Surface tension and its temperature coefficient for liquid metals

Temperature-dependent surface tension gamma(lv)(T) and its temperature coefficient (T) [=dgamma(lv)(T)/dT] for liquid metals are thermodynamically determined on the basis of an established model for surface energy of crystals. The model predictions correspond to the available experimental or theoretical results. It is found that for metallic liquids gamma(lv)(T(m)) proportional, variant H(v)/V(m)(2/3), gamma(lv)(T) proportional, variant T, and (T) proportional, variant T over a certain temperature range (including T < T(m) and T >/= T(m)), where H(v) and V(m) are the liquid-vapor transition enthalpy at boiling temperature T(b) and the atomic volume at melting temperature T(m), respectively. Furthermore, T(m)(T(m))/gamma(lv)(T(m)) almost remains constant, which gives a way to estimates of (T(m)) values when T(m) and gamma(lv)(T(m)) are known.     

Body-centered-cubic Ni and its magnetic properties

The body-centered-cubic (bcc) phase of Ni, which does not exist in nature, has been achieved as a thin film on GaAs(001) at 170 K via molecular beam epitaxy. The bcc Ni is ferromagnetic with a Curie temperature of 456 K and possesses a magnetic moment of 0.52+/-0.08 micro(B)/atom. The cubic magnetocrystalline anisotropy of bcc Ni is determined to be +4.0x10(5) ergs x cm(-3), as opposed to -5.7x10(4) ergs x cm(-3) for the naturally occurring face-centered-cubic (fcc) Ni. This sharp contrast in the magnetic anisotropy is attributed to the different electronic band structures between bcc Ni and fcc Ni, which are determined using angle-resolved photoemission with synchrotron radiation.     

Single-beam and enhanced two-beam second-harmonic generation from silicon nanocrystals by use of spatially inhomogeneous femtosecond pulses

Optical second-harmonic generation (SHG) is used as a noninvasive probe of the interfaces of Si nanocrystals (NCs) embedded uniformly in an SiO2 matrix. Measurements of the generated SH mode verify that the second-harmonic polarization has a nonlocal dipole form proportional to (E x Delta inverted) E that depends on inhomogeneities in the incident field E, as proposed in recent models based on a locally noncentrosymmetric dipolar response averaged over the spherical NC interfaces. A two-beam SHG geometry is found to enhance this polarization greatly compared to single-beam SHG, yielding strong signals useful for scanning, spectroscopy, and real-time monitoring. This configuration provides a general strategy for enhancing the second-order nonlinear response of centrosymmetric samples, as demonstrated here for both Si nanocomposites and their glass substrates.     

Imaging of small nanoparticle-containing objects by finite-element-based photoacoustic tomography

We present semiquantitative photoacoustic images of small nanoparticle-containing objects having a wide range of contrast levels relative to the background. The images are obtained by a finite-element reconstruction algorithm that is based on the Helmholtz-like photoacoustic wave equation in the frequency domain. Our reconstruction approach is an iterative Newton method coupled with combined Marquardt and Tikhonov regularizations that can extract the spatial distribution of relative optical absorption property in heterogeneous media. We demonstrate experimental images in single- and multiple-object configurations with a circular scanning photoacoustic tomographic system. The results obtained show that millimeter-size nanoparticle-containing objects can be clearly detected in terms of position, size, and relative optical properties.     

Scaling of directed dynamical small-world networks with random responses

A dynamical model of small-world networks, with directed links which describe various correlations in social and natural phenomena, is presented. Random responses of sites to the input message are introduced to simulate real systems. The interplay of these ingredients results in the collective dynamical evolution of a spinlike variable S(t) of the whole network. The global average spreading length (s) and average spreading time (s) are found to scale as p(-alpha)ln(N with different exponents. Meanwhile, S(t) behaves in a duple scaling form for N>N(*): S approximately f(p(-beta)q(gamma)t), where p and q are rewiring and external parameters, alpha, beta, and gamma are scaling exponents, and f(t) is a universal function. Possible applications of the model are discussed.     

Two-stage aggregate formation via streams in myxobacteria

In response to adverse conditions, myxobacteria form aggregates that develop into fruiting bodies. We model myxobacteria aggregation with a lattice cell model based entirely on short-range (nonchemotactic) cell-cell interactions. Local rules result in a two-stage process of aggregation mediated by transient streams. Aggregates resemble those observed in experiment and are stable against even very large perturbations. Noise in individual cell behavior increases the effects of streams and results in larger, more stable aggregates.     

Synthesis and the magnetoresistance effect of layered-perovskite La<Subscript>2.5-x</Subscript>K<Subscript>0.5+x</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7+δ</Subscript> (0<<Emphasis Type="Italic">x</Emphasis><0.5) polycrystals

Polycrystalline layered-perovskites La_2.5-xK_0.5+x·Mn₂O₇₊ (0<x<0.5) have been prepared by a modified sol-gel method and their structures, and magnetic and electric properties have been studied. Experimental results show that these materials exhibit ferromagnetic-to-paramagnetic phase transitions at 200, 225, 235, 247, and 253 K for x=0.05, 0.15, 0.25, 0.35, and 0.45, respectively. A large deviation between the metal-insulator transition temperature (T) and the magnetic transition temperature (T_C) is observed, and a large magnetoresistance (MR) effect with /₀ of 40% at 12 kOe is obtained over a wide temperature range. These behaviors are quite different from those observed in the well-knownABO₃-type perovskite manganites. PACS 75.30.Gw; 75.30.Et; 81.20.-n     

Macrocycle-Derived Functional Xanthenes and Progress Towards Concurrent Detection of Glucose and Fructose

The detection of saccharides in biological media is of great current importance for the monitoring of disease states. We have previously reported that solutions of boronic acid-functionalized macrocycles form acyclic oligomeric materials in situ. The oligomers contain fluorescent xanthene moieties. Current efforts are aimed at modulating the spectroscopic responses of these materials for the analysis of specific sugars. We describe conditions whereby the xanthene boronic acids exhibit high colorimetric fructose selectivity. In contrast, at physiological levels selective glucose monitoring can be achieved via fluorescence. Additionally, we describe a method which exhibits promise for detecting both glucose and fructose at dual wavelengths in the UV-Vis region. Mechanistic rationale for each of these findings is presented.     

A crisis of a stochastic web

In a kicked rotor subjected to a piecewise-continuous force field, it is observed that a stochastic web and the chaotic diffusion on it suddenly change to transients when an adjustable parameter drives the dissipation. This phenomenon appears to be a new crisis type, which occurs in systems where conservative dynamics may be converted to a dissipative one with a contraction rate showing linear time dependence. It is analytically and numerically shown that, in the crisis, the lifetime dependence obeys universal scaling law suggested by Grebogy, Ott, and Yorke [Phys. Rev. Lett. 57, 1284 (1986)], and the scaling exponent takes a special value, 1, due to the dissipation characteristics. Additionally presented is another power law that describes, from another viewpoint, the transition of a conservative stochastic web (which is a fat fractal) to a non-attracting thin fractal (the strange repeller).Received: 13 December 2003, Published online: 9 March 2004PACS: 05.45.Ac Low-dimensional chaos