal velocities. The correlation length of the turbulence is taken into account and thus we fill the gap between the microturbulence (vanishing correlation length) and the macroturbulence (infinite correlation length). The medium is assumed to be divided into uncorrelated rigid cells. The essence of the approximation we introduce is the following: once a photon has been absorbed in a cell, we assume that it forgets the cells previously crossed. This permits us to replace the initial cells by “effective cells” and to work only with the averaged intensity of the radiation field. We show that this “effective cells approximation” is extremely well adapted to the method of addition of layers. Illustrative numerical examples are given for two-level atoms in finite slabs with two directions of propagation. The results are easily described as being a continuous sequence from the macroturbulent case up to the microturbulent case when the number of turbulent eddies increases. The transitions from one case to the other involve the consideration of three scales: the eddy size, the thermalization length and the mean free path of a photon at the center of the line.     

Translational free random walk of spins in the presence of a parabolic magnetic field

The free random walk approach has been used to analyze the attenuation of the NMR signal due to spin dephasing in the presence of a constant and pulsed parabolic magnetic field. The spin echo sequence was chosen to examine the attenuation of the NMR signal resulting from self-diffusion. In the framework of the gaussian approach, the long-time limit calculations predict more pronounced diffusion weighting for the parabolic field than for linear magnetic field. Analytical results were obtained and compared with those from other approaches based on a variety of different of approximations.     

Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by different methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of films deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel films with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed films, because of the interaction between clusters. Using an effective medium model, it is demonstrated that the random dispersion of clusters in implantation films causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed films induces a screening effect between the clusters of largest size. Received 10 January 2002 / Received in final form 26 April 2002 Published online 19 July 2002