High pressure electrical transport behavior in SrF_2 nanoplates

The charge transport behavior of strontium fluoride nanocrystals has been investigated by in situ impedance measurement up to 35 GPa.It was found that the parameters changed discontinuously at each phase transition.The charge carriers in SrF_2 nanocrystals include both F~-ions and electrons.In the Fm3 m phase,pressure makes the electronic transport easier,while makes it more difficult in the Pnma phase.The defects at grain boundaries dominate the electronic transport process.Pressure could make the charge-discharge processes in the Fm3 m phase much easier,but make it more difficult in the Pnma phase.     

Energy transfer studies in krypton-xenon mixtures excited in a cooled DC discharge

The VUV spectrum of gaseous mixtures of krypton with a small amount of xenon added was investigated in the range 115-200 nm. The mixtures were excited in a capillary DC discharge where the capillary could be cooled by using liquid nitrogen. The mixed molecule band around the XeI resonance line at nm and the mixed molecule continuum to the long wavelength side from the line were analysed. The band around nm was identified as transitions between a weakly bound excited state and the weakly bound ground state of XeKr molecules. When cooling the capillary wall, the appearance of the Xe₂ continuum was observed. The effect is ascribed to energy transfer between molecular states as a consequence of radiation trapping in the band around nm. The role of the mixed molecule in the formation of the VUV spectrum of the gas mixture is discussed and underlined. Received 12 May 1999 and Received in final form 27 August 1999     

A variational approach to a quasi-one-dimensional blume-emery-griffiths model

Summary The system considered consists of spin-1 objects arranged on a three-dimensional lattice. It is characterized by an anisotropic Blume-Emery-Griffiths Hamiltonian in which the dipolar and quadrupolar intrachain coupling constants are much stronger than the corresponding interchain coupling constants so that the system can be considered quasi-one-dimensional. The method of analysis utilizes first the exact solution of the one-dimensional case and then the Bogolubov variational principle. The critical properties are examined and the phase diagram derived.

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Riassunto Il sistema considerato consiste di oggetti a spin-1 disposti su un reticolo tridimensionale ed è caratterizzato da una Hamiltoniana anisotropa in cui le costanti dipolari e quadrupolari di accoppiamento all’interno delle catene sono molto maggiori delle corrispondenti costanti di accoppiamento fra catene, cosicché il sistema può essere considerato quasi unidimensionale. Il metodo di analisi impiegato utilizza anzitutto la soluzione esatta del caso unidimensionale e successivamente il principio variazionale di Bogolubov. Si esaminano le proprietà critiche e si ricava il diagramma di fase.

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Резюме Рассматривается система, которая состоит из объектов со спином 1, образующих трехмерную решетку. Эта система характеризуется Гамильтонианом Ълуме-Эмери-Гриффитса, в котором дипольные и квадрапольные внутрицепочечные постоянные связи являются более сильными, чем соответствующие постоянные связи между цепочками, так что система может быть рассмотрена, как квази-одномерная. Метод анализа использует сначала точное решение для одномерного случая, а затем вариационный принцип Ъоголюбова. Исслеледуются критические свойства и выводится фазовая диаграмма.

     

Criticality and phase transitions in ionic fluids

Recent experimental investigations of criticality and phase separation in ionic fluids have revealed behavior of great theoretical interest. In seeking to understand the experiments, some of which appear to exhibit argonlike criticality and some of which exhibit classical (mean-field) criticality, a convenient starting point is the restricted primitive model (RPM) of symmetrically charged hard spheres, all of equal diameter , each sphere bearing a positive or negative charge of magnitudeq. There is overall charge neutrality, so that the expected number densities of the anions and cations are equal, ₊= _-. Studies of RPM charge-charge and density-density correlation functions indicate that the fluctuation-suppressing mechanism that yields mean-field critical behavior in nonionic systems with long-range interparticle potentials is not operative in the RPM. On the basis of plausible assumptions, Ising-like behavior is instead expected. The above work is summarized. New work of Zhang and the author is outlined, showing that when one loses the RPM symmetry (through, e.g., different valence, diameter, or dipole moment of anions and cations) a strong coupling between charge-charge and density-density correlation ensues. The way in which this can be expected to give rise to mean-field or mean-field-like behavior is noted. Other new observations concern the mean-field analogy found by Høye and the author between the parameter 2/(d–2) (d is the dimensionality) in that model and the monomer number in high polymers, with respect to the coexistence-curve shape dependence on those parameters.     

Optical characterization and manipulation of alkali metal nanoparticles in porous silica

Rubidium and cesium metal nanoparticles were grown in nanoporous silica samples placed in alkali vapor cells. Their size and shape were investigated by measuring the sample optical transmittance. Spectral changes due to photodesorption processes activated by weak light were also analyzed. Alkali atoms photoejected from the silica walls diffuse through and out of the nanopores, modifying both the nanoparticle distribution in the silica matrix and the atomic vapor pressure in the cell volume. The number of rubidium and cesium atoms burst out of the samples was measured as a function of photon energy and fluence. The optical absorption measurements together with the analysis of the photodesorption yield give a complete picture of the processes triggered by light inside the nanopores. We show that atomic photodesorption, upon proper choice of light frequency and intensity, induces either growth or evaporation of nanosized alkali metal clusters. Cluster size and shape are determined by the host-guest interaction.     

Electronic transitions of fluorene, dibenzofuran, carbazole, and dibenzothiophene: From the onset of absorption to the ionization threshold

A comparative study of the electronic transitions of fluorene and its hetero-analogues dibenzofuran, carbazole, and dibenzothiophene was performed in a wide energy range. Gas phase, crystal phase, and linear dichroism electronic transmittance spectra were measured with synchrotron radiation. Electronic transitions to excited singlet states were predicted with time-dependent density functional theory, TD-B3LYP/6-31+G(d,p). Based on the experimental and theoretical results, symmetry assignments of electronic transitions in the vacuum and near-UV region are suggested. The correspondence between excited states in these molecules, similarities, and differences between their electronic spectra are discussed.     

Dielectric, thermal and optical study of an unusually shaped liquid crystal

Dielectric measurements have been carried out for the determination of real and imaginary parts of the permittivity of a newly synthesized, unusually shaped liquid crystal. The sample has been investigated in the frequency range from 100 Hz to 10 MHz within a temperature range 80-130 °C. The dielectric measurements in the smectic A phase indicate a Cole-Cole type of dispersion, and the activation energy was found to be 5.5 meV by using the Arrhenius plot of relaxation time. In addition to this, thermal and optical transmittance studies have also been conducted in the above mentioned temperature range, and the temperature dependence of these parameters has been discussed in detail. The phase transition temperature obtained from a differential scanning calorimetry (DSC) study matches within 2 °C that was obtained from an optical transmittance study. The dielectric and optical behavior of the unusually shaped liquid crystal has been explained on the basis of a proposed theoretical model in which a sample possesses two different conformers having induced polarizations in opposite directions.     

Nonlinear hydrodynamic phenomena in Stokes flow regime

We investigate nonlinear phenomena in dispersed two-phase systems under creeping-flow conditions. We consider nonlinear evolution of a single deformed drop and collective dynamics of arrays of hydrodynamically coupled particles. To explore physical mechanisms of system instabilities, chaotic drop evolution, and structural transitions in particle arrays we use simple models, such as small-deformation equations and effective-medium theory. We find numerical and analytical solutions of the simplified governing equations. The small-deformation equations for drop dynamics are analyzed using results of dynamical systems theory. Our investigations shed new light on the dynamics of complex fluids, where the nonlinearity often stems from the evolving boundary conditions in Stokes flow.