230 research outputs found
Downscaling heavy rainfall in the subtropics ? a simple approach for dynamical nesting
International audienceThe simulation of local scale precipitation with nested models often suffers from large errors in the boundary rows. Advection of precipitation into the model domain of the small scale model can lead to an overestimation of precipitation in the boundary grid cells of the nested model and a drying of the interior grid area. Consequently, the finer scale structure of rainfall events of the small scale model can not evolve. These errors result from three main sources: "dynamical", "scale", and "parameterization" problems. As a first step to reduce the "parameterization" boundary errors, we propose a nesting procedure where rainwater from the driving larger scale model is converted to cloud water in the smaller scale model. The nesting method is applied to a case study of heavy rainfall in semi-arid southern Morocco. The results show the elimination of erroneous excessive rainfall in the boundary rows and slightly enhanced rainfall in the interior of the nested model domain. Additionally, fine scale structures in the precipitation patterns develop. The excessive surface runoff is clearly diminished in comparison to the standard nesting procedure. The proposed approach enables scale consistent precipitation patterns resulting from model physics and grid-resolution of the smaller scale model for the complete model domain
Observation of Andreev bound states in YBaCuO/Au/Nb ramp-type Josephson junctions
We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb
ramp junctions of several geometries. Macroscopically, tunneling occurs in the
ab-plane of YBCO either in the (100) and (010) direction, or in the (110)
direction. These junctions have a stable and macroscopically well defined
geometry. This allows systematic investigations of both quasiparticle and
Josephson tunneling over a wide range of temperature and magnetic field. With
Nb superconducting, its gap appears in the quasiparticle conductance spectra as
Nb coherence peaks and a dip at the center of a broadened zero-bias conductance
peak (ZBCP). As we increase the temperature or an applied magnetic field both
the Nb coherence peaks and the dip get suppressed and the ZBCP fully develops,
while states are conserved. With Nb in the normal state the ZBCP is observed up
to about 77 K and is almost unaffected by an increasing field up to 7 T. The
measurements are consistent with a convolution of density of states with
broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces.
Since junctions with different geometries are fabricated on the same substrate
under the same conditions one expects to extract reliable tunneling information
that is crystallographic direction sensitive. In high contrast to Josephson
tunneling, however, the quasiparticle conductance spectra are crystallographic
orientation insensitive: independent whether the tunneling occurs in the (100)
or (110) directions, a pronounced ZBCP is always observed, consistent with
microscopic roughness of the junction interfaces. Qualitatively, all these
particularities regarding quasiparticle spectra hold regardless whether the
YBCO thin film is twinned or untwinned.Comment: 13 pages, 10 figure
Numerical simulations of two dimensional magnetic domain patterns
I show that a model for the interaction of magnetic domains that includes a
short range ferromagnetic and a long range dipolar anti-ferromagnetic
interaction reproduces very well many characteristic features of
two-dimensional magnetic domain patterns. In particular bubble and stripe
phases are obtained, along with polygonal and labyrinthine morphologies. In
addition, two puzzling phenomena, namely the so called `memory effect' and the
`topological melting' observed experimentally are also qualitatively described.
Very similar phenomenology is found in the case in which the model is changed
to be represented by the Swift-Hohenberg equation driven by an external
orienting field.Comment: 8 pages, 8 figures. Version to appear in Phys. Rev.
Current-induced vortex dynamics in Josephson-junction arrays: Imaging experiments and model simulations
We study the dynamics of current-biased Josephson-junction arrays with a
magnetic penetration depth smaller than the lattice spacing. We compare the
dynamics imaged by low-temperature scanning electron microscopy to the vortex
dynamics obtained from model calculations based on the resistively-shunted
junction model, in combination with Maxwell's equations. We find three bias
current regions with fundamentally different array dynamics. The first region
is the subcritical region, i.e. below the array critical current I_c. The
second, for currents I above I_c, is a "vortex region", in which the response
is determined by the vortex degrees of freedom. In this region, the dynamics is
characterized by spatial domains where vortices and antivortices move across
the array in opposite directions in adjacent rows and by transverse voltage
fluctuations. In the third, for still higher currents, the dynamics is
dominated by coherent-phase motion, and the current-voltage characteristics are
linear.Comment: 10 pages, with eps figures. To appear in Phys. Rev.
A smooth cascade of wrinkles at the edge of a floating elastic film
The mechanism by which a patterned state accommodates the breaking of
translational symmetry by a phase boundary or a sample wall has been addressed
in the context of Landau branching in type-I superconductors, refinement of
magnetic domains, and compressed elastic sheets. We explore this issue by
studying an ultrathin polymer sheet floating on the surface of a fluid,
decorated with a pattern of parallel wrinkles. At the edge of the sheet, this
corrugated profile meets the fluid meniscus. Rather than branching of wrinkles
into generations of ever-smaller sharp folds, we discover a smooth cascade in
which the coarse pattern in the bulk is matched to fine structure at the edge
by the continuous introduction of discrete, higher wavenumber Fourier modes.
The observed multiscale morphology is controlled by a dimensionless parameter
that quantifies the relative strength of the edge forces and the rigidity of
the bulk pattern.Comment: 4 pages, 4 figure
Behavior of vortices near twin boundaries in underdoped
We use scanning SQUID microscopy to investigate the behavior of vortices in
the presence of twin boundaries in the pnictide superconductor
Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries.
Individual vortices move in a preferential way when manipulated with the SQUID:
they tend to not cross a twin boundary, but rather to move parallel to it. This
behavior can be explained by the observation of enhanced superfluid density on
twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin
boundaries may be a mechanism for enhanced critical currents observed in
twinned samples in pnictides and other superconductors
Type-1.5 Superconductors
We demonstrate the existence of a novel superconducting state in high quality
two-component MgB2 single crystalline superconductors where a unique
combination of both type-1 (kappa_1 0.707)
superconductor conditions is realized for the two components of the order
parameter. This condition leads to a vortex-vortex interaction attractive at
long distances and repulsive at short distances, which stabilizes
unconventional stripe- and gossamer-like vortex patterns that we have
visualized in this type-1.5 superconductor using Bitter decoration and also
reproduced in numerical simulations.Comment: accepted in Phys. Rev. Let
Model independent approach to studies of the confining dual Abrikosov vortex in SU(2) lattice gauge theory
We address the problem of determining the type I, type II or borderline dual
superconductor behavior in maximal Abelian gauge SU(2) through the study of the
dual Abrikosov vortex. We find that significant electric currents in the
simulation data call into question the use of the dual Ginzburg Landau Higgs
model in interpreting the data. Further, two definitions of the penetration
depth parameter take two different values. The splitting of this parameter into
two is intricately connected to the existence of electric currents. It is
important in our approach that we employ definitions of flux and electric and
magnetic currents that respect Maxwell equations exactly for lattice averages
independent of lattice spacings. Applied to specific Wilson loop sizes, our
conclusions differ from those that use the dual GLH model.Comment: 18 pages, 14 figures, change title, new anaylysis with more figure
On expansion in the width for domain walls
The well-known idea to construct domain wall type solutions of field
equations by means of an expansion in the width of the domain wall is
reexamined. We observe that the problem involves singular perturbations.
Hilbert-Chapman-Enskog method is used to construct a consistent perturbative
expansion. We obtain the solutions to the second order in the width without
introducing an effective action for the domain wall. We find that zeros of the
scalar field in general do not lie on a Nambu-Goto trajectory. As examples we
consider cylindrical and spherical domain walls. We find that the spherical
domain wall, in contradistinction to the cylindrical one, shows an effective
rigidity.Comment: 21 pages, LaTe
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