Heterogeneity in structurally arrested hard spheres

When cooled or compressed sufficiently rapidly, a liquid vitrifies into a glassy amorphous state. Vitrification in a dense liquid is associated with jamming of the particles. For hard spheres, the density and degree of order in the final structure depend on the compression rate: simple intuition suggests, and previous computer simulation demonstrates, that slower compression results in states that are both denser and more ordered. In this work, we use the Lubachevsky-Stillinger algorithm to generate a sequence of structurally arrested hard-sphere states by varying the compression rate. We find that while the degree of order, as measured by both bond-orientation and translation order parameters, increases monotonically with decreasing compression rate, the density of the arrested state first increases, then decreases, then increases again, as the compression rate decreases, showing a minimum at an intermediate compression rate. Examination of the distribution of the local order parameters and the distribution of the root-mean-square fluctuation of the particle positions, as well as direct visual inspection of the arrested structures, reveal that they are structurally heterogeneous, consisting of disordered, amorphous regions and locally ordered crystal-like domains. In particular, the low-density arrested states correspond with many interconnected small crystal clusters that form a polycrystalline network interspersed in an amorphous background, suggesting that jamming by the domains may be an important mechanism for these states

Shear viscosity of neutron-rich nucleonic matter near its liquid-gas phase transition

Within a relaxation time approach using free nucleon-nucleon cross sections modified by the in-medium nucleon masses that are determined from an isospin- and momentum-dependent effective nucleon-nucleon interaction, we investigate the specific shear viscosity ($\eta/s$) of neutron-rich nucleonic matter near its liquid-gas phase transition. It is found that as the nucleonic matter is heated at fixed pressure or compressed at fixed temperature, its specific shear viscosity shows a valley shape in the temperature or density dependence, with the minimum located at the boundary of the phase transition. Moreover, the value of $\eta/s$ drops suddenly at the first-order liquid-gas phase transition temperature, reaching as low as $4\sim5$ times the KSS bound of $\hbar/4\pi$. However, it varies smoothly for the second-order liquid-gas phase transition. Effects of the isospin degree of freedom and the nuclear symmetry energy on the value of $\eta/s$ are also discussed.Comment: 6 pages, 5 figure

Energy dependence of pion in-medium effects on \pi^-/\pi^+ ratio in heavy-ion collisions

Within the framework of a thermal model with its parameters fitted to the results from an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we study the pion in-medium effect on the charged-pion ratio in heavy-ion collisions at various energies. We find that due to the cancellation between the effects from pion-nucleon s-wave and p-wave interactions in nuclear medium, the \pi^-/\pi^+ ratio generally decreases after including the pion in-medium effect. The effect is larger at lower collision energies as a result of narrower pion spectral functions at lower temperatures.Comment: 4 pages, 4 figures, 1 table, minor modifications, version to appear in Physical Review

The ultrastructural study situation of aging brain nerve tissue

本文简述了衰老脑组织的样本来源、取样部位及意义，重点阐述了电镜下衰老脑组织神经元及神经胶质细胞的退行性改变。脑衰老时神经元胞体的退变集中体现在细胞器、细胞核、染色质、细胞骨架等的形态结构改变之上，如线粒体的肿胀、脊和膜消失，出现空泡样变，粗面内质网扩张、脱颗粒，微丝、微管溶解数目减少，染色质凝固，核膜皱缩等；胞突与细胞体是相连通的，关系十分密切，因而上述细胞体所见病理变化有时在树突、轴突和突触亦可见到。神经元在退化的同时具有一定的可塑能力，但这一能力随脑衰老的进展而减退。胶质细胞在脑衰老时主要出现代偿性增生及上述失代偿性退变两种形态结构的改变。如何增强胶质细胞的代偿修复能力，以及如何促进树突棘及突触的再发育，改善神经元的可塑性，成为延缓脑衰老的研究热点。In this paper, the sample source, sampling location and significance of aging brain tissue were introduced, and the degeneration of neurons and glial cells in aging brain were reviewed. Aging brain degeneration of neuronal cell body embodies a concentrated reflection in the organelles and the nucleus, chromatin and cytoskeleton of morphological structure changes, such as mitochondria swelling disappeared, ridge and film, a cavity sample, rough endoplasmic reticulum and degranulation, reduced Numbers of microfilament, microtubule dissolved, chromatin solidification, nuclear membrane shrinkage, etc. Cytoplasmic processes is connected with the cell body, very close. The pathological changes of cell body can also be seen in the dendrites and axons and synapses. Neurons in the degradation also possess a certain ability of plasticity, but the ability decreased along with the progress of brain aging. Glial cells in brain aging and compensatory hyperplasia mainly appear at the time of the compensatory hyperplasia, and two forms of degenerative changes. How to enhance the capacity of compensatory of glial cells to repair, and how to promote dendritic spines and synaptic development, improve the plasticity of neurons, become the focus in the slow brain aging