Unusual temperature dependence of the positron lifetime in a polymer of intrinsic microporosity

The performance of polymeric membranes for gas separation is mainly determined by the free volume. Polymers of intrinsic microporosity are interesting due to the high abundance of accessible free volume. We performed measurements of the temperature dependence of the positron lifetime, generally accepted for investigation of free volume, in two polymers of intrinsic microporosity (PIM‐1 and PIM‐7) in the range from 143 to 523 K. The mean value of the free volume calculated from the ortho‐positronium lifetime is in the range of typical values for high free volume polymers. However, the temperature dependence of the local free volume is non‐monotonous in contrast to the macroscopic thermal expansion. The explanation is linked to the spirocenters in the polymer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discrete moving breather collisions in a Klein-Gordon chain of oscillators

We study the collisions of moving breathers with the same frequency, traveling with opposite directions within a Klein-Gordon chain of oscillators. Two types of collisions have been analyzed: symmetric and non-symmetric, head-on collisions. For low enough frequency the outcome is strongly dependent of the dynamical states of the two colliding breathers just before the collision. For symmetric collisions, several results can be observed: breather generation, with the formation of a trapped breather and two new moving breathers; breather reflection; generation of two new moving breathers; and breather fusion bringing about a trapped breather. For non-symmetric collisions some possible results are: breather generation, with the formation of three new moving breathers; breather fusion, originating a new moving breather; breather trapping with breather reflection; generation of two new moving breathers; and two new moving breathers traveling as a bound state. Breather annihilation has never been observed.     

The scientific contributions of Karl Frederick Canter (1944-2006)

The purpose of this paper is to remember as many good qualities of our friend Karl Canter as I can, as an antidote to the loss of such a magnanimous, generous, inventive and amusing person. The paper endeavours to distill the essence in simple terms of some of Karl's important scientific contributions and say what I know of how these accomplishments were made possible and molded by his various character traits, illustrated with the few anecdotes that I remember.     

On the dynamics of exotic matter: Towards creation of Perpetuum Mobile of third kind

The one-dimensional dynamics of a classical ideal ‘exotic’ fluid with equation of state p=p(?)<0$p=p(\mathrm{?})<0$ violating the weak energy condition is discussed. Under certain assumptions it is shown that the well-known Hwa–Bjorken exact solution of one-dimensional relativistic hydrodynamics is confined within the future/past light cone. It is also demonstrated that the total energy of such a solution is equal to zero and that there are regions within the light cone with negative (−)$(-)$ and positive (+)$(+)$ total energies. For certain equations of state there is a continuous energy transfer from the (−)$(-)$-regions to the (+)$(+)$-regions resulting in indefinite growth of energy in the (+)$(+)$-regions with time, which may be interpreted as action of a specific ‘Perpetuum Mobile’ (Perpetuum Motion). It is speculated that if it is possible to construct a three-dimensional non-stationary flow of an exotic fluid having a finite negative value of energy such a situation would also occur. Such a flow may continuously transfer positive energy to gravitational waves, resulting in a runaway. It is conjectured that theories plagued by such solutions should be discarded as inherently unstable.     

The effect of competition between the frequency of the field and the frequency of the spin flipping on the kinetics of Ising metamagnet

We consider the effect of a variable representing the competition between the frequency of the field and the frequency of the spin flipping (Ω) on the dynamics of the metamagnetic Ising model in a cubic lattice under the presence of a time varying (oscillating) external magnetic field. The system is modelled with a formalism of master equation at a mean-field level. The time averaged staggered magnetization (Ms) acts as the order parameter and divides temperature field plane into three regions: anti-ferromagnetic, paramagnetic and coexistence of anti-ferromagnetic and paramagnetic phases. It is observed that the topology of the dynamical phase diagram depends strongly on Ω as well as the ratio between interlayer and intralayer couplings.     

The effect of randomly oriented anisotropy on the zero-field-cooled magnetization of a non-interacting magnetic nanoparticle assembly

A model based on localized partition function and master equation was set up to calculate the zero-field-cooled (ZFC) and field-cooled (FC) curves of a non-interacting magnetic nanoparticle assembly with randomly oriented anisotropy. The peak temperature of the ZFC curve corresponds to the highest energy barrier that acts against the unblocking process, and could be described well by an equation covering the heating rate effect. The predicted H^2/3 field dependence of the peak temperature is in good agreement with published results.     

Approach to the rotation driven vibrational to axially rotational shape phase transition along the yrast line of a nucleus

By analyzing the potential energy surface, the shape phase diagram and the energy spectrum of the nucleus in U(5)$\mathrm{U}(5)$ symmetry in the IBM, we propose that the U(5)$\mathrm{U}(5)$ symmetry with parameters (A+B)<0$(A+B)<0$ may be a model to describe the rotation driven vibrational to axially rotational phase transition along the yrast line. With such a model, we have described successfully the observed rotation driven shape phase transition along the yrast line and proposed some other empirical evidences.     

Temperature Dependence of Thermal Conductivity of Nanofluids

Mechanism of thermal conductivity of nanofluids is analysed and calculated, including Brownian motion effects, particle agglomeration and viscosity, together influenced by temperature. The results show that only Brown- Jan motion as reported is not enough to describe the temperature dependence of the thermal conductivity of nanofluids. The change of particle agglomeration and viscosity with temperature are also important factors. As temperature increases, the reduction of the particle surface energy would decrease the agglomeration of nanopartieles, and the reduction of viscosity would improve the Brownish motion. The results egree well with the experimental data reported.     

Valence transition behavior of the doped Falicov-Kimball model at nonzero temperatures

The extrapolation of small-cluster exact-diagonalization calculations is used to study the influence of doping on valence transitions in the spinless Falicov-Kimball model at nonzero temperatures. Two types of doping are examined, and namely, the substitution of rare-earth ions by nonmagnetic ions that introduce (i) one or (ii) none additional electron (per nonmagnetic ion) into the conduction band. It is found that the first type of substitution increases the average f-state occupancy of rare-earth ions, whereas the second type of substitution has the opposite effect. The results obtained are used to describe valence transition behavior of samarium in the hexaboride solid solutions Sm_1−xM_xB₆ (M=Y³⁺, La³⁺, Sr²⁺, Yb²⁺) and a very good agreement of theoretical and experimental results is found.     

Electronic and magnetic properties of CeAl2 nanoparticles

We presented the X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) studies of heavy fermion compound CeAl₂ bulk and 8 nm nanoparticles, performed at the Ce M_4,5- and L₃- absorption edges. XMCD and XAS revealed that Ce in bulk CeAl₂ exhibits localized 4f¹ character with magnetic ordering. The Ce in nanoparticles, on the other hand, shows a small amount delocalized 4f⁰ character with non-magnetic Kondo behavior. By applying general sum rules, an estimation of the orbital and spin contribution to those Ce 4f moments can be obtained. Our results also demonstrated that the magnetic behavior in CeAl₂ is very sensitive to the degree of localization of the 4f electrons.     

Fully Developed Convective Heat Transfer of Power Law Fluids in a Circular Tube

We present a theoretical analysis for fully developed convective beat transfer in a circular tube for power law fluids by assuming that the thermal diffusivity is a function of temperature gradient. The analytical eolution is obtained and the heat transfer behaviour is investigated under a constant heat flux boundary condition. It is shown that the Nusselt number strongly depends on the value of power law index n. The Nusselt number sharply decreases in the range of 0 〈 n 〈 0.1. However, for n 〉 0.5, the Nusselt number decreases monotonically with the increasing n, and for n 〉 20, the values of Nusselt number approach a constant.     

Critical behavior of sound attenuation in a metamagnetic Ising system

In this study, the sound attenuation coefficient of a spin- metamagnetic Ising system is calculated by the method of thermodynamics of irreversible processes. The behavior of sound attenuation near the phase transition temperatures is analyzed according to various values of phenomenological rate coefficients (γij). For all γm and γs values it is found that sound attenuation peaks occur below TN(H) and depend on frequency ω and the value of the off-diagonal rate coefficient γ. On the other hand, the critical behavior of the sound attenuation in the hydrodynamic regime is obtained analytically via the critical exponents. Moreover, the behavior of the sound attenuation as a function of frequency is also investigated and ω² dependence is observed for the attenuation coefficient. These results are in a good agreement with ultrasonic investigations of magnetic systems.     

Effects of dynamic response time in an evolving market

Considering the time scales of global information and personal reaction, we study the role of dynamic response time in the evolution of collective behavior in an evolving market. The insensitiveness to the market information makes the population cluster around a kind of extreme behavior, in which they always follow what happened last time, while a sensitive population tends to self-segregate into opposing groups. The average success rate R among all the agents is found to have a nonmonotonic dependence on the time scale parameter q. There exists a critical value below (above) which R decreases (increases) with the rise of q. Theoretical analyses demonstrate that the higher efficiency found in the market with an insensitive population is related to its predictability. In such a predictable market, the agents following current prediction have a higher winning probability than those rejecting it. Analytical calculations are in good agreement with numerical data.     

In Vivo Monitoring of Neovascularization in Tumour Angiogenesis by Photoacoustic Tomography

Photoacoustic tomography （PAT） is presented to in vivo monitor neovascularization in tumour angiogenesis with high resolution and high contrast images in a rat. With a circular scan system, the photoacoustic signal, generated by laser pulses at a wavelength of 532nm from a Q-switched Nd：YAG laser is captured by a hydrophone with a diameter of 1 mm and a sensitivity of 850nV/Pa. The vascular structure around the rat tumour is imaged clearly, with optimal contrast, because blood has strong absorption near this wavelength. Serial noninvasive photoacoustic images of neovascularization in tumour angiogenesis are also obtained consecutively from a growing tumour implanted under the skin of a rat over a period of two weeks. This work demonstrates that PAT can potentially provide a powerful tool for tumour angiogenesis detection in cancer research. It will bring us closer to clinical applications for tumour diagnosis and treatment monitoring.     

Detection of Isotopes of Mercury Ions by Resonant Ejection in Paul Trap

A simple method to detect mercury ions confined in a Paul trap has been developed by resonant ejection. In this method, frequency of the additional ejection ac voltage is scanned instead of the amplitude of the rf drive voltage in conventional methods. It is possible not only to observe the spectra of the secular oscillation of the trapped ions directly, but also to eject the confined ions from the trap mass-selectively.     

Thermal stability of nonlinear structures: A Gibbs analysis of the semiclassical system

With the help of an approach based on an analytic study of the partition function and related observable expectation values, we address the problem of the thermal stability of nonlinear structures. We develop an useful analogy with magnetic systems, present explicit calculations for a simple case, and address the existing debate in the literature concerning whether thermal interaction aids or hampers the existence of selftrapped states. An unambiguous demonstration of selftrapping being aided in one regime but hampered in another emerges from our analysis.Work supported in part by the NATO Scientific Affairs Division     

Linearization of Systems of Nonlinear Diffusion Equations

We investigate the linearization of systems of n-component nonlinear diffusion equations; such systems have physical applications in soil science, mathematical biology and invariant curve flows. Equivalence transformations of their auxiliary systems are used to identify the systems that can be linearized. We also provide several examples of systems with two-component equations, and show how to linearize them by nonlocal mappings.     

Topological Properties of Urban Public Traffic Networks in Chinese Top-Ten Biggest Cities

We investigate the topological characteristics of complex networks as exemplified by the urban public traffic network （UPTN） in Chinese top-ten biggest cities. It is found that the UPTNs have small world behaviour, by the examination of their topological parameters. The quantitative analysis of the transport efficiency of the UPTNs reveals their higher local efficiency El and lower global efficiency Eg, which coincide well with the status quo of those Chinese cities still at their developing stage. Furthermore, the topological properties of efficiency in the UPTNs are also examined, and the findings indicate that, on the one hand, the UPTNs show robustness to random attacks and frangibility to malicious attacks on a global scale; on the other hand, the interrelation between UPTN efficiency and network motifs deserves our attention. The motifs which interrelate the UPTN efficiency are always triangular-formed patterns, e.g. motifs ID 238, ID 174 and ID 102, etc.     

Modelling of Phase Change Heat Transfer System for Micro-channel and Chaos Simulation

The dynamic properties for the micro-channel phase change heat transfer system are studied by theoretical method combined with experiment. Liquid-vapour interface dynamic systems are obtained by introducing disjoining pressure produced by three phase molecular interactions and Lie algebra analysis. Experiments for 0.6 mm×2 mm rectangular micro-channel are carried out to obtain the pressure time serials. Power spectrum density analysis for these serials shows that the system is in chaotic state if the frequency is above 7.39 Hz. The result indicates that the high heat transfer performance of the micro channel phase change system may relate to the characteristics of chaos, The chaos attractor is drawn by the simulation of the obtained differential dynamic system under the conditions of our experiment.