J. D. Gunton

Diversity-Induced Resonance

We present conclusive evidence showing that different sources of diversity, such as those represented by quenched disorder or noise, can induce a resonant collective behavior in an ensemble of coupled bistable or excitable systems. Our analytical and numerical results show that when such systems are subjected to an external subthreshold signal, their response is optimized for an intermediate value of the diversity. These findings show that intrinsic diversity might have a constructive role and suggest that natural systems might profit from their diversity in order to optimize the response to an external stimulus.

Phase behavior of short-range square-well model

Various Monte Carlo techniques are used to determine the complete phase diagrams of the square-well model for the attractive ranges lambda = 1.15 and lambda = 1.25. The results for the latter case are in agreement with earlier Monte Carlo simulations for the fluid-fluid coexistence curve and yield new results for the liquidus-solidus lines. Our results for lambda = 1.15 are new. We find that the fluid-fluid critical point is metastable for both cases, with the case lambda = 1.25 being just below the threshold value for metastability. We compare our results with prior studies and with experimental results for the gamma(II)-crystallin.

Protein condensation : kinetic pathways to crystallization and disease

Preface 1. Introduction 2. Globular protein structure 3. Experimental methods 4. Thermodynamics and statistical mechanics 5. Protein-protein interactions 6. Theoretical studies of equilibrium 7. Nucleation theory 8. Experimental studies of nucleation 9. Lysozyme 10. Some other globular proteins 11. Membrane proteins 12. Crystallins and cataracts 13. Sickle hemoglobin and sickle cell anemia 14, Alzheimers disease Index.

Coherence Resonance in Chaotic Systems

We show that it is possible for chaotic systems to display the main features of coherence resonance. In particular, we show that a Chua model, operating in a chaotic regime and in the presence of noise, can exhibit oscillations whose regularity is optimal for some intermediate value of the noise intensity. We find that the power spectrum of the signal develops a peak at finite frequency at intermediate values of the noise. These are all signatures of coherence resonance. We also experimentally study a Chua circuit and corroborate the above simulation results. Finally, we analyze a simple model composed of two separate limit cycles which still exhibits coherence resonance, and show that its behavior is qualitatively similar to that of the chaotic Chua system.

Dynamics of phase separation in a binary polymer blend of critical composition

We report results of a detailed numerical study of the phase separation process in a three dimensional model of polymer blends. The model system considered by us is described by Flory–Huggins–de Gennes free‐energy functional. For a critical quench, we numerically integrate the corresponding time evolution equation for the conserved order parameter based on the above free‐energy functional. We study the time dependence of the characteristic domain size as well as the pair correlation function and the structure factor for several quench temperatures. The results indicate that the growth law for the characteristic domain size is given by a modified Lifshitz–Slyozov law and the growth law exponent is independent of the quench temperature. We also find that both the pair correlation function and the structure factors show dynamical scaling at late times.

Three-state lattice gas on a triangular lattice as a model for multicomponent adsorption

Abstract We consider a three-state lattice gas with nearest-neighbor interactions on a triangular lattice as a model for multicomponent chemi- and physisorption. By varying the lateral interaction constants between the asorbate particles, this model can be made to exhibit either enhanced adsorption or poisoning (inhibited adsorption). Following a ground-state calculation and a discussion of the conditions on the interaction constants that lead to enhanced adsorption and poisoning, we calculate phase diagrams and adsorption isotherms by the finite-size scaling transfer-matrix method. Detailed numerical studies of the phase diagrams are performed for two models, one that exhibits enhanced adsorption, and one that exhibits poisoning. Finally we consider a simple model for the coadsorption of hydrogen and sulphur on a platinum (111) surface, with interaction constants estimated from experimental data. The resulting adsorption isotherms are in good agreement with experimental results. We find that transfer-matrix calculations provide an efficient method for obtaining adsorption isotherms for complicated systems with a surprisingly modest computational effort.

Time‐dependent investigation of the resonant tunneling in a double‐barrier quantum well

The temporal development of the wave function within a double‐barrier quantum well is studied by numerically solving the time‐dependent Schrodinger equation. One‐dimensional AlGaAs‐GaAs‐AlGaAs and AlAs‐GaAs‐AlAs structures are considered in this study. The build‐up time and the decay constant of the resonant probability amplitude have been computed as functions of the system parameters. In general, these times are considerably different in magnitude. The amplitude of the wave function trapped inside the well is found to depend crucially on the initial energy spread. The results are in good agreement with those of a recent photoluminescence experiment.

Self-assembly of Janus Ellipsoids

We propose a primitive model of Janus ellipsoids that represents particles with an ellipsoidal core and two semisurfaces coded with dissimilar properties, for example, hydrophobicity and hydrophilicity, respectively. We investigate the effects of the aspect ratio on the self-assembly morphology and aggregation processes using Monte Carlo simulations. We also discuss certain differences between our results and those of earlier results for Janus spheres. In particular, we find that the size and structure of the aggregate can be controlled by the aspect ratio.

Structural phase transitions of the Si(100) surface

Abstract A Monte Carlo study of the structural phase transitions on the Si(100) surface for both the “(2×1)” and “c(2×2)” reconstruction families is carried out using the asymmetric dimer model. A second order phase transition is observed at about 250 K from a p(2×2) (layered antiferromagnetic) to a disordered (paramagnetic) state in the “2×1” family. A similar transition is observed about 800 K from a c(2×2) (ferromagnetic) to a disordered state in the “c(2×2)” family. These results are in agreement with real-space renormalization group results. The variation of the specific heat, susceptability and the absolute value of the order parameter as a function of temperature is obtained in the range 200 to 300 K for the “2×1” family and 750 to 850 K for the “c(2×2)” family. Also, the order parameter correlation function is computed for ten different temperatures in the above ranges.

Crystal nucleation for a model of globular proteins

A continuum model of globular proteins proposed by Talanquer and Oxtoby [J. Chem. Phys. 109, 223 (1998)] is investigated numerically, with particular emphasis on the region near the metastable fluid-fluid coexistence curve. Classical nucleation theory is shown to be invalid not only in the vicinity of the metastable critical point but also close to the liquidus line. An approximate analytic solution is also presented for the shape and properties of the nucleating crystal droplet.