Topological thermal instability and length of proteins

We present an analysis of the effects of global topology on the structural stability of folded proteins in thermal equilibrium with a heat bath. For a large class of single domain proteins, we computed the harmonic spectrum within the Gaussian Network Model (GNM) and determined the spectral dimension, a parameter describing the low frequency behaviour of the density of modes. We find a surprisingly strong correlation between the spectral dimension and the number of amino acids of the protein. Considering that larger spectral dimension value relate to more topologically compact folded state, our results indicate that for a given temperature and length of the protein, the folded structure corresponds to the less compact folding compatible with thermodynamic stability.Comment: 15 pages, 6 eps figures, 2 table

Thermally induced directed currents in hard rod systems

We study the non equilibrium statistical properties of a one dimensional hard-rod fluid undergoing collisions and subject to a spatially non uniform Gaussian heat-bath and periodic potential. The system is able to sustain finite currents when the spatially inhomogeneous heat-bath and the periodic potential profile display an appropriate relative phase shift, $\phi$. By comparison with the collisionless limit, we determine the conditions for the most efficient transport among inelastic, elastic and non interacting rods. We show that the situation is complex as, depending on shape of the temperature profile, the current of one system may outperform the others.Comment: 5 pages, 2 figure

Refurbishing Voyager 1 & 2 Planetary Radio Astronomy (PRA) Data

Voyager/PRA (Planetary Radio Astronomy) data from digitized tapes archived at CNES have been reprocessed and recalibrated. The data cover the Jupiter and Saturn flybys of both Voyager probes. We have also reconstructed goniopolarimetric datasets (flux and polarization) at full resolution. These datasets are currently not available to the scientific community, but they are of primary interest for the analysis of the Cassini data at Saturn, and the Juno data at Jupiter, as well as for the preparation of the JUICE mission. We present the first results derived from the re-analysis of this dataset.Comment: Accepted manuscript for PRE8 (Planetary Radio Emission VIII conference) proceeding

Sensorless variable speed single-phase induction motor drive system based on direct rotor flux orientation

The single-phase induction motor (SPIM) is one of the electrical machines more used in the World, and can be found in several fractional and sub-fractional horsepower applications in houses, offices, shoppings, farms, and industries. The introduction of more sophisticated applications has required the use of variable speed drives for SPIM, where the adoption of sensorless techniques is the more reasonable option for speed control due to the low cost of this electrical machine. A proposal for sensorless variable speed SPIM drive based on direct rotor field orientation techniques is presented in this paper. None transformation is used in order to eliminate the asymmetry of the stator windings of the SPIM. The rotor speed is estimated from an flux observer, which is based on two independent linear feedback control systems. The speed and flux estimatives are used in two control loop based on PID regulators, which determine the voltages to be applied to the SPIM windings by a three-legs VSI inverter. Using computer simulations, two situations are considered in order to demonstrate the satisfactory performance of the proposed sensorless speed control for SPIM drives: variations on rotor speed reference and the application of mechanical load

Macroscopic detection of the strong stochasticity threshold in Fermi-Pasta-Ulam chains of oscillators

The largest Lyapunov exponent of a system composed by a heavy impurity embedded in a chain of anharmonic nearest-neighbor Fermi-Pasta-Ulam oscillators is numerically computed for various values of the impurity mass $M$. A crossover between weak and strong chaos is obtained at the same value $\epsilon_{_T}$ of the energy density $\epsilon$ (energy per degree of freedom) for all the considered values of the impurity mass $M$. The threshold \epsi lon_{_T} coincides with the value of the energy density $\epsilon$ at which a change of scaling of the relaxation time of the momentum autocorrelation function of the impurity ocurrs and that was obtained in a previous work ~[M. Romero-Bastida and E. Braun, Phys. Rev. E {\bf65}, 036228 (2002)]. The complete Lyapunov spectrum does not depend significantly on the impurity mass $M$. These results suggest that the impurity does not contribute significantly to the dynamical instability (chaos) of the chain and can be considered as a probe for the dynamics of the system to which the impurity is coupled. Finally, it is shown that the Kolmogorov-Sinai entropy of the chain has a crossover from weak to strong chaos at the same value of the energy density that the crossover value $\epsilon_{_T}$ of largest Lyapunov exponent. Implications of this result are discussed.Comment: 6 pages, 5 figures, revtex4 styl

Thermodynamic formalism for the Lorentz gas with open boundaries in dd dimensions

A Lorentz gas may be defined as a system of fixed dispersing scatterers, with a single light particle moving among these and making specular collisions on encounters with the scatterers. For a dilute Lorentz gas with open boundaries in $d$ dimensions we relate the thermodynamic formalism to a random flight problem. Using this representation we analytically calculate the central quantity within this formalism, the topological pressure, as a function of system size and a temperature-like parameter \ba. The topological pressure is given as the sum of the topological pressure for the closed system and a diffusion term with a \ba-dependent diffusion coefficient. From the topological pressure we obtain the Kolmogorov-Sinai entropy on the repeller, the topological entropy, and the partial information dimension.Comment: 7 pages, 5 figure

Transport properties in chaotic and non-chaotic many particles systems

Two deterministic models for Brownian motion are investigated by means of numerical simulations and kinetic theory arguments. The first model consists of a heavy hard disk immersed in a rarefied gas of smaller and lighter hard disks acting as a thermal bath. The second is the same except for the shape of the particles, which is now square. The basic difference of these two systems lies in the interaction: hard core elastic collisions make the dynamics of the disks chaotic whereas that of squares is not. Remarkably, this difference is not reflected in the transport properties of the two systems: simulations show that the diffusion coefficients, velocity correlations and response functions of the heavy impurity are in agreement with kinetic theory for both the chaotic and the non-chaotic model. The relaxation to equilibrium, however, is very sensitive to the kind of interaction. These observations are used to reconsider and discuss some issues connected to chaos, statistical mechanics and diffusion.Comment: 23 pgs with 8 Figure

N-tree approximation for the largest Lyapunov exponent of a coupled-map lattice

The N-tree approximation scheme, introduced in the context of random directed polymers, is here applied to the computation of the maximum Lyapunov exponent in a coupled map lattice. We discuss both an exact implementation for small tree-depth $n$ and a numerical implementation for larger $n$s. We find that the phase-transition predicted by the mean field approach shifts towards larger values of the coupling parameter when the depth $n$ is increased. We conjecture that the transition eventually disappears.Comment: RevTeX, 15 pages,5 figure

Diffusion, peer pressure and tailed distributions

We present a general, physically motivated non-linear and non-local advection equation in which the diffusion of interacting random walkers competes with a local drift arising from a kind of peer pressure. We show, using a mapping to an integrable dynamical system, that on varying a parameter, the steady state behaviour undergoes a transition from the standard diffusive behavior to a localized stationary state characterized by a tailed distribution. Finally, we show that recent empirical laws on economic growth can be explained as a collective phenomenon due to peer pressure interaction.Comment: RevTex: 4 pages + 3 eps-figures. Minor Revision and figure 3 replaced. To appear in Phys. Rev. Letter