Non-Arrhenius Behavior of Secondary Relaxation in Supercooled Liquids

Dielectric relaxation spectroscopy (1 Hz - 20 GHz) has been performed on supercooled glass-formers from the temperature of glass transition (T_g) up to that of melting. Precise measurements particularly in the frequencies of MHz-order have revealed that the temperature dependences of secondary beta-relaxation times deviate from the Arrhenius relation in well above T_g. Consequently, our results indicate that the beta-process merges into the primary alpha-mode around the melting temperature, and not at the dynamical transition point T which is approximately equal to 1.2 T_g.Comment: 4 pages, 4 figures, revtex

Dielectric and conductivity relaxation in mixtures of glycerol with LiCl

We report a thorough dielectric characterization of the alpha relaxation of glass forming glycerol with varying additions of LiCl. Nine salt concentrations from 0.1 - 20 mol% are investigated in a frequency range of 20 Hz - 3 GHz and analyzed in the dielectric loss and modulus representation. Information on the dc conductivity, the dielectric relaxation time (from the loss) and the conductivity relaxation time (from the modulus) is provided. Overall, with increasing ion concentration, a transition from reorientationally to translationally dominated behavior is observed and the translational ion dynamics and the dipolar reorientational dynamics become successively coupled. This gives rise to the prospect that by adding ions to dipolar glass formers, dielectric spectroscopy may directly couple to the translational degrees of freedom determining the glass transition, even in frequency regimes where usually strong decoupling is observed.Comment: 8 pages, 7 figure

Bulk Viscous LRS Biachi-I Universe with variable GG and decaying Λ\Lambda

The present study deals with spatially homogeneous and totally anisotropic locally rotationally symmetric (LRS) Bianchi type I cosmological model with variable $G$ and $\Lambda$ in presence of imperfect fluid. To get the deterministic model of Universe, we assume that the expansion $(\theta)$ in the model is proportional to shear $(\sigma)$. This condition leads to $A=\ell B^{n}$, where $A$,\;$B$ are metric potential. The cosmological constant $\Lambda$ is found to be decreasing function of time and it approaches a small positive value at late time which is supported by recent Supernovae Ia (SN Ia) observations. Also it is evident that the distance modulus curve of derived model matches with observations perfectly.Comment: 11 pages, 4 figures and 1 table, Accepted for publication in Astrophysics and Space Scienc

FORTE satellite constraints on ultra-high energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy (UHE) neutrino fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE neutrinos. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE neutrino shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.Comment: added a table, updated references and Figure 8, this version is submitted to Phys. Rev.

Entropy Crisis, Ideal Glass Transition and Polymer Melting: Exact Solution on a Husimi Cactus

We introduce an extension of the lattice model of melting of semiflexible polymers originally proposed by Flory. Along with a bending penalty, present in the original model and involving three sites of the lattice, we introduce an interaction energy that corresponds to the presence of a pair of parallel bonds and a second interaction energy associated with the presence of a hairpin turn. Both these new terms represent four-site interactions. The model is solved exactly on a Husimi cactus, which approximates a square lattice. We study the phase diagram of the system as a function of the energies. For a proper choice of the interaction energies, the model exhibits a first-order melting transition between a liquid and a crystalline phase. The continuation of the liquid phase below this temperature gives rise to a supercooled liquid, which turns continuously into a new low-temperature phase, called metastable liquid. This liquid-liquid transition seems to have some features that are characteristic of the critical transition predicted by the mode-coupling theory.Comment: To be published in Physical Review E, 68 (2) (2003

Low-frequency molecular relaxations in disordered solids

Motions of atoms or molecules in the glassy state of isotropic and anisotropic liquids and in the glass-like state of orientationally disordered crystals can be generally observed by dielectric and mechanical relaxation spectroscopy in the frequency range 10-4 Hz — 107 Hz. These translational and/or rotational diffusions of atoms or molecules in the otherwise rigid matrix of a glass show relaxational characteristics quite different from those associated with the diffusional motion of atoms or molecules whose freezing out causes a liquid to become macroscopically rigid at the glass transition temperature. This availability of localized configurational states (to an ensemble of atoms or molecules) also has a thermodynamic consequence, for an analysis of the heat capacity and entropy of the glassy and disorder frozen-in state of materials shows a substantial nonvibrational contribution to these thermodynamic properties. The number of atomic or molecular groups involved in such motions increases with temperature, but decreases on the physical ageing of a glass. The effect of densification on these motions during the physical ageing of a glass differs from that of compression. The existence of these localized motions is likely to be due to the presence of loosely packed regions, caused by the freezing-in of the density fluctuations at the glass transition temperature which leads to a heterogeneous structure at a molecular level in a glass

STRUCTURAL CHANGES AND MOLECULAR MOBILITY DURING PHYSICAL AGEING OF GLASSES

L'amplitude de la relaxation secondaire et la chaleur spécifique d'un grand nombre de verres décroissent au cours des traitements thermiques. Ceci suggère une élimination des régions à forte concentration de volume libre et d'entropie dans les structures. La densification correspondante affecte les propriétés cinétiques, vibrationnelles et électroniques d'un verre de manière différente de ce qui résulte d'une compression. Les régions éliminées correspondent aux centres d'effet tunnel liés au comportement des verres à basse température. Le vieillissement d'un verre fait évoluer sa structure vers celle d'un milieu élastique continu. Ces résultats suggèrent l'existence d'une microstructure hétérogène.The magnitude of secondary relaxation and heat capacity of a variety of glasses decrease on physical ageing. The decrease suggests a collapse of localized high-volume, high-entropy regions in its structure. Densification on ageing affects the kinetic, vibrational and electronic properties of a glass differently than does densification on compression. Tunneling centres responsible for the low-temperature behaviour of glasses are identified with the localized regions. Ageing causes a glass to approach a structure with properties of an isotropic elastic continuum. The results support a microscopically heterogeneous structure

The electrostatic interaction energy of water molecules in polymorphs of ice

The Coulombic interaction energy of water molecules in hexagonal and cubic ice and in ice III, IV, V and VI has been calculated. The calculations take into account nearest neighbours up to the third shell and the probability of each orientation of these neighbours in a dendritic manner. The energy is found to increase when mutual polarization of molecules is taken into account. It is highest for molecules in cubic ice and lowest in ice VI. For a polymorph of ice the energy depends significantly upon interactions of a molecule with its second and third neighbours, with the result that breaking of some H-bonds in the ices leads to the weakening of others. The interactions with the second and third neighbours increases the energy of an isolated pair by 20 % to 35 %. Implications of these results for the use of pair-additive potentials in computer simulations of water and ice and for our understanding of the mechanical deformation of ice are pointed out

The dielectric properties of dry and water-saturated nylon-12

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