3,207 research outputs found
Optical guiding in meter-scale plasma waveguides
We demonstrate a new highly tunable technique for generating meter-scale low
density plasma waveguides. Such guides can enable electron acceleration to tens
of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by
optical field ionization induced by two time-separated Bessel beam pulses: The
first pulse, a J_0 beam, generates the core of the waveguide, while the delayed
second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We
demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths
with on axis plasma densities as low as N_e0=5x10^16 cm^-3
Multiple Invaded Consolidating Materials
We study a multiple invasion model to simulate corrosion or intrusion
processes. Estimated values for the fractal dimension of the invaded region
reveal that the critical exponents vary as function of the generation number
, i.e., with the number of times the invasion process takes place. The
averaged mass of the invaded region decreases with a power-law as a
function of , , where the exponent . We
also find that the fractal dimension of the invaded cluster changes from
to . This result confirms that the
multiple invasion process follows a continuous transition from one universality
class (NTIP) to another (optimal path). In addition, we report extensive
numerical simulations that indicate that the mass distribution of avalanches
has a power-law behavior and we find that the exponent
governing the power-law changes continuously as a
function of the parameter . We propose a scaling law for the mass
distribution of avalanches for different number of generations .Comment: 8 pages and 16 figure
Roughening of Fracture Surfaces: the Role of Plastic Deformations
Post mortem analysis of fracture surfaces of ductile and brittle materials on
the m-mm and the nm scales respectively, reveal self affine graphs with an
anomalous scaling exponent . Attempts to use elasticity
theory to explain this result failed, yielding exponent up
to logarithms. We show that when the cracks propagate via plastic void
formations in front of the tip, followed by void coalescence, the voids
positions are positively correlated to yield exponents higher than 0.5.Comment: 4 pages, 6 figure
Fracture Surfaces as Multiscaling Graphs
Fracture paths in quasi-two-dimenisonal (2D) media (e.g thin layers of
materials, paper) are analyzed as self-affine graphs of height as a
function of length . We show that these are multiscaling, in the sense that
order moments of the height fluctuations across any distance
scale with a characteristic exponent that depends nonlinearly on the order of
the moment. Having demonstrated this, one rules out a widely held conjecture
that fracture in 2D belongs to the universality class of directed polymers in
random media. In fact, 2D fracture does not belong to any of the known kinetic
roughening models. The presence of multiscaling offers a stringent test for any
theoretical model; we show that a recently introduced model of quasi-static
fracture passes this test.Comment: 4 pages, 5 figure
Stretched exponentials and power laws in granular avalanching
We introduce a model for granular avalanching which exhibits both stretched exponential and power law avalanching over its parameter range. Two modes of transport are incorporated, a rolling layer consisting of individual particles and the overdamped, sliding motion of particle clusters. The crossover in behaviour observed in experiments on piles of rice is attributed to a change in the dominant mode of transport. We predict that power law avalanching will be observed whenever surface flow is dominated by clustered motion.
Comment: 8 pages, more concise and some points clarified
Universal quantum computation by discontinuous quantum walk
Quantum walks are the quantum-mechanical analog of random walks, in which a
quantum `walker' evolves between initial and final states by traversing the
edges of a graph, either in discrete steps from node to node or via continuous
evolution under the Hamiltonian furnished by the adjacency matrix of the graph.
We present a hybrid scheme for universal quantum computation in which a quantum
walker takes discrete steps of continuous evolution. This `discontinuous'
quantum walk employs perfect quantum state transfer between two nodes of
specific subgraphs chosen to implement a universal gate set, thereby ensuring
unitary evolution without requiring the introduction of an ancillary coin
space. The run time is linear in the number of simulated qubits and gates. The
scheme allows multiple runs of the algorithm to be executed almost
simultaneously by starting walkers one timestep apart.Comment: 7 pages, revte
Skeleton and fractal scaling in complex networks
We find that the fractal scaling in a class of scale-free networks originates
from the underlying tree structure called skeleton, a special type of spanning
tree based on the edge betweenness centrality. The fractal skeleton has the
property of the critical branching tree. The original fractal networks are
viewed as a fractal skeleton dressed with local shortcuts. An in-silico model
with both the fractal scaling and the scale-invariance properties is also
constructed. The framework of fractal networks is useful in understanding the
utility and the redundancy in networked systems.Comment: 4 pages, 2 figures, final version published in PR
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