3,482 research outputs found
Shear viscosity to entropy density ratio in nuclear multifragmentation
Nuclear multifragmentation in intermediate energy heavy ion collisions has
long been associated with liquid-gas phase transition. We calculate the shear
viscosity to entropy density ratio eta/s for an equilibrated system of nucleons
and fragments produced in multifragmentation within an extended statistical
multifragmentation model. The temperature dependence of eta/s exhibits
surprisingly similar behavior as that for water. In the coexistence phase of
fragments and light particles, the ratio eta/s reaches a minimum of comparable
depth as that for water in the vicinity of the critical temperature for
liquid-gas phase transition. The effects of freeze-out volume and surface
symmetry energy on eta/s in multifragmentation are studied.Comment: 5 pages, 5 figures, to appear in PR
Parallel Batch-Dynamic Graph Connectivity
In this paper, we study batch parallel algorithms for the dynamic
connectivity problem, a fundamental problem that has received considerable
attention in the sequential setting. The most well known sequential algorithm
for dynamic connectivity is the elegant level-set algorithm of Holm, de
Lichtenberg and Thorup (HDT), which achieves amortized time per
edge insertion or deletion, and time per query. We
design a parallel batch-dynamic connectivity algorithm that is work-efficient
with respect to the HDT algorithm for small batch sizes, and is asymptotically
faster when the average batch size is sufficiently large. Given a sequence of
batched updates, where is the average batch size of all deletions, our
algorithm achieves expected amortized work per
edge insertion and deletion and depth w.h.p. Our algorithm
answers a batch of connectivity queries in expected
work and depth w.h.p. To the best of our knowledge, our algorithm
is the first parallel batch-dynamic algorithm for connectivity.Comment: This is the full version of the paper appearing in the ACM Symposium
on Parallelism in Algorithms and Architectures (SPAA), 201
Negative Interactions in Irreversible Self-Assembly
This paper explores the use of negative (i.e., repulsive) interaction the
abstract Tile Assembly Model defined by Winfree. Winfree postulated negative
interactions to be physically plausible in his Ph.D. thesis, and Reif, Sahu,
and Yin explored their power in the context of reversible attachment
operations. We explore the power of negative interactions with irreversible
attachments, and we achieve two main results. Our first result is an
impossibility theorem: after t steps of assembly, Omega(t) tiles will be
forever bound to an assembly, unable to detach. Thus negative glue strengths do
not afford unlimited power to reuse tiles. Our second result is a positive one:
we construct a set of tiles that can simulate a Turing machine with space bound
s and time bound t, while ensuring that no intermediate assembly grows larger
than O(s), rather than O(s * t) as required by the standard Turing machine
simulation with tiles
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
Quark-Hadron Phase Transitions in Viscous Early Universe
Based on hot big bang theory, the cosmological matter is conjectured to
undergo QCD phase transition(s) to hadrons, when the universe was about s old. In the present work, we study the quark-hadron phase transition, by
taking into account the effect of the bulk viscosity. We analyze the evolution
of the quantities relevant for the physical description of the early universe,
namely, the energy density , temperature , Hubble parameter and
scale factor before, during and after the phase transition. To study the
cosmological dynamics and the time evolution we use both analytical and
numerical methods. By assuming that the phase transition may be described by an
effective nucleation theory (prompt {\it first-order} phase transition), we
also consider the case where the universe evolved through a mixed phase with a
small initial supercooling and monotonically growing hadronic bubbles. The
numerical estimation of the cosmological parameters, and for instance,
makes it clear that the time evolution varies from phase to phase. As the QCD
era turns to be fairly accessible in the high-energy experiments and the
lattice QCD simulations, the QCD equation of state is very well defined. In
light of this, we introduce a systematic study of the {\it cross-over}
quark-hadron phase transition and an estimation for the time evolution of
Hubble parameter.Comment: 27 pages, 17 figures, revtex style (To appear in Phys. Rev. D). arXiv
admin note: text overlap with arXiv:gr-qc/040404
How Much do Heavy Quarks Thermalize in a Heavy Ion Collision?
We investigate the thermalization of charm quarks in high energy heavy ion
collisions. To this end, we calculate the diffusion coefficient in the
perturbative Quark Gluon Plasma and relate it to collisional energy loss and
momentum broadening. We then use these transport properties to formulate a
Langevin model for the evolution of the heavy quark spectrum in the hot medium.
The model is strictly valid in the non-relativistic limit and for all
velocities \gamma v < \alphas^{-1/2} to leading logarithm in . The
corresponding Fokker-Planck equation can be solved analytically for a Bjorken
expansion and the solution gives a simple estimate for the medium modifications
of the heavy quark spectrum as a function of the diffusion coefficient. Finally
we solve the Langevin equations numerically in a hydrodynamic simulation of the
heavy ion reaction. The results of this simulation are the medium modifications
of the charm spectrum and the expected elliptic flow as a
function of the diffusion coefficient.Comment: 34 pages, 9 figures. Inculdes a detailed comparison with Boltzmann
simulation
Discontinuous percolation transitions in real physical systems
We study discontinuous percolation transitions (PT) in the diffusion-limited
cluster aggregation model of the sol-gel transition as an example of real
physical systems, in which the number of aggregation events is regarded as the
number of bonds occupied in the system. When particles are Brownian, in which
cluster velocity depends on cluster size as with
, a larger cluster has less probability to collide with other
clusters because of its smaller mobility. Thus, the cluster is effectively more
suppressed in growth of its size. Then the giant cluster size increases
drastically by merging those suppressed clusters near the percolation
threshold, exhibiting a discontinuous PT. We also study the tricritical
behavior by controlling the parameter , and the tricritical point is
determined by introducing an asymmetric Smoluchowski equation.Comment: 5 pages, 5 figure
Phase transition from quark-meson coupling hyperonic matter to deconfined quark matter
We investigate the possibility and consequences of phase transitions from an
equation of state (EOS) describing nucleons and hyperons interacting via mean
fields of sigma, omega, and rho mesons in the recently improved quark-meson
coupling (QMC) model to an EOS describing a Fermi gas of quarks in an MIT bag.
The transition to a mixed phase of baryons and deconfined quarks, and
subsequently to a pure deconfined quark phase, is described using the method of
Glendenning. The overall EOS for the three phases is calculated for various
scenarios and used to calculate stellar solutions using the
Tolman-Oppenheimer-Volkoff equations. The results are compared with recent
experimental data, and the validity of each case is discussed with consequences
for determining the species content of the interior of neutron stars.Comment: 12 pages, 14 figures; minor typos correcte
Collective pinning of imperfect vortex lattices by material line defects in extreme type-II superconductors
The critical current density shown by a superconductor at the extreme type-II
limit is predicted to follow an inverse square-root power law with external
magnetic field if the vortex lattice is weakly pinned by material line defects.
It acquires an additional inverse dependence with thickness along the line
direction once pinning of the interstitial vortex lines by material point
defects is included. Moderate quantitative agreement with the critical current
density shown by second-generation wires of high-temperature superconductors in
kG magnetic fields is achieved at liquid-nitrogen temperature.Comment: 10 pages, 3 figures, 2 tables. To appear in Physical Review
BLITZEN: A highly integrated massively parallel machine
The architecture and VLSI design of a new massively parallel processing array chip are described. The BLITZEN processing element array chip, which contains 1.1 million transistors, serves as the basis for a highly integrated, miniaturized, high-performance, massively parallel machine that is currently under development. Each processing element has 1K bits of static RAM and performs bit-serial processing with functional elements for arithmetic, logic, and shifting
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