797 research outputs found
Effective grain surface area in the formation of molecular hydrogen in interstellar clouds
In the interstellar clouds, molecular hydrogens are formed from atomic
hydrogen on grain surfaces. An atomic hydrogen hops around till it finds
another one with which it combines. This necessarily implies that the average
recombination time, or equivalently, the effective grain surface area depends
on the relative numbers of atomic hydrogen influx rate and the number of sites
on the grain. Our aim is to discover this dependency. We perform a numerical
simulation to study the recombination of hydrogen on grain surfaces in a
variety of cloud conditions. We use a square lattice (with a periodic boundary
condition) of various sizes on two types of grains, namely, amorphous carbon
and olivine. We find that the steady state results of our simulation match very
well with those obtained from a simpler analytical consideration provided the
`effective' grain surface area is written as , where, is
the actual physical grain area and is a function of the flux of atomic
hydrogen which is determined from our simulation. We carry out the simulation
for various astrophysically relevant accretion rates. For high accretion rates,
small grains tend to become partly saturated with and and the
subsequent accretion will be partly inhibited. For very low accretion rates,
the number of sites to be swept before a molecular hydrogen can form is too
large compared to the actual number of sites on the grain, implying that
is greater than unity.Comment: 8 pages, 5 figures in eps forma
Dynamic transitions and hysteresis
When an interacting many-body system, such as a magnet, is driven in time by
an external perturbation, such as a magnetic field,the system cannot respond
instantaneously due to relaxational delay. The response of such a system under
a time-dependent field leads to many novel physical phenomena with intriguing
physics and important technological applications. For oscillating fields, one
obtains hysteresis that would not occur under quasistatic conditions in the
presence of thermal fluctuations. Under some extreme conditions of the driving
field, one can also obtain a non-zero average value of the variable undergoing
such dynamic hysteresis. This non-zero value indicates a breaking of symmetry
of the hysteresis loop, around the origin. Such a transition to the
spontaneously broken symmetric phase occurs dynamically when the driving
frequency of the field increases beyond its threshold value which depends on
the field amplitude and the temperature. Similar dynamic transitions also occur
for pulsed and stochastically varying fields. We present an overview of the
ongoing researches in this not-so-old field of dynamic hysteresis and
transitions.Comment: 30 Pages Revtex, 10 Postscript figures. To appear in Reviews of
Modern Physics, April, 199
QPOs from Radial and Vertical Oscillation of Shocks in Advective Accretion Flows
We present results of several numerical simulations of two dimensional
advective flows which include cooling processes. We show that the computed
light curve is similar to the state in GRS 1915+105. The power density
spectrum (PDS) also shows presence of QPOs near the break frequency.Comment: 4 pages, 2 figures To be published in the Proceedings of 10th Marcel
Grossman Meeting, Ed. R. Ruffini et al. (World Scientific: Singapore
Dynamic Response of Ising System to a Pulsed Field
The dynamical response to a pulsed magnetic field has been studied here both
using Monte Carlo simulation and by solving numerically the meanfield dynamical
equation of motion for the Ising model. The ratio R_p of the response
magnetisation half-width to the width of the external field pulse has been
observed to diverge and pulse susceptibility \chi_p (ratio of the response
magnetisation peak height and the pulse height) gives a peak near the
order-disorder transition temperature T_c (for the unperturbed system). The
Monte Carlo results for Ising system on square lattice show that R_p diverges
at T_c, with the exponent , while \chi_p shows a peak at
, which is a function of the field pulse width . A finite size
(in time) scaling analysis shows that , with
. The meanfield results show that both the divergence of R
and the peak in \chi_p occur at the meanfield transition temperature, while the
peak height in , for small values of
. These results also compare well with an approximate analytical
solution of the meanfield equation of motion.Comment: Revtex, Eight encapsulated postscript figures, submitted to Phys.
Rev.
Spin-Reversal Transition in Ising Model under Pulsed Field
In this communication we report the existence of a dynamic ``spin-reversal''
transition in an Ising system perturbed by a pulsed external magnetic field.
The transition is achieved by tuning the strength () and/or the duration
() of the pulse which is applied in a direction opposite to the
existing order. We have studied this transition in the kinetic Ising Model in
two dimension using Monte Carlo technique, and solved numerically the mean
field equation of motion. The transition is essentially dynamic in nature and
it takes the system from one ordered equilibrium phase to another by means of
the growth of opposite spin domains (in the kinetic Ising case) induced during
the period when the pulsed field is applied.Comment: 19 pages, Latex, 6 eps figures, to appear in Physica A Subject-Class:
Statistical Physic
Dynamic Magnetization-Reversal Transition in the Ising Model
We report the results of mean field and the Monte Carlo study of the dynamic
magnetization-reversal transition in the Ising model, brought about by the
application of an external field pulse applied in opposition to the existing
order before the application of the pulse. The transition occurs at a
temperature T below the static critical temperature T_c without any external
field. The transition occurs when the system, perturbed by the external field
pulse competing with the existing order, jumps from one minimum of free energy
to the other after the withdrawal of the pulse. The parameters controlling the
transition are the strength h_p and the duration Delta t of the pulse. In the
mean field case, approximate analytical expression is obtained for the phase
boundary which agrees well with that obtained numerically in the small Delta t
and large T limit. The order parameter of the transition has been identified
and is observed to vary continuously near the transition. The order parameter
exponent beta was estimated both for the mean field (beta =1) and the Monte
Carlo beta = 0.90 \pm 0.02 in two dimension) cases. The transition shows a
"critical slowing-down" type behaviour near the phase boundary with diverging
relaxation time. The divergence was found to be logarithmic in the mean field
case and exponential in the Monte Carlo case. The finite size scaling technique
was employed to estimate the correlation length exponent nu (= 1.5 \pm 0.3 in
two dimension) in the Monte Carlo case.Comment: 13 pages, latex, 8 figure
Noninteracting Fermions in infinite dimensions
Usually, we study the statistical behaviours of noninteracting Fermions in
finite (mainly two and three) dimensions. For a fixed number of fermions, the
average energy per fermion is calculated in two and in three dimensions and it
becomes equal to 50 and 60 per cent of the fermi energy respectively. However,
in the higher dimensions this percentage increases as the dimensionality
increases and in infinite dimensions it becomes 100 per cent. This is an
intersting result, at least pedagogically. Which implies all fermions are
moving with Fermi momentum. This result is not yet discussed in standard text
books of quantum statistics. In this paper, this fact is discussed and
explained. I hope, this article will be helpful for graduate students to study
the behaviours of free fermions in generalised dimensionality.Comment: To appear in European Journal of Physics (2010
Growth of Breakdown Susceptibility in Random Composites and BTW Model : Prediction of Dielectric Breakdown and Other Catastrophes
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