Beam-beam and impedance

As two counter-rotating beams interact they can give rise to coherent dipole modes. Under the influence of impedance these coherent beam-beam modes can couple to higher order head-tail modes and lead to strong instabilities. A fully self-consistent approach including beam-beam and impedance was used to characterize this new coupled mode instability and study possible cures such as a transverse damper and high chromaticity.Comment: 9 pages, contribution to the ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders, CERN, Geneva, Switzerland, 18-22 Mar 201

Coherent beam-beam mode in the LHC

Observations of single bunch beam-beam coherent modes during dedicated experiments in the LHC are presented. Their role in standard operation for physics is discussed and, in particular, candidates of beam-beam coherent mode driven unstable by the machine impedance are presented.Comment: 4 pages, contribution to the ICFA Mini-Workshop on Beam-Beam Effects in Hadron Colliders, CERN, Geneva, Switzerland, 18-22 Mar 201

Temporal Dissection of Stimulus-Driven and Task-Driven Processes during Perceptual Decision about 3D SFM Stimuli

International audienceAnalyzing the visual properties of a stimulus, such as its shape or its motion, involves a temporally con-strained cascade of processes including sensory integration, attentional selection and perceptual decision. In natural condi-tions, these processing stages may temporally overlap. In this study, we used a time constrained paradigm designed to segre-gate these stages, and recorded MEG activity to characterize their cortical correlates. The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time. The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape. Subjects' responses were delayed to the end of the trials us-ing a random stimulus-response mapping. The analysis of the evoked magnetic fields reveals temporal-ly non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction reveals that the occipital, dorsal and ventral stimulus-related areas follow different temporal pro-files, The response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response, may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1]

Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters: Surface-Induced Mechanism

The freezing behavior of gold nanoclusters was studied by employing molecular dynamics simulations based on a semi-empirical embedded-atom method. Investigations of the gold nanoclusters revealed that, just after freezing, ordered nano-surfaces with a fivefold symmetry were formed with interior atoms remaining in the disordered state. Further lowering of temperatures induced nano-crystallization of the interior atoms that proceeded from the surface towards the core region, finally leading to an icosahedral structure. These dynamic processes explain why the icosahedral cluster structure is dominantly formed in spite of its energetic metastability.Comment: 9 pages, 4 figures(including 14 eps-files

Melting Point and Lattice Parameter Shifts in Supported Metal Nanoclusters

The dependencies of the melting point and the lattice parameter of supported metal nanoclusters as functions of clusters height are theoretically investigated in the framework of the uniform approach. The vacancy mechanism describing the melting point and the lattice parameter shifts in nanoclusters with decrease of their size is proposed. It is shown that under the high vacuum conditions (p<10^-7 torr) the essential role in clusters melting point and lattice parameter shifts is played by the van der Waals forces of cluster-substrate interation. The proposed model satisfactorily accounts for the experimental data.Comment: 6 pages, 3 figures, 1 tabl

Spontaneous alloying in binary metal microclusters - A molecular dynamics study -

Microcanonical molecular dynamics study of the spontaneous alloying(SA), which is a manifestation of fast atomic diffusion in a nano-sized metal cluster, is done in terms of a simple two dimensional binary Morse model. Important features observed by Yasuda and Mori are well reproduced in our simulation. The temperature dependence and size dependence of the SA phenomena are extensively explored by examining long time dynamics. The dominant role of negative heat of solution in completing the SA is also discussed. We point out that a presence of melting surface induces the diffusion of core atoms even if they are solid-like. In other words, the {\it surface melting} at substantially low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.

Żabińskiite, ideally Ca(Al_(0.5)Ta_(0.5))(SiO_4)O, a new mineral of the titanite group from the Piława Górna pegmatite, the Góry Sowie Block, southwestern Poland

Żabińskiite, ideally Ca(Al_(0.5)Ta_(0.5))(SiO_4)O, was found in a Variscan granitic pegmatite at Piława Górna, Lower Silesia, SW Poland. The mineral occurs along with (Al,Ta,Nb)- and (Al,F)-bearing titanites, a pyrochlore-supergroup mineral and a K-mica in compositionally inhomogeneous aggregates, ∼120 μm × 70 μm in size, in a fractured crystal of zircon intergrown with polycrase-(Y) and euxenite-(Y). Żabińskiite is transparent, brittle, brownish, with a white streak, vitreous lustre and a Mohs hardness of ∼5. The calculated density for the refined crystal is equal to 3.897 g cm^(–3), but depends strongly on composition. The mineral is non-pleochroic, biaxial (–), with mean refractive indices ≥1.89. The (Al,Ta,Nb)-richest żabińskiite crystal, (Ca_(0.980)Na_(0.015))Σ=0.995(Al_(0.340) Fe^(3+)_(0.029) Ti_(0.298)V_(0.001)Zr_(0.001)Sn_(0.005)Ta_(0.251)Nb_(0.081))Σ=1.005[(Si_(0.988)Al_0.012)O_(4.946)F_(0.047)(OH)_(0.007))Σ=5.000]; 60.7 mol.% Ca[Al_(0.5)(Ta,Nb)_(0.5)](SiO_4)O; is close in composition to previously described synthetic material. Żabińskiite is triclinic (space group symmetry Ai and has unit-cell parameters a = 7.031(2) Å, b = 8.692(2) Å, c = 6.561(2) Å, α = 89.712(11)°, β = 113.830(13)°, γ = 90.352(12)° and V = 366.77 (11) Å3. It is isostructural with triclinic titanite and bond-topologically identical with titanite and other minerals of the titanite group. Żabińskiite crystallized along with (Al,Ta,Nb)-bearing titanites at increasing Ti and Nb, and decreasing Ta activities, almost coevally with polycrase-(Y) and euxenite-(Y) from Ca-contaminated fluxed melts or early hydrothermal fluids

Molecular dynamics simulations of lead clusters

Molecular dynamics simulations of nanometer-sized lead clusters have been performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf 269/270}, 1109 (1992)). The binding energies of clusters forming crystalline (fcc), decahedron and icosahedron structures are compared, showing that fcc cuboctahedra are the most energetically favoured of these polyhedral model structures. However, simulations of the freezing of liquid droplets produced a characteristic form of ``shaved'' icosahedron, in which atoms are absent at the edges and apexes of the polyhedron. This arrangement is energetically favoured for 600-4000 atom clusters. Larger clusters favour crystalline structures. Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect fcc Wulff particle, containing a number of parallel stacking faults. The effects of temperature on the preferred structure of crystalline clusters below the melting point have been considered. The implications of these results for the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other minor changes for publicatio