Nonlinear electrical conductivity in a 1D granular medium

We report on observations of the electrical transport within a chain of metallic beads (slightly oxidized) under an applied stress. A transition from an insulating to a conductive state is observed as the applied current is increased. The voltage-current (U-I) characteristics are nonlinear and hysteretic, and saturate to a low voltage per contact (0.4 V). Our 1D experiment allows us to understand phenomena (such as the Branly effect) related to this conduction transition by focusing on the nature of the contacts instead of the structure of the granular network. We show that this transition comes from an electro-thermal coupling in the vicinity of the microcontacts between each bead - the current flowing through these contact points generates their local heating which leads to an increase of their contact areas, and thus enhances their conduction. This current-induced temperature rise (up to 1050 C) results in the microsoldering of the contact points (even for voltages as low as 0.4 V). Based on this self-regulated temperature mechanism, an analytical expression for the nonlinear U-I back trajectory is derived, and is found to be in very good agreement with the experiments. In addition, we can determine the microcontact temperature with no adjustable parameters. Finally, the stress dependence of the resistance is found to be strongly non-hertzian due to the presence of the surface films. This dependence cannot be usually distinguished from the one due to the disorder of the granular contact network in 2D or 3D experiments.Received: 7 November 2003, Published online: 28 May 2004PACS: 45.70.-n Granular systems - 72.80.-r Conductivity of specific materials     

Double-diffractive $\chi$ meson production at the hadron colliders

The double-diffractive production of and mesons, with a rapidity gap on either side, is studied, using both the Regge formalism and the perturbative QCD approach. Due to the rather low scale, the exclusive double-diffractive process is predicted to dominate, whereas the probability that the incoming protons dissociate is expected to be relatively small. We evaluate the corresponding production cross sections at the Tevatron and LHC energies. For the double-diffractive process with proton dissociation, it is possible to select events with large transverse momenta transferred through the rapidity gaps, by measuring the transverse energy, , flows in the proton fragmentation regions. Then the large provides a scale to justify the use of perturbative QCD, and to allow a spin-parity analysis of the centrally produced system to be performed, by studying the azimuthal angular correlations between the directions of the forward and backward flows. The central production of the new X(3872) charmonium state is considered.Received: 24 March 2004, Published online: 11 May 2004     

Active thermal convection in vibrofluidized granular systems

We present a granular-hydrodynamic model that captures the essence of convection in a fully vibrofluidized granular system. The steady temperature distribution is solved analytically. Numerical simulation shows that the convection always develops through a supercritical bifurcation, with its energy about of the random (heat) one. A comparison calculation is performed for a normal fluid. The convection roll, or an active roll as we call it, has an angular velocity gradient from its interior to exterior. We conclude that active rolls are universal.Received: 25 March 2004, Published online: 9 September 2004PACS: 45.70.Mg Granular flow: mixing, segregation and stratification - 47.20.Bp Buoyancy-driven instability - 47.27.Te Convection and heat transfer     

Stationary shear flows of dense granular materials: a tentative continuum modelling

We propose a simple continuum model to interpret the shearing motion of dense, dry and cohesion-less granular media. Compressibility, dilatancy and Coulomb-like friction are the three basic ingredients. The granular stress is split into a rate-dependent part representing the rebound-less impacts between grains and a rate-independent part associated with long-lived contacts. Because we consider stationary flows only, the grain compaction and the grain velocity are the two main variables. The predicted velocity and compaction profiles are in apparent qualitative agreement with most of the experimental or numerical results concerning free-surface shear flows as well as confined shear flows.Received: 24 March 2004, Published online: 29 June 2004PACS: 45.70.Ht Avalanches - 45.70.-n Granular systems - 83.80.Fg Granular solids     

Transients in sheared granular matter

As dense granular materials are sheared, a shear band and an anisotropic force network form. The approach to steady-state behavior depends on the history of the packing and the existing force and contact network. We present experiments on shearing of dense granular matter in a 2D Couette geometry in which we probe the history and evolution of shear bands by measuring particle trajectories and stresses during transients. We find that when shearing is stopped and restarted in the same direction, steady-state behavior is immediately reached, in agreement with the typical assumption that the system is quasistatic. Although some relaxation of the force network is observed when shearing is stopped, quasistatic behavior is maintained because the contact network remains essentially unchanged. When the direction of shear is reversed, a transient occurs in which stresses initially decrease, changes in the force network reach further into the bulk, and particles far from the wheel become more mobile. This occurs because the force network is fragile to changes transverse to the force network established under previous shear; particles must rearrange before becoming jammed again, thereby providing resistance to shear in the reversed direction. The strong force network is re-established after displacing the shearing surface , where d is the mean grain diameter. Steady-state velocity profiles are reached after a shear of . Particles immediately outside of the shear band move on average less than 1 diameter before becoming jammed again. We also examine particle rotation during this transient and find that mean particle spin decreases during the transient, which is related to the fact that grains are not interlocked as strongly.Received: 5 March 2004, Published online: 24 August 2004PACS: 45.70.-n Granular systems - 83.80.Fg Granular solids     

Engineering quantum decoherence of charge qubit via a nanomechanical resonator

We propose a theoretical scheme to observe the loss of quantum coherence through the coupling of the superconducting charge qubit system to a nanomechanical resonator (NAMR), which has already been successfully fabricated in experiment and is convenient to manipulate. With a similar form to the usual cavity QED system, this qubit-NAMR composite system with engineered coupling exhibits the collapse and revival phenomenon in a progressive decoherence process. Corresponding to the two components of superposition of the two charge eigenstates, the state of the nanomechanical resonator evolves simultaneously towards two distinct quasi-classical states. Therefore the generalized which way detection by the NAMR induces the quantum decoherence of the charge qubit.Received: 21 May 2004, Published online: 9 September 2004PACS: 03.65.-w Quantum mechanics - 74.50. + r Tunneling phenomena; point contacts, weak links, Josephson effects - 03.67.Lx Quantum computation - 85.25.Dq Superconducting quantum interference devices (SQUIDs)     

Energy and power fluctuations in vibrated granular gases

Using two-dimensional numerical simulations of a granular gas excited by vibrating one of the container boundaries, we study the fluctuations of its total kinetic energy, of the power injected into the gas by the moving boundary and of the power dissipated by inelastic collisions. We show that an effective number N _f of degrees of freedom that depends on the inelasticity of collisions can be extracted from the probability density function (PDF) of the fluctuations of the total kinetic energy E. is then an intensive variable contrary to the usually defined granular temperature . We then show that an intensive temperature can also be calculated from the probability of certain large deviations of the injected power. Finally, we show that the fluctuations of injected and dissipated power are related such that their ratio is inversely proportional to the square-root of the ratio of their correlation times. This allows to define a quantity homogenous to a temperature that is intensive and conserved in the process of energy dynamics from its injection by the driving piston to its dissipation by inelastic collisions.Received: 3 August 2004, Published online: 14 December 2004PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 05.45.-a Nonlinear dynamics and nonlinear dynamical systems     

Threshold corrections in bottom and charm quark hadroproduction at next-to-next-to-leading order

We calculate threshold soft-gluon corrections to total cross sections and transverse momentum distributions for bottom and charm quark production in fixed target experiments, considering both pp and interactions. We investigate the quality of the near-threshold soft-gluon approximation at next-to-leading order (NLO) and calculate next-to-next-to-leading order (NNLO) corrections through next-to-next-to-next-to-leading-logarithmic (NNNLL) accuracy, including some virtual terms. We find that the NNLO threshold corrections reduce the factorization and renormalization scale dependence of the cross sections.Received: 26 April 2004, Published online: 2 July 2004     

Lattice-BGK approach to simulating granular flows

Many continuum theories for granular flow produce an equation of motion for the fluctuating kinetic energy density (granular temperature) that accounts for the energy lost in inelastic collisions. Apart from the presence of an extra dissipative term, this equation is very similar in form to the usual temperature equation in hydrodynamics. It is shown how a lattice-kinetic model based on the Bhatnagar-Gross-Krook (BGK) equation that was previously derived for a miscible two-component fluid may be modified to model the continuum equations for granular flow. This is done by noting that the variable corresponding to the concentration of one species follows an equation that is essentially analogous to the granular temperature equation. A simulation of an unforced granular fluid using the modified model reproduces the phenomenon of clustering instability, namely the spontaneous agglomeration of particles into dense clusters, which occurs generically in all granular flows. The success of the continuum theory in capturing the gross features of this basic phenomenon is discussed. Some shear flow simulations are also presented.     

On the number of contacts of two polymer chains situated on fractal structures

We study the critical behavior of the number of monomer-monomer contacts for two polymers in a good solvent. Polymers are modeled by two self-avoiding walks situated on fractals that belong to the checkerboard (CB) and X family. Each member of a family is labeled by an odd integer b, . By applying the exact Renormalization Group (RG) method, we establish the relevant phase diagrams whereby we calculate the contact critical exponents (for the CB and X fractals with b = 5 and b = 7). The critical exponent is associated with power law of the number of sites at which the two polymers are touching each other.Received: 12 March 2004, Published online: 3 August 2004PACS: 64.60.Ak Renormalization-group, fractal, and percolation studies of phase transitions - 36.20.Ey Conformation (statistics and dynamics)     

Darboux coordinates and isospectral Hamiltonian flows for the massive thirring model

The two-dimensional massive Thirring model is described as the integrability condition of a pair of commuting completely integrable isospectral Hamiltonian flows in the dual (2)^+* of the positive part (2)⁺ of the twisted loop algebra (2). Action-angle coordinates corresponding to the spectral invariants are derived on rational coadjoint orbits and a linearization of the flows obtained in the Jacobi variety of the underlying invariant spectral curve through a Liouville generating function for canonical coordinates.Research supported in part by the Natural Sciences Engineering Research Council of Canada and the Fonds FCAR du Québec.     

Production of $\eta_c$ and dimuon enhancement in heavy ion collisions

Dilepton production in heavy ion collisions in the intermediate mass region (IMR) has consistently shown an excess over theoretical estimates. An attempt to understand this discrepancy between the observed dilepton pairs and the theoretical estimate is made here through the production of the meson and estimates obtained by NRQCD calculations. We find that production offers a satisfactory quantitative picture for explaining the discrepancy.Received: 13 August 2003, Revised: 9 February 2004, Published online: 23 March 2004     

Complete description of polarization effects in emission of a photon by an electron in the field of a strong laser wave

We consider the emission of a photon by an electron in the field of a strong laser wave. Polarization effects in this process are important for a number of physical problems. The probability of this process for circularly or linearly polarized laser photons and for arbitrary polarization of all other particles is calculated. We obtain the complete set of functions which describe such a probability in a compact invariant form. Besides this, we discuss in some detail the polarization effects in the kinematics relevant to the problem of conversion at and colliders.Received: 13 February 2004, Published online: 23 June 2004     

On the use of STM superconducting tips at very low temperatures

We report on high quality local tunnel spectroscopy measurements in superconductors using in-situ fabricated superconducting tips as counterelectrode. The experiments were made at very low temperatures using a dilution refrigerator and a ³He cryostat. Spectra obtained with superconducting tip and sample of Al show that the spectroscopic resolution of our set-up is of 15 eV. Following the observation of Josephson current in tunnelling regime (with tips of Pb and of Al), we discuss the feasibility of Scanning Josephson Spectroscopy with atomic size resolution. Experiments showing new applications of these superconducting tips under applied external magnetic fields are also reported.Received: 12 May 2004, Published online: 7 September 2004PACS: 73.63.Rt Nanoscale contacts - 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.) - 74.25.Ha Magnetic properties - 74.50. + r Tunnelling phenomena; point contacts, weak links, Josephson effects - 74.78.Na Mesoscopic and nanoscale systems - 74.80.Fp Point contacts; SN and SNS junctions     

Hunting a light <Emphasis Type="Italic">CP</Emphasis>-violating Higgs boson via diffraction at the LHC

We study the central diffractive production of the (three neutral) Higgs bosons, with a rapidity gap on either side, in an MSSM scenario with CP-violation. We consider the and decay for the light H₁ boson and the four b-jet final state for the heavy H₂ and H₃ bosons, and discuss the corresponding backgrounds. A direct indication of the existence of CP-violation can come from the observation of either an azimuthal asymmetry in the angular distribution of the tagged forward protons (for the exclusive process) or of a contribution in the azimuthal correlation between the transverse energy flows in the proton fragmentation regions for the process with the diffractive dissociation of both incoming protons ( ). We emphasise the advantage of reactions with the rapidity gaps (that is, production by pomeron-pomeron fusion) to probe CP-parity and to determine the quantum numbers of the produced central object.Received: 16 January 2004, Published online: 31 March 2004     

Spectra and decays of $\pi \pi$ and $\pi K$ atoms

We describe the spectra and decays of and atoms within a non-relativistic effective field theory. The evaluations of the energy shifts and widths are performed at next-to-leading order in isospin symmetry breaking. We provide general formulae for all S-states, and discuss the states with angular momentum one in some detail. The prediction for the lifetime of the atom in its ground state yields s.Received: 9 May 2004, Published online: 23 July 2004     

Stability of oscillatory flows past compliant surfaces

The stability of oscillatory flows over compliant surfaces is studied analytically and numerically. The types of compliant surfaces studied are the spring backed wall model, which permits tangential motion of the surface, and the incompressible viscoelastic gel model. The stability is determined using the Floquet analysis, where amplitude of perturbations at time intervals separated by one time period is examined to determine whether perturbations grow or decay. The oscillatory flows past both the spring backed wall model and the viscoelastic gel model exhibit an instability in the limit of zero Reynolds number, and the transition amplitude of the oscillatory velocity increases with the frequency of oscillations. The transition amplitude has a minimum at zero wave number for the spring backed plate model, whereas the minimum occurs at finite wavenumber for the viscoelastic gel model. For the spring backed plate model, it is shown that the instability due to steady mean flow and the purely oscillatory instability reinforce each other, and the regions of instability are mapped in the ( ) plane, where is the steady strain rate and A is the oscillatory strain rate. For the viscoelastic gel model, the instability is found to depend strongly on the gel viscosity , and the effect of oscillations on the continuation of viscous modes at intermediate Reynolds number shows a complicated dependence on the oscillation frequency.Received: 17 March 2004, Published online: 30 September 2004PACS: 47.20.Ft Instability of shear flows - 83.50.-v Deformation and flow - 87.19.Tt Rheology of body fluids     

Off equilibrium dynamics in the 3d-XY system

We investigate through Monte Carlo simulations the non-equilibrium behaviour of the three-dimensional XY-model quenched from a high temperature state to its ferromagnetic and critical phases. The two-times autocorrelation and response functions are determined in the asymptotic (scaling) regime, from which the nonequilibrium exponents and critical are extracted. The form of the scaling function is in agreement with the prediction of local scale-invariance. The so-called limit fluctuation-dissipation ratio is shown to vanish in the ordered phase and to reach a constant value around 0.43 for the critical quench.Received: 12 July 2004, Published online: 30 September 2004PACS: 75.40.Gb Dynamic properties - 05.70.Ln Non-equilibrium and irreversible thermodynamics