647 research outputs found
Mechanism for nonequilibrium symmetry breaking and pattern formation in magnetic films
Magnetic thin films exhibit a strong variation in properties depending on
their degree of disorder. Recent coherent x-ray speckle experiments on magnetic
films have measured the loss of correlation between configurations at opposite
fields and at the same field, upon repeated field cycling. We perform finite
temperature numerical simulations on these systems that provide a comprehensive
explanation for the experimental results. The simulations demonstrate, in
accordance with experiments, that the memory of configurations increases with
film disorder. We find that non-trivial microscopic differences exist between
the zero field spin configuration obtained by starting from a large positive
field and the zero field configuration starting at a large negative field. This
seemingly paradoxical beahvior is due to the nature of the vector spin dynamics
and is also seen in the experiments. For low disorder, there is an instability
which causes the spontaneous growth of line-like domains at a critical field,
also in accord with experiments. It is this unstable growth, which is highly
sensitive to thermal noise, that is responsible for the small correlation
between patterns under repeated cycling. The domain patterns, hysteresis loops,
and memory properties of our simulated systems match remarkably well with the
real experimental systems.Comment: 12 pages, 10 figures Added comparison of results with
cond-mat/0412461 and some more discussio
Hysteresis multicycles in nanomagnet arrays
We predict two new physical effects in arrays of single-domain nanomagnets by
performing simulations using a realistic model Hamiltonian and physical
parameters. First, we find hysteretic multicycles for such nanomagnets. The
simulation uses continuous spin dynamics through the Landau-Lifshitz-Gilbert
(LLG) equation. In some regions of parameter space, the probability of finding
a multicycle is as high as ~0.6. We find that systems with larger and more
anisotropic nanomagnets tend to display more multicycles. This result
demonstrates the importance of disorder and frustration for multicycle
behavior. We also show that there is a fundamental difference between the more
realistic vector LLG equation and scalar models of hysteresis, such as Ising
models. In the latter case, spin and external field inversion symmetry is
obeyed but in the former it is destroyed by the dynamics, with important
experimental implications.Comment: 7 pages, 2 figure
Massive Parallel Quantum Computer Simulator
We describe portable software to simulate universal quantum computers on
massive parallel computers. We illustrate the use of the simulation software by
running various quantum algorithms on different computer architectures, such as
a IBM BlueGene/L, a IBM Regatta p690+, a Hitachi SR11000/J1, a Cray X1E, a SGI
Altix 3700 and clusters of PCs running Windows XP. We study the performance of
the software by simulating quantum computers containing up to 36 qubits, using
up to 4096 processors and up to 1 TB of memory. Our results demonstrate that
the simulator exhibits nearly ideal scaling as a function of the number of
processors and suggest that the simulation software described in this paper may
also serve as benchmark for testing high-end parallel computers.Comment: To appear in Comp. Phys. Com
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