Finite-Temperature Quasicontinuum: Molecular Dynamics without All the Atoms

Using a combination of statistical mechanics and finite-element interpolation, we develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nanoindentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter

Implications of quantum ambiguities in k = 1 loop quantum cosmology: Distinct quantum turnarounds and the super-Planckian regime

The spatially closed Friedmann-Lema\^{i}tre-Robertson-Walker model in loop quantum cosmology admits two inequivalent consistent quantizations: one based on expressing the field strength in terms of the holonomies over closed loops, and, another using a connection operator and open holonomies. Using the effective dynamics, we investigate the phenomenological differences between the two quantizations for the single fluid and the two fluid scenarios with various equations of state, including the phantom matter. We show that a striking difference between the two quantizations is the existence of two distinct quantum turnarounds, either bounces or recollapses, in the connection quantization, in contrast to a single distinct quantum bounce or a recollapse in the holonomy quantization. These results generalize an earlier result on the existence of two distinct quantum bounces for stiff matter by Corichi and Karami. However, we find that in certain situations two distinct quantum turnarounds can become virtually indistinguishable. And depending on the initial conditions, a pure quantum cyclic universe can also exist undergoing a quantum bounce and a quantum recollapse. We show that for various equations of states, connection based quantization leads to super-Planckian values of the energy density and the expansion scalar at quantum turnarounds. Interestingly, we find that very extreme energy densities can also occur for the holonomy quantization, breaching the maximum allowed density in the spatially flat loop quantized model. However, the expansion scalar in all these cases is bounded by a universal value.Comment: Revised version with expanded results and discussion on the role of inverse volume effects. Four figures added. To appear in Phys. Rev.

Hysteresis and beats in loop quantum cosmology

Differences in pressure during expansion and contraction stages in cosmic evolution can result in a hysteresis-like phenomena in non-singular cyclic models sourced with scalar fields. We discuss this phenomena for spatially closed isotropic spacetime in loop quantum cosmology (LQC) for a quadratic and a cosh-like potential, with and without a negative cosmological constant using effective spacetime description of the underlying quantum geometry. Two inequivalent loop quantizations of this spacetime, one based on holonomies of the Ashtekar-Barbero connection using closed loops, and another based on the connection operator, are discussed. Due to quantum geometric effects, both models avoid classical singularities, but unlike the holonomy based quantization, connection based quantization results in two quantum bounces. In spite of differences in non-singular effective dynamics of both the models, the phenomena of hysteresis is found to be robust for the $\phi^2$ potential. Quasi-periodic beats exist for a cosh-like potential, irrespective of the nature of classical recollapse whether by spatial curvature, or a negative cosmological constant. An interplay of negative cosmological constant and spatial curvature in presence of potentials results in rich features such as islands of cluster of bounces separated by accelerated expansion, and a universe which either undergoes a step like expansion with multiple turnarounds or quasi-periodic beats depending on a "tuning" of the steepness parameter of the potential.Comment: 12 pages, 17 figure

HYDROBIA ULVAE: A DEPOSIT-FEEDER FOR CLEANING LIVING HARD-SHELLED FORAMINIFERA

International audienceThis study proposes a new method for fast and inexpensive extraction of a large number of living foraminifera for laboratory cultures. The method is a significant improvement over current extraction methods, which are highly time-consuming. Several treatments were designed to test the method. Sediment bearing foraminifera from Brouage Mudflat (Atlantic coast of France) was washed through a 50-µm sieve and distributed in glass Petri dishes with 20, 40 and 80 specimens of Hydrobia ulvae, a common gastropod from European intertidal mudflats. As a control experiment, one dish was treated similarly but maintained without Hydrobia. After two days, most of the sediment in the Hydrobia treatments was compacted into small cylindrical gastropod feces and the tests of living benthic foraminifera (Ammonia tepida and Haynesina germanica) were clean and easily visible. Additional experiments showed that the foraminifera were not ingested by Hydrobia ulvae, and could be picked quickly and easily