Size segregation and convection

The size segregation of granular materials in a vibrating container is investigated using Molecular Dynamics. We find that the rising of larger particles is accompanied by the existence of convection cells even in the case of the lowest possible frequencies. The convection can, however, also be triggered by the larger particle itself. The possibility of rising through this mechanism strongly depends on the depth of the larger particle.Comment: 7 pages, 4 figure

Gate fidelity and coherence of an electron spin in a Si/SiGe quantum dot with micromagnet

The gate fidelity and the coherence time of a qubit are important benchmarks for quantum computation. We construct a qubit using a single electron spin in a Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of $\approx$ 99$\%$ using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in substrate. The coherence time measured using dynamical decoupling extends up to $\approx$ 400 $\mu$s for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10 kHz $-$ 1 MHz range, possibly because charge noise affecting the spin via the micromagnet gradient. This work shows that an electron spin in a Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification

Asymmetric expression of Argonautes in reproductive tissues

The Arabidopsis genome encodes ten Argonautes proteins showing distinct expression pattern as well as intracellular localisation during sexual reproduction

No self-similar aggregates with sedimentation

Two-dimensional cluster-cluster aggregation is studied when clusters move both diffusively and sediment with a size dependent velocity. Sedimentation breaks the rotational symmetry and the ensuing clusters are not self-similar fractals: the mean cluster width perpendicular to the field direction grows faster than the height. The mean width exhibits power-law scaling with respect to the cluster size, ~ s^{l_x}, l_x = 0.61 +- 0.01, but the mean height does not. The clusters tend to become elongated in the sedimentation direction and the ratio of the single particle sedimentation velocity to single particle diffusivity controls the degree of orientation. These results are obtained using a simulation method, which becomes the more efficient the larger the moving clusters are.Comment: 10 pages, 10 figure

Influence of Hydrodynamic Interactions on the Kinetics of Colloidal Particle's Adsorption

The kinetics of irreversible adsorption of spherical particles onto a flat surface is theoretically studied. Previous models, in which hydrodynamic interactions were disregarded, predicted a power-law behavior $t^{-2/3}$ for the time dependence of the coverage of the surface near saturation. Experiments, however, are in agreement with a power-law behavior of the form $t^{-1/2}$. We outline that, when hydrodynamic interactions are considered, the assymptotic behavior is found to be compatible with the experimental results in a wide region near saturation.Comment: 4 pages, 1 figures, Phys. Rev. Lett. (in press

Numerical model for granular compaction under vertical tapping

A simple numerical model is used to simulate the effect of vertical taps on a packing of monodisperse hard spheres. Our results are in agreement with an experimantal work done in Chicago and with other previous models, especially concerning the dynamics of the compaction, the influence of the excitation strength on the compaction efficiency, and some ageing effects. The principal asset of the model is that it allows a local analysis of the packings. Vertical and transverse density profiles are used as well as size and volume distributions of the pores. An interesting result concerns the appearance of a vertical gradient in the density profiles during compaction. Furthermore, the volume distribution of the pores suggests that the smallest pores, ranging in size between a tetrahedral and an octahedral site, are not strongly affected by the tapping process, in contrast to the largest pores which are more sensitive to the compaction of the packing.Comment: 8 pages, 15 figures (eps), to be published in Phys. Rev. E. Some corrections have been made, especially in paragraph IV

Symmetry and dimension of the magnon dispersion of inorganic spin-Peierls systems

The data on the dispersion of the magnetic excitations of CuGeO_3 in the spin-Peierls dimerized phase are analyzed. On the basis of the lattice structure it is shown that even along the chains the $d=2$ character cannot be neglected. The symmetry of the dispersion differs from the one assumed so far. The magnetic resonance data is reinterpreted. The possibility of interchain rather than intrachain frustration is discussed.Comment: 4 pages, Revtex, to appear in PR

Computer investigation of the energy landscape of amorphous silica

The multidimensional topography of the collective potential energy function of a so-called strong glass former (silica) is analyzed by means of classical molecular dynamics calculations. Features qualitatively similar to those of fragile glasses are recovered at high temperatures : in particular an intrinsic characteristic temperature $T_c\simeq 3500$K is evidenced above which the system starts to investigate non-harmonic potential energy basins. It is shown that the anharmonicities are essentially characterized by a roughness appearing in the potential energy valleys explored by the system for temperatures above $T_c$.Comment: 5 pages; accepted for publication in PR

Vorticity statistics in the two-dimensional enstrophy cascade

We report the first extensive experimental observation of the two-dimensional enstrophy cascade, along with the determination of the high order vorticity statistics. The energy spectra we obtain are remarkably close to the Kraichnan Batchelor expectation. The distributions of the vorticity increments, in the inertial range, deviate only little from gaussianity and the corresponding structure functions exponents are indistinguishable from zero. It is thus shown that there is no sizeable small scale intermittency in the enstrophy cascade, in agreement with recent theoretical analyses.Comment: 5 pages, 7 Figure