327 research outputs found
Intensity of Brillouin light scattering from spin waves in magnetic multilayers with noncollinear spin configurations: Theory and experiment
The scattering of photons from spin waves (Brillouin light scattering -- BLS)
is a well-established technique for the study of layered magnetic systems. The
information about the magnetic state and properties of the sample is contained
in the frequency position, width, and intensity of the BLS peaks. Previously
[Phys. Rev. B 67, 184404 (2003)], we have shown that spin wave frequencies can
be conveniently calculated within the ultrathin film approach, treating the
intralayer exchange as an effective bilinear interlayer coupling between thin
virtual sheets of the ferromagnetic layers. Here we give the consequent
extension of this approach to the calculation of the Brillouin light scattering
(BLS) peak intensities. Given the very close relation of the BLS cross-section
to the magneto-optic Kerr effect (MOKE), the depth-resolved longitudinal and
polar MOKE coefficients calculated numerically via the usual magneto-optic
formalism can be employed in combination with the spin wave precessional
amplitudes to calculate full BLS spectra for a given magnetic system. This
approach allows an easy calculation of BLS intensities even for noncollinear
spin configurations including the exchange modes. The formalism is applied to a
Fe/Cr/Fe/Ag/Fe trilayer system with one antiferromagnetically coupling spacer
(Cr). Good agreement with the experimental spectra is found for a wide variety
of spin configurations.Comment: 19 pages, 5 figure
Serializing the Parallelism in Parallel Communicating Pushdown Automata Systems
We consider parallel communicating pushdown automata systems (PCPA) and
define a property called known communication for it. We use this property to
prove that the power of a variant of PCPA, called returning centralized
parallel communicating pushdown automata (RCPCPA), is equivalent to that of
multi-head pushdown automata. The above result presents a new sub-class of
returning parallel communicating pushdown automata systems (RPCPA) called
simple-RPCPA and we show that it can be written as a finite intersection of
multi-head pushdown automata systems
On the Size Complexity of Non-Returning Context-Free PC Grammar Systems
Improving the previously known best bound, we show that any recursively
enumerable language can be generated with a non-returning parallel
communicating (PC) grammar system having six context-free components. We also
present a non-returning universal PC grammar system generating unary languages,
that is, a system where not only the number of components, but also the number
of productions and the number of nonterminals are limited by certain constants,
and these size parameters do not depend on the generated language
On the Shuffle Automaton Size for Words
We investigate the state size of DFAs accepting the shuffle of two words. We
provide words u and v, such that the minimal DFA for u shuffled with v requires
an exponential number of states. We also show some conditions for the words u
and v which ensure a quadratic upper bound on the state size of u shuffled with
v. Moreover, switching only two letters within one of u or v is enough to
trigger the change from quadratic to exponential
Small Universal Accepting Networks of Evolutionary Processors with Filtered Connections
In this paper, we present some results regarding the size complexity of
Accepting Networks of Evolutionary Processors with Filtered Connections
(ANEPFCs). We show that there are universal ANEPFCs of size 10, by devising a
method for simulating 2-Tag Systems. This result significantly improves the
known upper bound for the size of universal ANEPFCs which is 18.
We also propose a new, computationally and descriptionally efficient
simulation of nondeterministic Turing machines by ANEPFCs. More precisely, we
describe (informally, due to space limitations) how ANEPFCs with 16 nodes can
simulate in O(f(n)) time any nondeterministic Turing machine of time complexity
f(n). Thus the known upper bound for the number of nodes in a network
simulating an arbitrary Turing machine is decreased from 26 to 16
Cytokinetic astralogy
Division plane specification in animal cells has long been presumed to involve direct contact between microtubules of the anaphase mitotic spindle and the cell cortex. In this issue, von Dassow et al. (von Dassow et al. 2009. J. Cell. Biol. doi:10.1083/jcb.200907090) challenge this assumption by showing that spindle microtubules can effectively position the division plane at a distance from the cell cortex
Translation from Classical Two-Way Automata to Pebble Two-Way Automata
We study the relation between the standard two-way automata and more powerful
devices, namely, two-way finite automata with an additional "pebble" movable
along the input tape. Similarly as in the case of the classical two-way
machines, it is not known whether there exists a polynomial trade-off, in the
number of states, between the nondeterministic and deterministic pebble two-way
automata. However, we show that these two machine models are not independent:
if there exists a polynomial trade-off for the classical two-way automata, then
there must also exist a polynomial trade-off for the pebble two-way automata.
Thus, we have an upward collapse (or a downward separation) from the classical
two-way automata to more powerful pebble automata, still staying within the
class of regular languages. The same upward collapse holds for complementation
of nondeterministic two-way machines.
These results are obtained by showing that each pebble machine can be, by
using suitable inputs, simulated by a classical two-way automaton with a linear
number of states (and vice versa), despite the existing exponential blow-up
between the classical and pebble two-way machines
Bounded Languages Meet Cellular Automata with Sparse Communication
Cellular automata are one-dimensional arrays of interconnected interacting
finite automata. We investigate one of the weakest classes, the real-time
one-way cellular automata, and impose an additional restriction on their
inter-cell communication by bounding the number of allowed uses of the links
between cells. Moreover, we consider the devices as acceptors for bounded
languages in order to explore the borderline at which non-trivial decidability
problems of cellular automata classes become decidable. It is shown that even
devices with drastically reduced communication, that is, each two neighboring
cells may communicate only constantly often, accept bounded languages that are
not semilinear. If the number of communications is at least logarithmic in the
length of the input, several problems are undecidable. The same result is
obtained for classes where the total number of communications during a
computation is linearly bounded
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