278 research outputs found
Multifractal current distribution in random diode networks
Recently it has been shown analytically that electric currents in a random
diode network are distributed in a multifractal manner [O. Stenull and H. K.
Janssen, Europhys. Lett. 55, 691 (2001)]. In the present work we investigate
the multifractal properties of a random diode network at the critical point by
numerical simulations. We analyze the currents running on a directed
percolation cluster and confirm the field-theoretic predictions for the scaling
behavior of moments of the current distribution. It is pointed out that a
random diode network is a particularly good candidate for a possible
experimental realization of directed percolation.Comment: RevTeX, 4 pages, 5 eps figure
Crossover of conductance and local density of states in a single-channel disordered quantum wire
The probability distribution of the mesoscopic local density of states (LDOS)
for a single-channel disordered quantum wire with chiral symmetry is computed
in two different geometries. An approximate ansatz is proposed to describe the
crossover of the probability distributions for the conductance and LDOS between
the chiral and standard symmetry classes of a single-channel disordered quantum
wire. The accuracy of this ansatz is discussed by comparison with a
large-deviation ansatz introduced by Schomerus and Titov in Phys. Rev. B
\textbf{67}, 100201(R) (2003).Comment: 19 pages, 5 eps figure
Scaling for the Percolation Backbone
We study the backbone connecting two given sites of a two-dimensional lattice
separated by an arbitrary distance in a system of size . We find a
scaling form for the average backbone mass: , where
can be well approximated by a power law for : with . This result implies that for the entire range . We also propose a scaling
form for the probability distribution of backbone mass for a given
. For is peaked around , whereas for decreases as a power law, , with . The exponents and satisfy the relation
, and is the codimension of the backbone,
.Comment: 3 pages, 5 postscript figures, Latex/Revtex/multicols/eps
Electronic and Magnetic Properties of Nanographite Ribbons
Electronic and magnetic properties of ribbon-shaped nanographite systems with
zigzag and armchair edges in a magnetic field are investigated by using a tight
binding model. One of the most remarkable features of these systems is the
appearance of edge states, strongly localized near zigzag edges. The edge state
in magnetic field, generating a rational fraction of the magnetic flux (\phi=
p/q) in each hexagonal plaquette of the graphite plane, behaves like a
zero-field edge state with q internal degrees of freedom. The orbital
diamagnetic susceptibility strongly depends on the edge shapes. The reason is
found in the analysis of the ring currents, which are very sensitive to the
lattice topology near the edge. Moreover, the orbital diamagnetic
susceptibility is scaled as a function of the temperature, Fermi energy and
ribbon width. Because the edge states lead to a sharp peak in the density of
states at the Fermi level, the graphite ribbons with zigzag edges show
Curie-like temperature dependence of the Pauli paramagnetic susceptibility.
Hence, it is shown that the crossover from high-temperature diamagnetic to
low-temperature paramagnetic behavior of the magnetic susceptibility of
nanographite ribbons with zigzag edges.Comment: 13 pages including 19 figures, submitted to Physical Rev
Global talent management and staffing in MNEs: An introduction to the edited volume of international business and management
Driven interfaces in random media at finite temperature : is there an anomalous zero-velocity phase at small external force ?
The motion of driven interfaces in random media at finite temperature and
small external force is usually described by a linear displacement at large times, where the velocity vanishes according to the
creep formula as for . In this paper,
we question this picture on the specific example of the directed polymer in a
two dimensional random medium. We have recently shown (C. Monthus and T. Garel,
arxiv:0802.2502) that its dynamics for F=0 can be analyzed in terms of a strong
disorder renormalization procedure, where the distribution of renormalized
barriers flows towards some "infinite disorder fixed point". In the present
paper, we obtain that for small , this "infinite disorder fixed point"
becomes a "strong disorder fixed point" with an exponential distribution of
renormalized barriers. The corresponding distribution of trapping times then
only decays as a power-law , where the exponent
vanishes as as . Our
conclusion is that in the small force region , the divergence of
the averaged trapping time induces strong
non-self-averaging effects that invalidate the usual creep formula obtained by
replacing all trapping times by the typical value. We find instead that the
motion is only sub-linearly in time , i.e. the
asymptotic velocity vanishes V=0. This analysis is confirmed by numerical
simulations of a directed polymer with a metric constraint driven in a traps
landscape. We moreover obtain that the roughness exponent, which is governed by
the equilibrium value up to some large scale, becomes equal to
at the largest scales.Comment: v3=final versio
Bioadsorption of Pb
Herein, the efficiency of Eucalyptus camaldulensis leaves as biosorbent for lead and copper was investigated. The particle size distribution was determined by Granulometric analysis and the functional groups were identified by FT-IR spectroscopy. The effects of contact time, pH and initial metal ions concentration were investigated. The experimental kinetic data were well fitted by the pseudo-second order kinetic model and Langmuir isotherm with a maximum adsorption capacity up to 71 mg g-1 and 37 mg g-1 for Cu2+ and Pb2+ respectively. The selectivity was examined in a binary ions solution where the adsorbent showed preference for lead over copper
Where two fractals meet: the scaling of a self-avoiding walk on a percolation cluster
The scaling properties of self-avoiding walks on a d-dimensional diluted
lattice at the percolation threshold are analyzed by a field-theoretical
renormalization group approach. To this end we reconsider the model of Y. Meir
and A. B. Harris (Phys. Rev. Lett. 63:2819 (1989)) and argue that via
renormalization its multifractal properties are directly accessible. While the
former first order perturbation did not agree with the results of other
methods, we find that the asymptotic behavior of a self-avoiding walk on the
percolation cluster is governed by the exponent nu_p=1/2 + epsilon/42 +
110epsilon^2/21^3, epsilon=6-d. This analytic result gives an accurate numeric
description of the available MC and exact enumeration data in a wide range of
dimensions 2<=d<=6.Comment: 4 pages, 2 figure
Extreme events driven glassy behaviour in granular media
Motivated by recent experiments on the approach to jamming of a weakly forced
granular medium using an immersed torsion oscillator [Nature 413 (2001) 407],
we propose a simple model which relates the microscopic dynamics to macroscopic
rearrangements and accounts for the following experimental facts: (1) the
control parameter is the spatial amplitude of the perturbation and not its
reduced peak acceleration; (2) a Vogel-Fulcher-Tammann-like form for the
relaxation time. The model draws a parallel between macroscopic rearrangements
in the system and extreme events whose probability of occurrence (and thus the
typical relaxation time) is estimated using extreme-value statistics. The range
of validity of this description in terms of the control parameter is discussed
as well as the existence of other regimes.Comment: 7 pages, to appear in Europhys. Let
Coarsening on percolation clusters: out-of-equilibrium dynamics versus non linear response
We analyze the violations of linear fluctuation-dissipation theorem (FDT) in
the coarsening dynamics of the antiferromagnetic Ising model on percolation
clusters in two dimensions. The equilibrium magnetic response is shown to be
non linear for magnetic fields of the order of the inverse square root of the
number of sites. Two extreme regimes can be identified in the thermoremanent
magnetization: (i) linear response and out-of-equilibrium relaxation for small
waiting times (ii) non linear response and equilibrium relaxation for large
waiting times. The function characterizing the deviations from linear
FDT cross-overs from unity at short times to a finite positive value for longer
times, with the same qualitative behavior whatever the waiting time. We show
that the coarsening dynamics on percolation clusters exhibits stronger
long-term memory than usual euclidian coarsening.Comment: 17 pages, 10 figure
- …