7 research outputs found
Single-particle model for a granular ratchet
A simple model for a granular ratchet corresponding to a single grain
bouncing off a vertically vibrating sawtooth-like base is studied. Depending on
the vibration strength, the sawtooth roughness and the restitution coefficient,
horizontal transport in both the preferred and unfavoured directions is
observed. A phase diagram indicating the regions in parameter space where each
of the three possible regimes (no current, normal current, and current
reversal) occurs is presented.Comment: 7 pages, 3 figures, submitted to Physica
Vortex Lines or Vortex-Line Chains at the Lower Critical Field in Anisotropic Superconductors?
The vortex state at the lower critical field, H_{c1}, in clean anisotropic
superconductors placed in an external field tilted with respect to the axis of
anisotropy (c-axis) is considered assuming two possible arrangements: dilute
vortex-lines or dilute vortex-line chains. By minimizing the Gibbs free
energies in the London limit for each possibility we obtain the corresponding
lower critical fields as a function of the tilt angle. The equilibrium
configuration at H_{c1} for a given tilt angle is identified with that for
which H_{c1} is the smallest. We report results for parameter values typical of
strong and moderate anisotropy. We find that for strong anisotropy vortex-line
chains are favored for small tilt angles (< 7.9^o) and that at 7.9^o there is
coexistence between this configuration and a vortex-line one. For moderate
anisotropy we find that there is little difference between the vortex-line and
the vortex-chain lower critical fields.Comment: 5 pages, 4 figures, accepted to appear on Physica
Brownian Dynamics, Time-averaging and Colored Noise
We propose a method to obtain the equilibrium distribution for positions and
velocities of a one-dimensional particle via time-averaging and Laplace
transformations. We apply it to the case of a damped harmonic oscillator in
contact with a thermal bath. The present method allows us to treat, among other
cases, a Gaussian noise function exponentially correlated in time, e.g.,
Gaussian colored noise. We obtain the exact equilibrium solution and study some
of its properties.Comment: 1 figur
Numerical simulation of vibrated granular gases under realistic boundary conditions
A variant of the Direct Simulation Monte Carlo method is used to study the
behavior of a granular gas, in two and three dimensions, under varying density,
restitution coefficient, and inelasticity regimes, for realistic vibrating wall
conditions. Our results agree quite well with recent experimental data. The
role of the energy injection mechanism is discussed, as well as the behavior of
state-functions, such as pressure, under realistic boundary conditions. Upon a
density increase, we find signals of a clustering transition.Comment: Latex, 12 EPS figures (to appear in Physica A
Hydrodynamics of driven granular gases
Hydrodynamic equations for granular gases driven by the Fokker-Planck
operator are derived. Transport coefficients appeared in Navier-Stokes order
change from the values of a free cooling state to those of a steady state.Comment: 5 pages, 3 figure
Josephson vortices and solitons inside pancake vortex lattice in layered superconductors
In very anisotropic layered superconductors a tilted magnetic field generates
crossing vortex lattices of pancake and Josephson vortices (JVs). We study the
properties of an isolated JV in the lattice of pancake vortices. JV induces
deformations in the pancake vortex crystal, which, in turn, substantially
modify the JV structure. The phase field of the JV is composed of two types of
phase deformations: the regular phase and vortex phase. The phase deformations
with smaller stiffness dominate. The contribution from the vortex phase
smoothly takes over with increasing magnetic field. We find that the structure
of the cores experiences a smooth yet qualitative evolution with decrease of
the anisotropy. At large anisotropies pancakes have only small deformations
with respect to position of the ideal crystal while at smaller anisotropies the
pancake stacks in the central row smoothly transfer between the neighboring
lattice positions forming a solitonlike structure. We also find that even at
high anisotropies pancake vortices strongly pin JVs and strongly increase their
viscous friction.Comment: 22 pages, 11 figures, to appear in Phys. Rev.
Onset of fluidization in vertically shaken granular material
When granular material is shaken vertically one observes convection, surface
fluidization, spontaneous heap formation and other effects. There is a
controversial discussion in literature whether there exists a threshold for the
Froude number below which these effects cannot be
observed anymore. By means of theoretical analysis and computer simulation we
find that there is no such single threshold. Instead we propose a modified
criterion which coincides with critical Froude number for small
driving frequency .Comment: 7 pages, 5 figure