6,065 research outputs found
Monte Carlo Renormalization Group Study of the d=1 XXZ Model
We report current progress on the synthesis of methods to alleviate two major
difficulties in implementing a Monte Carlo Renormalization Group (MCRG) for
quantum systems. In particular, we have utilized the loop-algorithm to reduce
critical slowing down, and we have implemented an MCRG method in which the
symmetries of the classical equivalent model need not be fully understood,
since the Renormalization Group is given by the Monte Carlo simulation. We
report preliminary results obtained when the resulting MCRG method is applied
to the d=1 XXZ model. Our results are encouraging. However, since this model
has a Kosterlitz-Thouless transition, it does not yet provide a full test of
our MCRG method.Comment: To appear in "Quantum Monte Carlo Methods in Condensed Matter
Physics", ed.\ M. Suzuki, World Scientific, 1993. 14 pages, LaTeX, (3 figures
available on request), FSU-SCRI-93-11
Extreme Long-time Dynamic Monte Carlo Simulations
We study the extreme long-time behavior of the metastable phase of the
three-dimensional Ising model with Glauber dynamics in an applied magnetic
field and at a temperature below the critical temperature. For these
simulations we use the advanced simulation method of projective dynamics. The
algorithm is described in detail, together with its application to the escape
from the metastable state. Our results for the field dependence of the
metastable lifetime are in good agreement with theoretical expectations and
span more than fifty decades in time.Comment: 13 pages with embedded eps figures. Int. J. Mod. Phys. C, in pres
Development of a meteoroid penetration distributed transducer Third quarterly report
Impact calibration tests in development of meteoroid penetration distributed transduce
A projection method for statics and dynamics of lattice spin systems
A method based on Monte Carlo sampling of the probability flows projected
onto the subspace of one or more slow variables is proposed for investigation
of dynamic and static properties of lattice spin systems. We illustrate the
method by applying it, with projection onto the order-parameter subspace, to
the three-dimensional 3-state Potts model in equilibrium and to metastable
decay in a three-dimensional 3-state kinetic Potts model.Comment: 4 pages, 3 figures, RevTex, final version to appear in Phys. Rev.
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Going through Rough Times: from Non-Equilibrium Surface Growth to Algorithmic Scalability
Efficient and faithful parallel simulation of large asynchronous systems is a
challenging computational problem. It requires using the concept of local
simulated times and a synchronization scheme. We study the scalability of
massively parallel algorithms for discrete-event simulations which employ
conservative synchronization to enforce causality. We do this by looking at the
simulated time horizon as a complex evolving system, and we identify its
universal characteristics. We find that the time horizon for the conservative
parallel discrete-event simulation scheme exhibits Kardar-Parisi-Zhang-like
kinetic roughening. This implies that the algorithm is asymptotically scalable
in the sense that the average progress rate of the simulation approaches a
non-zero constant. It also implies, however, that there are diverging memory
requirements associated with such schemes.Comment: to appear in the Proceedings of the MRS, Fall 200
Suppressing Roughness of Virtual Times in Parallel Discrete-Event Simulations
In a parallel discrete-event simulation (PDES) scheme, tasks are distributed
among processing elements (PEs), whose progress is controlled by a
synchronization scheme. For lattice systems with short-range interactions, the
progress of the conservative PDES scheme is governed by the Kardar-Parisi-Zhang
equation from the theory of non-equilibrium surface growth. Although the
simulated (virtual) times of the PEs progress at a nonzero rate, their standard
deviation (spread) diverges with the number of PEs, hindering efficient data
collection. We show that weak random interactions among the PEs can make this
spread nondivergent. The PEs then progress at a nonzero, near-uniform rate
without requiring global synchronizations
Two Modes of Magnetization Switching in a Simulated Iron Nanopillar in an Obliquely Oriented Field
Finite-temperature micromagnetics simulations are employed to study the
magnetization-switching dynamics driven by a field applied at an angle to the
long axis of an iron nanopillar. A bi-modal distribution in the switching times
is observed, and evidence for two competing modes of magnetization-switching
dynamics is presented. For the conditions studied here, temperature K
and the reversal field 3160 Oe at an angle of 75 to the long axis,
approximately 70% of the switches involve unstable decay (no free-energy
barrier) and 30% involve metastable decay (a free-energy barrier is crossed).
The latter are indistinguishable from switches which are constrained to start
at a metastable free-energy minimum. Competition between unstable and
metastable decay could greatly complicate applications involving magnetization
switches near the coercive field.Comment: 19 pages, 7 figure
Magnetization switching in nanoscale ferromagnetic grains: simulations with heterogeneous nucleation
We present results obtained with various types of heterogeneous nucleation in
a kinetic Ising model of magnetization switching in single-domain ferromagnetic
nanoparticles. We investigate the effect of the presence of the system boundary
and make comparison with simulations on periodic lattices. We also study
systems with bulk disorder and compare how two different types of disorder
influence the switching behavior.Comment: 3 pages, 4 Postscript figure
Quantum Decoherence at Finite Temperatures
We study measures of decoherence and thermalization of a quantum system
in the presence of a quantum environment (bath) . The whole system is
prepared in a canonical thermal state at a finite temperature. Applying
perturbation theory with respect to the system-environment coupling strength,
we find that under common Hamiltonian symmetries, up to first order in the
coupling strength it is sufficient to consider the uncoupled system to predict
decoherence and thermalization measures of . This decoupling allows closed
form expressions for perturbative expansions for the measures of decoherence
and thermalization in terms of the free energies of and of . Numerical
results for both coupled and decoupled systems with up to 40 quantum spins
validate these findings.Comment: 5 pages, 3 figure
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