A new conjecture extends the GM law for percolation thresholds to dynamical situations

The universal law for percolation thresholds proposed by Galam and Mauger (GM) is found to apply also to dynamical situations. This law depends solely on two variables, the space dimension d and a coordinance numberq. For regular lattices, q reduces to the usual coordination number while for anisotropic lattices it is an effective coordination number. For dynamical percolation we conjecture that the law is still valid if we use the number q_2 of second nearest neighbors instead of q. This conjecture is checked for the dynamic epidemic model which considers the percolation phenomenon in a mobile disordered system. The agreement is good.Comment: 8 pages, latex, 3 figures include

High-contrast Ultrabroadband Frontend Source for High Intensity Few-Cycle Lasers

An ultrabroadband seed source for high-power, high-contrast OPCPA systems at 800 nm is presented. The source is based on post compression in a hollow-core fiber followed by crossed polarized waves (XPW) filtering and is capable of delivering 80$\mu$J, 5fs, CEP-stable (0.3rad RMS) pulses with excellent spectral and temporal qualit

Kondo spin liquid and magnetically long-range ordered states in the Kondo necklace model

A simplified version of the symmetric Kondo lattice model, the Kondo necklace model, is studied by using a representation of impurity and conduction electron spins in terms of local Kondo singlet and triplet operators. Within a mean field theory, a spin gap always appears in the spin triplet excitation spectrum in 1D, leading to a Kondo spin liquid state for any finite values of coupling strength $t/J$ (with $t$ as hopping and $J$ as exchange); in 2D and 3D cubic lattices the spin gaps are found to vanish continuously around $(t/J)_c\approx 0.70$ and $(t/J)_c\approx 0.38$, respectively, where quantum phase transitions occur and the Kondo spin liquid state changes into an antiferromagnetically long-range ordered state. These results are in agreement with variational Monte Carlo, higher-order series expansion, and recent quantum Monte Carlo calculations for the symmetric Kondo lattice modelComment: Revtex, four pages, three figures; to be published in Physical Review B1, 1 July (2000

Structure and dynamics of a model glass: influence of long-range forces

We vary the amplitude of the long-range Coulomb forces within a classical potential describing a model silica glass and study the consequences on the structure and dynamics of the glass, via molecular dynamics simulations. This model allows us to follow the variation of specific features such as the First Sharp Diffraction Peak and the Boson Peak in a system going continuously from a fragile (no Coulomb forces) to a strong (with Coulomb forces) glass. In particular we show that the characteristic features of a strong glass (existence of medium range order, bell-shaped ring size distribution, sharp Boson peak) appear as soon as tetrahedral units are formed.Comment: 5 pages, 4 figures. To be published in J.Phys.: C

Nonlinear optical transformation of the polarization state of circularly polarized light with holographic-cut cubic crystals

n/

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

Passive coherent combining of CEP-stable few-cycles pulses from a temporally divided hollow fiber compressor

International audienceWe demonstrate a simple and robust passive coherent combining technique for temporal compression of millijouleenergy laser pulses down to few-cycle duration in a gas-filled hollow fiber. High combining efficiency is achieved byusing carefully oriented calcite plates for temporal pulse division and recombination. Carrier-envelope phase (CEP)-stable, 6-fs, 800-nm pulses with more than 0.6 mJ energy are routinely generated. This method could aid in theenergy scaling of CEP-stable hollow-fiber compressor systems

Fractal formation and ordering in random sequential adsorption

We reveal the fractal nature of patterns arising in random sequential adsorption of particles with continuum power-law size distribution, $P(R)\sim R^{\alpha-1}$, $R \le R_{\rm max}$. We find that the patterns become more and more ordered as $\alpha$ increases, and that the Apollonian packing is obtained at $\alpha \to \infty$ limit. We introduce the entropy production rate as a quantitative criteria of regularity and observe a transition from an irregular regime of the pattern formation to a regular one. We develop a scaling theory that relates kinetic and structural properties of the system.Comment: 4 pages, RevTex, 4 postscript figures. To appear in Phys.Rev.Let

Spin Gap in a Doped Kondo Chain

We show that the Kondo chain away from half-filling has a spin gap upon the introduction of an additional direct Heisenberg coupling between localized spins. This is understood in the weak-Kondo-coupling limit of the Heisenberg-Kondo lattice model by bosonization and in the strong-coupling limit by a mapping to a modified t-J model. Only for certain ranges of filling and Heisenberg coupling does the spin gap phase extend from weak to strong coupling.Comment: 4 pages RevTeX including 4 eps figures; minor corrections and clarification