3,472 research outputs found
Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs
Pebble games are single-player games on DAGs involving placing and moving
pebbles on nodes of the graph according to a certain set of rules. The goal is
to pebble a set of target nodes using a minimum number of pebbles. In this
paper, we present a possibly simpler proof of the result in [CLNV15] and
strengthen the result to show that it is PSPACE-hard to determine the minimum
number of pebbles to an additive term for all , which improves upon the currently known additive constant hardness of
approximation [CLNV15] in the standard pebble game. We also introduce a family
of explicit, constant indegree graphs with nodes where there exists a graph
in the family such that using constant pebbles requires moves
to pebble in both the standard and black-white pebble games. This independently
answers an open question summarized in [Nor15] of whether a family of DAGs
exists that meets the upper bound of moves using constant pebbles
with a different construction than that presented in [AdRNV17].Comment: Preliminary version in WADS 201
Mini-Computer Control of Electrical Energy Demand
Electrical power demand control results in huge dollar savings for the medium to large user. Meramec Mining Company realizes an average 150,000.00 monthly charge because of demand control efforts.
This paper discusses the planning, designing, implementing, and auditing of the mini-computer system at Meramec Mining Company. No detailed discussion of the actual computer coding is included
Crosstalk between nanotube devices: contact and channel effects
At reduced dimensionality, Coulomb interactions play a crucial role in
determining device properties. While such interactions within the same carbon
nanotube have been shown to have unexpected properties, device integration and
multi-nanotube devices require the consideration of inter-nanotube
interactions. We present calculations of the characteristics of planar carbon
nanotube transistors including interactions between semiconducting nanotubes
and between semiconducting and metallic nanotubes. The results indicate that
inter-tube interactions affect both the channel behavior and the contacts. For
long channel devices, a separation of the order of the gate oxide thickness is
necessary to eliminate inter-nanotube effects. Because of an exponential
dependence of this length scale on dielectric constant, very high device
densities are possible by using high-k dielectrics and embedded contacts
Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations
The antivortex is a fundamental magnetization structure which is the
topological counterpart of the well-known magnetic vortex. We study here the
ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy
thin-film element. Using micromagnetic simulations we predict that the
antivortex response to an ultrashort external field pulse is characterized by
the production of a new antivortex as well as of a temporary vortex, followed
by an annihilation process. These processes are complementary to the recently
reported response of a vortex and, like for the vortex, lead to the reversal of
the orientation of the antivortex core region. In addition to its fundamental
interest, this dynamic magnetization process could be used for the generation
and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R
The magnetoelectrochemical switch
In the field of spintronics, the archetype solid-state two-terminal device is the spin valve, where the resistance is controlled by the magnetization configuration. We show here how this concept of spin-dependent switch can be extended to magnetic electrodes in solution, by magnetic control of their chemical environment. Appropriate nanoscale design allows a huge enhancement of the magnetic force field experienced by paramagnetic molecular species in solutions, which changes between repulsive and attractive on changing the electrodes' magnetic orientations. Specifically, the field gradient force created within a sub-100-nm-sized nanogap separating two magnetic electrodes can be reversed by changing the orientation of the electrodes' magnetization relative to the current flowing between the electrodes. This can result in a breaking or making of an electric nanocontact, with a change of resistance by a factor of up to 103. The results reveal how an external field can impact chemical equilibrium in the vicinity of nanoscale magnetic circuits
Angular-dependence of magnetization switching for a multi-domain dot: experiment and simulation
We have measured the in-plane angular variation of nucleation and
annihilation fields of a multi-domain magnetic single dot with a microsquid.
The dots are Fe/Mo(110) self-assembled in UHV, with sub-micron size and a
hexagonal shape. The angular variations were quantitatively reproduced by
micromagnetic simulations. Discontinuities in the variations are observed, and
shown to result from bifurcations related to the interplay of the non-uniform
magnetization state with the shape of the dot.Comment: 4 pages, 4 figures, for submission as a regular articl
Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands
We have investigated three-dimensional magnetization structures in numerous
mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism
combined with photoemission electron microscopy (XMCD-PEEM). The particles are
epitaxial islands with an elongated hexagonal shape with length of up to 2.5
micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal
asymmetric magnetization distributions at the surface of these particles.
Micromagnetic simulations are in excellent agreement with the observed magnetic
structures and provide information on the internal structure of the
magnetization which is not accessible in the experiment. It is shown that the
magnetization is influenced mostly by the particle size and thickness rather
than by the details of its shape. Hence, these hexagonal samples can be
regarded as model systems for the study of the magnetization in thick,
mesoscopic ferromagnets.Comment: 12 pages, 11 figure
- …