19 research outputs found
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
Landscape equivalent of the shoving model
It is shown that the shoving model expression for the average relaxation time
of viscous liquids follows largely from a classical "landscape" estimation of
barrier heights from curvature at energy minima. The activation energy involves
both instantaneous bulk and shear moduli, but the bulk modulus contributes less
than 8% to the temperature dependence of the activation energy. This reflects
the fact that the physics of the two models are closely related.Comment: 4 page
Public/Private Partnerships for Prescription Drug Coverage: Policy Formulation and Outcomes in Quebec's Universal Drug Insurance Program, with Comparisons to the Medicare Prescription Drug Program in the United States
Structural Dynamics of Materials Probed by X-Ray Photon Correlation Spectroscopy
In this chapter we discuss coherent X-ray scattering, photon statistics of speckle patterns, and X-ray photon correlation spectroscopy (XPCS). XPCS is a coherent X-ray scattering technique used to characterize dynamic properties of condensed matter by recording a fluctuating speckle pattern. In the experiments, the time correlation function of the scattered intensity is calculated at different momentum transfers Q and thereby detailed information about the dynamics is obtained. Recently, XPCS applications have broadened to include the study of nonequilibrium and heterogeneous dynamics, e.g., in systems close to jamming or at the glass transition. This is enabled through multi-speckle techniques where a 2D area detector (CCDs or pixel detectors) is employed, and the correlation function is evaluated by averaging over subsets of equivalent pixels (same Q ). In this manner time averaging can be avoided, and the time-dependent dynamics is quantified by the so-called two-times correlation functions. Higher-order correlation functions may also be calculated to investigate questions related to non-Gaussian dynamics and dynamical heterogeneity. We discuss recent forefront applications of XPCS in the study of soft and hard condensed matter dynamics, including phase-separation dynamics of colloid-polymer mixtures, motion of Au nanoparticles at the air-water interface, dynamics of atoms in metallic crystals and glasses, and domain coarsening in phase-ordering binary alloys