PubMed Health⌕ Search

Biomedical subjects

Robert L Jack

Publications and source records attributed to Robert L Jack.

7 recordsLinked to original sources
PubMed2006

Space-time thermodynamics and subsystem observables in a kinetically constrained model of glassy materials.

In a recent article [M. Merolle et al., Proc. Natl. Acad. Sci. U.S.A. 102, 10837 (2005)], it was argued that dynamic heterogeneity in d-dimensional glass formers is a manifestation of an order-disorder phenomenon in the d+1 dimensions of space time. By considering a dynamical analog of the free energy, evidence was found for phase coexistence between active and inactive regions of space time, and it was suggested that this phenomenon underlies the glass transition. Here we develop these ideas further by investigating in detail the one-dimensional Fredrickson-Andersen (FA) model, in which the active and inactive phases originate in the reducibility of the dynamics. We illustrate the phase coexistence by considering the distributions of mesoscopic space-time observables. We show how the analogy with phase coexistence can be strengthened by breaking microscopic reversibility in the FA model, leading to a nonequilibrium theory in the directed percolation universality class.

Journal Article↗
PubMed2005

Caging and mosaic length scales in plaquette spin models of glasses.

We consider two systems of Ising spins with plaquette interactions. They are simple models of glasses which have dual representations as kinetically constrained systems. These models allow an explicit analysis using the mosaic, or entropic droplet, approach of the random first-order transition theory of the glass transition. We show that the low-temperature states of these systems resemble glassy mosaic states, despite the fact that excitations are localized and that there are no static singularities. By means of finite-size thermodynamics we study a generalized caging effect whereby the system is frozen on short length scales, but free at larger length scales. We find that the freezing length scales obtained from statics coincide with those relevant to dynamic correlations, as expected in the mosaic view. The simple nucleation arguments of the mosaic approach, however, do not give the correct relation between freezing lengths and relaxation times, as they do not capture the transition states for relaxation. We discuss how these results make a connection between the mosaic and the dynamic facilitation views of glass formers.

Journal Article↗
PubMed2005

Static and dynamic length scales in a simple glassy plaquette model.

We study static and dynamic spatial correlations in a two-dimensional spin model with four-body plaquette interactions and standard Glauber dynamics by means of analytic arguments and Monte Carlo simulations. We study in detail the dynamical behavior which becomes glassy at low temperatures, due to the emergence of effective kinetic constraints in a dual representation where spins are mapped to plaquette variables. We study the interplay between nontrivial static correlations of the spins and the dynamic "four-point" correlations usually studied in the context of supercooled liquids. We show that slow dynamics is spatially heterogeneous due to the presence of diverging length scales and scaling, as is also found in kinetically constrained models. This analogy is illustrated by a comparative study of a froth model where the kinetic constraints are imposed.

Journal Article↗
PubMed2005

Glassy behavior in an exactly solved spin system with a ferromagnetic transition.

We show that applying simple dynamical rules to Baxter's eight-vertex model leads to a system which resembles a glass-forming liquid. There are analogies with liquid, supercooled liquid, glassy, and crystalline states. The disordered phases exhibit strong dynamical heterogeneity at low temperatures, which may be described in terms of an emergent mobility field. Their dynamics are well described by a simple model with trivial thermodynamics, but an emergent kinetic constraint. We show that the (second order) thermodynamic transition to the ordered phase may be interpreted in terms of confinement of the excitations in the mobility field. We also describe the aging of disordered states toward the ordered phase, in terms of simple rate equations.

Journal Article↗
PubMed2005

Anaphylactoid reaction after verteporfin therapy.

PURPOSE: To report a case of anaphylactoid reaction in a woman after initial exposure to verteporfin and to alert physicians of this potentially life-threatening reaction. DESIGN: Interventional case report. METHODS: An 80-year-old woman who was found to have exudative age-related macular degeneration on clinical examination and fluorescein angiography underwent treatment with verteporfin photodynamic therapy (PDT). Thirty minutes after verteporfin PDT infusion, the patient experienced throat constriction, swelling of her hands, and severe shortness of breath. She received immediate intravenous treatment with methylprednisolone, diphenhydramine, and famotidine and was admitted for observation. RESULTS: The patient's symptoms resolved, and she experienced no long-term side effects related to PDT. Routine noninvasive pulse oximetry, however, did result in finger discoloration and superficial burn. Evaluation revealed that her symptoms were noncardiac in origin. CONCLUSION: Verteporfin can result in an anaphylactoid reaction. Treating physicians should be advised of possible photosensitizing reactions once the patient has received verteporfin PDT.

Aged, 80 and over↗
PubMed2004

Dissipation and tunneling in quantum Hall bilayers.

We discuss the interplay between transport and intrinsic dissipation in quantum Hall bilayers, within the framework of a simple thought experiment. We compute, for the first time, quantum corrections to the semiclassical dynamics of this system. This allows us to reinterpret tunneling measurements on these systems. We find a strong peak in the zero-temperature tunneling current that arises from the decay of Josephson-like oscillations into incoherent charge fluctuations. In the presence of an in-plane field, resonances in the tunneling current develop an asymmetric line shape.

Journal Article↗