460 research outputs found
The Potential of Restarts for ProbSAT
This work analyses the potential of restarts for probSAT, a quite successful
algorithm for k-SAT, by estimating its runtime distributions on random 3-SAT
instances that are close to the phase transition. We estimate an optimal
restart time from empirical data, reaching a potential speedup factor of 1.39.
Calculating restart times from fitted probability distributions reduces this
factor to a maximum of 1.30. A spin-off result is that the Weibull distribution
approximates the runtime distribution for over 93% of the used instances well.
A machine learning pipeline is presented to compute a restart time for a
fixed-cutoff strategy to exploit this potential. The main components of the
pipeline are a random forest for determining the distribution type and a neural
network for the distribution's parameters. ProbSAT performs statistically
significantly better than Luby's restart strategy and the policy without
restarts when using the presented approach. The structure is particularly
advantageous on hard problems.Comment: Eurocast 201
Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets
In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are
non-planar vortices which exhibit a localized structure of the z-components of
the spins around the vortex center. We study how thermal noise induces a
transition of this structure from one polarization to the opposite one. We
describe the vortex core by a discrete Hamiltonian and consider a stationary
solution of the Fokker-Planck equation. We find a bimodal distribution function
and calculate the transition rate using Langer's instanton theory (1969). The
result is compared with Langevin dynamics simulations for the full many-spin
model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres
Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)
The transport properties of high-performance thin-film transistors (TFT) made
with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The
room-temperature field-effect mobility of the devices varied between 0.004
cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the
material, which modified the structural order. The transport properties of TFTs
were studied as a function of temperature. The field-effect mobility is
thermally activated in all films at T<200 K and the activation energy depends
on the charge density in the channel. The experimental data is compared to
theoretical models for transport, and we argue that a model based on the
existence of a mobility edge and an exponential distribution of traps provides
the best interpretation of the data. The differences in room-temperature
mobility are attributed to different widths of the shallow localized state
distribution at the edge of the valence band due to structural disorder in the
film. The free carrier mobility of the mobile states in the ordered regions of
the film is the same in all structural modifications and is estimated to be
between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure
Evidence for Different Freeze-Out Radii of High- and Low-Energy Pions Emitted in Au+Au Collisions at 1 GeV/nucleon
Double differential production cross sections of negative and positive pions
and the number of participating protons have been measured in central Au+Au
collisions at 1 GeV per nucleon incident energy. At low pion energies the pi^-
yield is strongly enhanced over the pi^+ yield. The energy dependence of the
pi^-/pi^+ ratio is assigned to the Coulomb interaction of the charged pions
with the protons in the reaction zone. The deduced Coulomb potential increases
with increasing pion c.m. energy. This behavior indicates different freeze-out
radii for different pion energies in the c.m.~frame.Comment: IKDA is the Institute for Nuclear Physics in Darmstadt/German
Zwischen Hang und Aue - Kohlenstoffdynamik im Einzugsgebiet des Otterbach
Die Nutzung und Umgestaltung der Landschaft durch den Menschen fĂŒhrt seit dem Neolithikum dazu, dass Umlagerungsprozesse z.B. von kohlenstoffhaltigen Sedimenten ausgelöst werden. In solchen dynamischen Landschaften ist der organische Kohlenstoff im Boden (SOC) sehr heterogen verteilt und zum Teil unter mĂ€chtigen Sedimentauflagen begraben. Ziel der Untersuchung war es herauszufinden, wo genau sich der SOC innerhalb eines Landschaftsausschnitts akkumuliert und wie sich einzelne GelĂ€ndepositionen voneinander hinsichtlich ihrer Kohlenstoffeigenschaften unterscheiden. Das Untersuchungsgebiet befindet sich in den AuslĂ€ufern des Bayerischen Waldes. Innerhalb des Einzugsgebietes des Otterbachs, einem TributĂ€r der Donau, wurden drei verschiedene GelĂ€ndepositionen beprobt: a) Unterhang, b) HangfuĂ und c) Aue. Innerhalb der jeweiligen GelĂ€ndepositionen wurden mehrere Profile angelegt, um die kleinrĂ€umige HeterogenitĂ€t der Böden abzubilden. Die Profile wurden horizontbezogen bis in eine Tiefe von 150 cm beprobt und auf ihre Lagerungsdichte, Kohlenstoffmenge (TC, IC und OC) und Textur analysiert. AnschlieĂend wurden Kohlenstoffgehalte und -vorrĂ€te berechnet. Eine zweistufige physikalische Dichtefraktionierung in Natriumpolywolframatlösung (1.8 g cm-Âł und 2.4 g cm-Âł) wurde angewandt, um die Anteile der verschiedenen Fraktionen der organischen Substanz am Gesamtkohlenstoffgehalt zu ermitteln. Die Analyse der chemischen Zusammensetzung der Fraktionen mit Kernspinresonanzspektroskopie (NMR) sowie ihres Alters mit Radiokohlenstoffdatierung (AMS C-14) ermöglichte eine genaue qualitative Differenzierung der organischen Bodensubstanz und lieĂ RĂŒckschlĂŒsse auf ihre StabilitĂ€t und ihren Abbaugrad zu. Durch die Kombination der Verfahren konnte ein genaues Bild der Verteilung des SOC in einem Landschaftsausschnitt gezeichnet und seine QualitĂ€t detailliert beschrieben werden. In den Ergebnissen zeigt sich, dass die mit rund 1000 Jahren relativ jungen Auenböden eine besondere Rolle bei der Speicherung von SOC spielen. Diese weisen signifikant höhere SOC VorrĂ€te auf als die Profile im Akkumulationsbereich des HangfuĂes. Auch verteilen sich die VorrĂ€te in den Auenprofilen ĂŒber die gesamte Profiltiefe. Im Unterhang und im HangfuĂ kann eine solche Verteilung nicht nachgewiesen werden, hier ist der GroĂteil des Kohlenstoffs nur in den obersten 30 cm gespeichert
Magnetic Vortex Core Reversal by Excitation of Spin Waves
Micron-sized magnetic platelets in the flux closed vortex state are
characterized by an in-plane curling magnetization and a nanometer-sized
perpendicularly magnetized vortex core. Having the simplest non-trivial
configuration, these objects are of general interest to micromagnetics and may
offer new routes for spintronics applications. Essential progress in the
understanding of nonlinear vortex dynamics was achieved when low-field core
toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was
established. At frequencies more than an order of magnitude higher vortex state
structures possess spin wave eigenmodes arising from the magneto-static
interaction. Here we demonstrate experimentally that the unidirectional vortex
core reversal process also occurs when such azimuthal modes are excited. These
results are confirmed by micromagnetic simulations which clearly show the
selection rules for this novel reversal mechanism. Our analysis reveals that
for spin wave excitation the concept of a critical velocity as the switching
condition has to be modified.Comment: Minor corrections and polishing of previous versio
QuiKey â An Efficient Semantic Command Line
Abstract. QuiKey is an interaction approach that offers interactive fine grained access to structured information sources in a light weight user interface. It is designed to be highly interaction efficient for searching, browsing and authoring semantic knowledge bases as well as incremen-tally constructing complex queries. Empirical evaluation using a compar-ative GOMS Analysis and a user study confirm interaction efficiency.
Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide
We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]
- âŠ