88,905 research outputs found
Vortex glass transition in a random pinning model
We study the vortex glass transition in disordered high temperature
superconductors using Monte Carlo simulations. We use a random pinning model
with strong point-correlated quenched disorder, a net applied magnetic field,
longrange vortex interactions, and periodic boundary conditions. From a finite
size scaling study of the helicity modulus, the RMS current, and the
resistivity, we obtain critical exponents at the phase transition. The new
exponents differ substantially from those of the gauge glass model, but are
consistent with those of the pure three-dimensional XY model.Comment: 7 pages RevTeX, 4 eps figure
Screened empirical bond-order potentials for Si-C
Typical empirical bond-order potentials are short ranged and give ductile
instead of brittle behavior for materials such as crystalline silicon or
diamond. Screening functions can be used to increase the range of these
potentials. We outline a general procedure to combine screening functions with
bond-order potentials that does not require to refit any of the potential's
properties. We use this approach to modify Tersoff's [Phys. Rev. B 39, 5566
(1989)], Erhart & Albe's [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.'s
[Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting
potential formulations correctly reproduce brittle materials response, and give
an improved description of amorphous phases
Attosecond screening dynamics mediated by electron-localization
Transition metals with their densely confined and strongly coupled valence
electrons are key constituents of many materials with unconventional
properties, such as high-Tc superconductors, Mott insulators and
transition-metal dichalcogenides. Strong electron interaction offers a fast and
efficient lever to manipulate their properties with light, creating promising
potential for next-generation electronics. However, the underlying dynamics is
a fast and intricate interplay of polarization and screening effects, which is
poorly understood. It is hidden below the femtosecond timescale of electronic
thermalization, which follows the light-induced excitation. Here, we
investigate the many-body electron dynamics in transition metals before
thermalization sets in. We combine the sensitivity of intra-shell transitions
to screening effects with attosecond time resolution to uncover the interplay
of photo-absorption and screening. First-principles time-dependent calculations
allow us to assign our experimental observations to ultrafast electronic
localization on d-orbitals. The latter modifies the whole electronic structure
as well as the collective dynamic response of the system on a timescale much
faster than the light-field cycle. Our results demonstrate a possibility for
steering the electronic properties of solids prior to electron thermalization,
suggesting that the ultimate speed of electronic phase transitions is limited
only by the duration of the controlling laser pulse. Furthermore, external
control of the local electronic density serves as a fine tool for testing
state-of-the art models of electron-electron interactions. We anticipate our
study to facilitate further investigations of electronic phase transitions,
laser-metal interactions and photo-absorption in correlated electron systems on
its natural timescale
Attraction between DNA molecules mediated by multivalent ions
The effective force between two parallel DNA molecules is calculated as a
function of their mutual separation for different valencies of counter- and
salt ions and different salt concentrations. Computer simulations of the
primitive model are used and the shape of the DNA molecules is accurately
modelled using different geometrical shapes. We find that multivalent ions
induce a significant attraction between the DNA molecules whose strength can be
tuned by the averaged valency of the ions. The physical origin of the
attraction is traced back either to electrostatics or to entropic
contributions. For multivalent counter- and monovalent salt ions, we find a
salt-induced stabilization effect: the force is first attractive but gets
repulsive for increasing salt concentration. Furthermore, we show that the
multivalent-ion-induced attraction does not necessarily correlate with DNA
overcharging.Comment: 51 pages and 13 figure
Finding the Important Factors in Large Discrete-Event Simulation: Sequential Bifurcation and its Applications
This contribution discusses experiments with many factors: the case study includes a simulation model with 92 factors.The experiments are guided by sequential bifurcation.This method is most efficient and effective if the true input/output behavior of the simulation model can be approximated through a first-order polynomial possibly augmented with two-factor interactions.The method is explained and illustrated through three related discrete-event simulation models.These models represent three supply chain configurations, studied for an Ericsson factory in Sweden.After simulating 21 scenarios (factor combinations) each replicated five times to account for noise a shortlist with the 11 most important factors is identified for the biggest of the three simulation models.simulation;bifurcation;supply;Sweden
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