13,224 research outputs found
Large cone angle magnetization precession of an individual nanomagnet with dc electrical detection
We demonstrate on-chip resonant driving of large cone-angle magnetization
precession of an individual nanoscale permalloy element. Strong driving is
realized by locating the element in close proximity to the shorted end of a
coplanar strip waveguide, which generates a microwave magnetic field. We used a
microwave frequency modulation method to accurately measure resonant changes of
the dc anisotropic magnetoresistance. Precession cone angles up to are
determined with better than one degree of resolution. The resonance peak shape
is well-described by the Landau-Lifshitz-Gilbert equation
A crossing probability for critical percolation in two dimensions
Langlands et al. considered two crossing probabilities, pi_h and pi_{hv}, in
their extensive numerical investigations of critical percolation in two
dimensions. Cardy was able to find the exact form of pi_h by treating it as a
correlation function of boundary operators in the Q goes to 1 limit of the Q
state Potts model. We extend his results to find an analogous formula for
pi_{hv} which compares very well with the numerical results.Comment: 8 pages, Latex2e, 1 figure, uuencoded compressed tar file, (1 typo
changed
Null vectors, 3-point and 4-point functions in conformal field theory
We consider 3-point and 4-point correlation functions in a conformal field
theory with a W-algebra symmetry. Whereas in a theory with only Virasoro
symmetry the three point functions of descendants fields are uniquely
determined by the three point function of the corresponding primary fields this
is not the case for a theory with algebra symmetry. The generic 3-point
functions of W-descendant fields have a countable degree of arbitrariness. We
find, however, that if one of the fields belongs to a representation with null
states that this has implications for the 3-point functions. In particular if
one of the representations is doubly-degenerate then the 3-point function is
determined up to an overall constant. We extend our analysis to 4-point
functions and find that if two of the W-primary fields are doubly degenerate
then the intermediate channels are limited to a finite set and that the
corresponding chiral blocks are determined up to an overall constant. This
corresponds to the existence of a linear differential equation for the chiral
blocks with two completely degenerate fields as has been found in the work of
Bajnok~et~al.Comment: 10 pages, LaTeX 2.09, DAMTP-93-4
Electrical detection of spin pumping: dc voltage generated by ferromagnetic resonance at ferromagnet/nonmagnet contact
We describe electrical detection of spin pumping in metallic nanostructures.
In the spin pumping effect, a precessing ferromagnet attached to a normal-metal
acts as a pump of spin-polarized current, giving rise to a spin accumulation.
The resulting spin accumulation induces a backflow of spin current into the
ferromagnet and generates a dc voltage due to the spin dependent conductivities
of the ferromagnet. The magnitude of such voltage is proportional to the
spin-relaxation properties of the normal-metal. By using platinum as a contact
material we observe, in agreement with theory, that the voltage is
significantly reduced as compared to the case when aluminum was used.
Furtheremore, the effects of rectification between the circulating rf currents
and the magnetization precession of the ferromagnet are examined. Most
significantly, we show that using an improved layout device geometry these
effects can be minimized.Comment: 9 pages, 11 figure
Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet
We report direct electrical detection of spin pumping, using a lateral normal
metal/ferromagnet/normal metal device, where a single ferromagnet in
ferromagnetic resonance pumps spin polarized electrons into the normal metal,
resulting in spin accumulation. The resulting backflow of spin current into the
ferromagnet generates a d.c. voltage due to the spin dependent conductivities
of the ferromagnet. By comparing different contact materials (Al and /or Pt),
we find, in agreement with theory, that the spin related properties of the
normal metal dictate the magnitude of the d.c. voltage
Temporal Correlations of Local Network Losses
We introduce a continuum model describing data losses in a single node of a
packet-switched network (like the Internet) which preserves the discrete nature
of the data loss process. {\em By construction}, the model has critical
behavior with a sharp transition from exponentially small to finite losses with
increasing data arrival rate. We show that such a model exhibits strong
fluctuations in the loss rate at the critical point and non-Markovian power-law
correlations in time, in spite of the Markovian character of the data arrival
process. The continuum model allows for rather general incoming data packet
distributions and can be naturally generalized to consider the buffer server
idleness statistics
Bump formation in a binary attractor neural network
This paper investigates the conditions for the formation of local bumps in
the activity of binary attractor neural networks with spatially dependent
connectivity. We show that these formations are observed when asymmetry between
the activity during the retrieval and learning is imposed. Analytical
approximation for the order parameters is derived. The corresponding phase
diagram shows a relatively large and stable region, where this effect is
observed, although the critical storage and the information capacities
drastically decrease inside that region. We demonstrate that the stability of
the network, when starting from the bump formation, is larger than the
stability when starting even from the whole pattern. Finally, we show a very
good agreement between the analytical results and the simulations performed for
different topologies of the network.Comment: about 14 page
Self-avoiding walks on scale-free networks
Several kinds of walks on complex networks are currently used to analyze
search and navigation in different systems. Many analytical and computational
results are known for random walks on such networks. Self-avoiding walks (SAWs)
are expected to be more suitable than unrestricted random walks to explore
various kinds of real-life networks. Here we study long-range properties of
random SAWs on scale-free networks, characterized by a degree distribution
. In the limit of large networks (system size ), the average number of SAWs starting from a generic site
increases as , with . For finite ,
is reduced due to the presence of loops in the network, which causes the
emergence of attrition of the paths. For kinetic growth walks, the average
maximum length, , increases as a power of the system size: , with an exponent increasing as the parameter is
raised. We discuss the dependence of on the minimum allowed degree in
the network. A similar power-law dependence is found for the mean
self-intersection length of non-reversal random walks. Simulation results
support our approximate analytical calculations.Comment: 9 pages, 7 figure
Scaling of critical connectivity of mobile ad hoc communication networks
In this paper, critical global connectivity of mobile ad hoc communication
networks (MAHCN) is investigated. We model the two-dimensional plane on which
nodes move randomly with a triangular lattice. Demanding the best communication
of the network, we account the global connectivity as a function of
occupancy of sites in the lattice by mobile nodes. Critical phenomena
of the connectivity for different transmission ranges are revealed by
numerical simulations, and these results fit well to the analysis based on the
assumption of homogeneous mixing . Scaling behavior of the connectivity is
found as , where , is
the length unit of the triangular lattice and is the scaling index in
the universal function . The model serves as a sort of site percolation
on dynamic complex networks relative to geometric distance. Moreover, near each
critical corresponding to certain transmission range , there
exists a cut-off degree below which the clustering coefficient of such
self-organized networks keeps a constant while the averaged nearest neighbor
degree exhibits a unique linear variation with the degree k, which may be
useful to the designation of real MAHCN.Comment: 6 pages, 6 figure
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