39,119 research outputs found
Levitation of Current Carrying States in the Lattice Model for the Integer Quantum Hall Effect
The disorder driven quantum Hall to insulator transition is investigated for
a two-dimensional lattice model. The Hall conductivity and the localization
length are calculated numerically near the transition. For uncorrelated and
weakly correlated disorder potentials the current carrying states are
annihilated by the negative Chern states originating from the band center. In
the presence of correlated disorder potentials with correlation length larger
than approximately half the lattice constant the floating up of the critical
states in energy without merging is observed. This behavior is similar to the
levitation scenario proposed for the continuum model.Comment: 4 pages incl. 4 eps-figures. Published versio
Neutrino-driven wind and wind termination shock in supernova cores
The neutrino-driven wind from a nascent neutron star at the center of a
supernova expands into the earlier ejecta of the explosion. Upon collision with
this slower matter the wind material is decelerated in a wind termination
shock. By means of hydrodynamic simulations in spherical symmetry we
demonstrate that this can lead to a large increase of the wind entropy,
density, and temperature, and to a strong deceleration of the wind expansion.
The consequences of this phenomenon for the possible r-process nucleosynthesis
in the late wind still need to be explored in detail. Two-dimensional models
show that the wind-ejecta collision is highly anisotropic and could lead to a
directional dependence of the nucleosynthesis even if the neutrino-driven wind
itself is spherically symmetric.Comment: 6 pages, 3 figures, International Symposium on Nuclear Astrophysics -
Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
Crystal Growth in Fluid Flow: Nonlinear Response Effects
We investigate crystal-growth kinetics in the presence of strong shear flow
in the liquid, using molecular-dynamics simulations of a binary-alloy model.
Close to the equilibrium melting point, shear flow always suppresses the growth
of the crystal-liquid interface. For lower temperatures, we find that the
growth velocity of the crystal depends non-monotonically on the shear rate.
Slow enough flow enhances the crystal growth, due to an increased particle
mobility in the liquid. Stronger flow causes a growth regime that is nearly
temperature-independent, in striking contrast to what one expects from the
thermodynamic and equilibrium kinetic properties of the system, which both
depend strongly on temperature. We rationalize these effects of flow on crystal
growth as resulting from the nonlinear response of the fluid to strong shearing
forces.Comment: to appear in Phys. Rev. Material
Impact of low-input meadows on arthropod diversity at habitat and landscape level
In Switzerland, in order to preserve and enhance arthopod diversity in grassland ecosystems (among others), farmers had to convert at least 7 % of their land to ecological compensation areas – ECA. Major ECA are low input grassland, traditional orchards, hedges and wild flower strips. In this paper the difference in species assemblages of 3 arthropod groups, namely spiders, carabid beetles and butterflies between intensively managed and low input meadows is stressed by means of multivariate statistics. On one hand, the consequences of these differences are analysed at the habitat level to promote good practices for the arthropod diversity in grassland ecosystems. On the other hand, the contribution of each meadow type to the regional diversity is investigated to widen the analysis at the landscape level
Concentration dependence of the transition temperature in metallic spin glasses
The dependence of the transition temperature in terms of the
concentration of magnetic impurities in spin glasses is explained on the
basis of a screened RKKY interaction. The two observed power laws, at
low and for intermediate , are described in a unified
approach.Comment: 4 page
Polar cap magnetic field reversals during solar grand minima: could pores play a role?
We study the magnetic flux carried by pores located outside active regions
with sunspots and investigate their possible contribution to the reversal of
the global magnetic field of the Sun. We find that they contain a total flux of
comparable amplitude to the total magnetic flux contained in polar caps. The
pores located at distances of 40--100~Mm from the closest active region have
systematically the correct sign to contribute to the polar cap reversal. These
pores can predominantly be found in bipolar magnetic regions. We propose that
during grand minima of solar activity, such a systematic polarity trend, akin
to a weak magnetic (Babcock-Leighton-like) source term could still be operating
but was missed by the contemporary observers due to the limited resolving power
of their telescopes.Comment: 11 pages, 9 figures, accepted for publication in
Astronomy&Astrophysic
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