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Register a new userPhase and glass transitions in short-range central potential model systems: The case of C60
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Extensive molecular dynamics simulations show that a short-range central potential, suited to model C60, undergoes a high temperature transition to a glassy phase characterized by the positional disorder of the constituent particles. Crystallization, melting and sublimation, which also take place during the simulation runs, are illustrated in detail. It turns out that vitrification and the mentioned phase transitions occur when the packing fraction of the system - defined in terms of an effective hard-core diameter - equals that of hard spheres at their own glass and melting transition, respectively. A close analogy also emerges between our findings and recent mode coupling theory calculations of structural arrest lines in a similar model of protein solutions. We argue that the conclusions of the present study might hold for a wide class of potentials currently employed to mimic interactions in complex fluids (some of which of biological interest), suggesting how to achieve at least qualitative predictions of vitrification and crystallization in those systems.
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We have investigated the phase diagram of the Girifalco model of C60 fullerene in the framework provided by the MHNC and the SCOZA liquid state theories, and by a Perturbation Theory (PT), for the free energy of the solid phase. We present an extended assessment of such theories as set against a recent Monte Carlo study of the same model [D. Costa et al, J. Chem. Phys. 118:304 (2003)]. We have compared the theoretical predictions with the corresponding simulation results for several thermodynamic properties. Then we have determined the phase diagram of the model, by using either the SCOZA, or the MHNC, or the PT predictions for one of the coexisting phases, and the simulation data for the other phase, in order to separately ascertain the accuracy of each theory. It turns out that the overall appearance of the phase portrait is reproduced fairly well by all theories, with remarkable accuracy as for the melting line and the solid-vapor equilibrium. The MHNC and SCOZA results for the liquid-vapor coexistence, as well as for the corresponding critical points, are quite accurate. All results are discussed in terms of the basic assumptions underlying each theory. We have selected the MHNC for the fluid and the first-order PT for the solid phase, as the most accurate tools to investigate the phase behavior of the model in terms of purely theoretical approaches. The overall results appear as a robust benchmark for further theoretical investigations on higher order C(n>60) fullerenes, as well as on other fullerene-related materials, whose description can be based on a modelization similar to that adopted in this work.
Computer simulations were used to study the gel transition occurring in colloidal systems with short range attractions. A colloid-polymer mixture was modelled and the results were compared with mode coupling theory expectations and with the results for other systems (hard spheres and Lennard Jones). The self-intermediate scattering function and the mean squared displacement were used as the main dynamical quantities. Two different colloid packing fractions have been studied. For the lower packing fraction, $alpha$-scaling holds and the wave-vector analysis of the correlation function shows that gelation is a regular non-ergodicity transition within MCT. The leading mechanism for this novel non-ergodicity transition is identified as bond formation caused by the short range attraction. The time scale and diffusion coefficient also show qualitatively the expected behaviour, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and very large correction to Gaussian behaviour found. The system with higher colloid packing fraction shows indications of a nearby high-order singularity, causing $alpha$-scaling to fail, but the general expectations for non-ergodicity transitions still hold.
We use Monte Carlo simulations to study the one-dimensional long-range diluted Heisenberg spin glass with interactions that fall as a power, sigma, of the distance. Varying the power is argued to be equivalent to varying the space dimension of a short-range model. We are therefore able to study both the mean-field and non-mean-field regimes. For one value of sigma, in the non-mean-field regime, we find evidence that the chiral glass transition temperature may be somewhat higher than the spin glass transition temperature. For the other values of sigma we see no evidence for this.
Using a microscopic numerical approach suitable to describe disordered antiferromagnets, with application to $Fe_{x}Zn_{1-x}F_{2}$, it is shown that the characteristics of the spin glass phase found for $x=0.25$ is much in agreement with the scenario predicted by the scaling theory of the droplet model.
Using confocal microscopy we investigate the effect of attraction on the packing of polydisperse emulsions under gravity. We find that the distributions of neighbors, coordination number, and local packing fraction as a function of attraction are captured by recently proposed geometrical modeling and statistical mechanics approaches to granular matter. This extends the range of applicability of these tools to polydisperse, attractive jammed packings. Furthermore, the dependence of packing density and average coordination number on the strength of attraction provides the first experimental test of a phase diagram of athermal jammed particles. The success of these theoretical frameworks in describing a new class of systems gives support to the much-debated statistical physics of jammed matter.
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