489 research outputs found
Fractal properties of relaxation clusters and phase transition in a stochastic sandpile automaton
We study numerically the spatial properties of relaxation clusters in a two
dimensional sandpile automaton with dynamic rules depending stochastically on a
parameter p, which models the effects of static friction. In the limiting cases
p=1 and p=0 the model reduces to the critical height model and critical slope
model, respectively. At p=p_c, a continuous phase transition occurs to the
state characterized by a nonzero average slope. Our analysis reveals that the
loss of finite average slope at the transition is accompanied by the loss of
fractal properties of the relaxation clusters.Comment: 11 page
Interface Motion in Disordered Ferromagnets
We consider numerically the depinning transition in the random-field Ising
model. Our analysis reveals that the three and four dimensional model displays
a simple scaling behavior whereas the five dimensional scaling behavior is
affected by logarithmic corrections. This suggests that d=5 is the upper
critical dimension of the depinning transition in the random-field Ising model.
Furthermore, we investigate the so-called creep regime (small driving fields
and temperatures) where the interface velocity is given by an Arrhenius law.Comment: some misprints correcte
Density fluctuations and phase separation in a traffic flow model
Within the Nagel-Schreckenberg traffic flow model we consider the transition
from the free flow regime to the jammed regime. We introduce a method of
analyzing the data which is based on the local density distribution. This
analyzes allows us to determine the phase diagram and to examine the separation
of the system into a coexisting free flow phase and a jammed phase above the
transition. The investigation of the steady state structure factor yields that
the decomposition in this phase coexistence regime is driven by density
fluctuations, provided they exceed a critical wavelength.Comment: in 'Traffic and Granular Flow 97', edited by D.E. Wolf and M.
Schreckenberg, Springer, Singapore (1998
Thermally activated interface motion in a disordered ferromagnet
We investigate interface motion in disordered ferromagnets by means of Monte
Carlo simulations. For small temperatures and driving fields a so-called creep
regime is found and the interface velocity obeys an Arrhenius law. We analyze
the corresponding energy barrier as well as the field and temperature
dependence of the prefactor.Comment: accepted for publication in Computer Physics Communication
Dissipative current in SIFS Josephson junctions
We investigate superconductor/insulator/ferromagnet/superconductor (SIFS)
tunnel Josephson junctions in the dirty limit, using the quasiclassical theory.
We consider the case of a strong tunnel barrier such that the left S layer and
the right FS bilayer are decoupled. We calculate quantitatively the density of
states (DOS) in the FS bilayer for arbitrary length of the ferromagnetic layer,
using a self-consistent numerical method. We compare these results with a known
analytical DOS approximation, which is valid when the ferromagnetic layer is
long enough. Finally we calculate quantitatively the current-voltage
characteristics of a SIFS junction.Comment: Proceedings of the Vortex VI conference, to be published in Physica
Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction
We theoretically investigate an opportunity to implement a source of highly
spin-polarized ac current on the basis of superconductor/weak
ferromagnet/superconductor (SFS) voltage-biased junction in the regime of
essential proximity effect and calculate the current flowing through the probe
electrode tunnel coupled to the ferromagnetic interlayer region. It is shown
that while the polarization of the dc current component is generally small in
case of weak exchange field of the ferromagnet, there is an ac component of the
current in the system. This ac current is highly spin-polarized and entirely
originated from the non-equilibrium proximity effect in the interlayer. The
frequency of the current is controlled by the voltage applied to SFS junction.
We discuss a possibility to obtain a source of coherent ac currents with a
certain phase shift between them by tunnel coupling two probe electrodes at
different locations of the interlayer region.Comment: 8 pages, 5 figure
Magnetic breakdown in a normal-metal - superconductor proximity sandwich
We study the magnetic response of a clean normal-metal slab of finite
thickness in proximity with a bulk superconductor. We determine its free energy
and identify two (meta-)stable states, a diamagnetic one where the applied
field is effectively screened, and a second state, where the field penetrates
the normal-metal layer. We present a complete characterization of the first
order transition between the two states which occurs at the breakdown field,
including its spinodals, the jump in the magnetization, and the latent heat.
The bistable regime terminates at a critical temperature above which the sharp
transition is replaced by a continuous cross-over. We compare the theory with
experiments on normal-superconducting cylinders.Comment: 7 pages Revtex, 3 Postscript figures, needs psfig.te
The Bak-Tang-Wiesenfeld sandpile model around the upper critical dimension
We consider the Bak-Tang-Wiesenfeld sandpile model on square lattices in
different dimensions (D>=6). A finite size scaling analysis of the avalanche
probability distributions yields the values of the distribution exponents, the
dynamical exponent, and the dimension of the avalanches. Above the upper
critical dimension D_u=4 the exponents equal the known mean field values. An
analysis of the area probability distributions indicates that the avalanches
are fractal above the critical dimension.Comment: 7 pages, including 9 figures, accepted for publication in Physical
Review
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