1,201 research outputs found
Significance of solutions of the inverse Biot-Savart problem in thick superconductors
The evaluation of current distributions in thick superconductors from field
profiles near the sample surface is investigated theoretically. A simple model
of a cylindrical sample, in which only circular currents are flowing, reduces
the inversion to a linear least squares problem, which is analyzed by singular
value decomposition. Without additional assumptions about the current
distribution (e.g. constant current over the sample thickness), the condition
of the problem is very bad, leading to unrealistic results. However, any
additional assumption strongly influences the solution and thus renders the
solutions again questionable. These difficulties are unfortunately inherent to
the inverse Biot-Savart problem in thick superconductors and cannot be avoided
by any models or algorithms
Dynamic roughening of the magnetic flux landscape in YBaCuO
We study the magnetic flux landscape in YBaCuO thin films as
a two dimensional rough surface. The vortex density in the superconductor forms
a self-affine structure in both space and time. This is characterized by a
roughness exponent and a growth exponent .
This is due to the structure and distribution of flux avalanches in the
self-organized critical state, which is formed in the superconductor. We also
discuss our results in the context of other roughening systems in the presence
of quenched disorder.Comment: 13 pages, 7 figures, accepted for publication in Physica
Halving the Casimir force with conductive oxides
The possibility to modify the strength of the Casimir effect by tailoring the
dielectric functions of the interacting surfaces is regarded as a unique
opportunity in the development of Micro- and NanoElectroMechanical Systems. In
air, however, one expects that, unless noble metals are used, the electrostatic
force arising from trapped charges overcomes the Casimir attraction, leaving no
room for exploitation of Casimir force engineering at ambient conditions. Here
we show that, in the presence of a conductive oxide, the Casimir force can be
the dominant interaction even in air, and that the use of conductive oxides
allows one to reduce the Casimir force up to a factor of 2 when compared to
noble metals.Comment: modified version, accepted for publication in Phys Rev Let
Magnetic flux density and the critical field in the intermediate state of type-I superconductors
To address unsolved fundamental problems of the intermediate state (IS), the
equilibrium magnetic flux structure and the critical field in a high purity
type-I superconductor (indium film) are investigated using magneto-optical
imaging with a 3D vector magnet and electrical transport measurements. The
least expected observation is that the critical field in the IS can be as small
as nearly 40% of the thermodynamic critical field . This indicates that
the flux density in the \textit{bulk} of normal domains can be
\textit{considerably} less than , in apparent contradiction with the long
established paradigm, stating that the normal phase is unstable below .
Here we present a novel theoretical model consistently describing this and
\textit{all} other properties of the IS. Moreover, our model, based the
rigorous thermodynamic treatment of observed laminar flux structure in a tilted
field, allows for a \textit{quantitative} determination of the domain-wall
parameter and the coherence length, and provides new insight into the
properties of all superconductors.Comment: 5 pages, 5 figure
Dynamics of stripe patterns in type-I superconductors subject to a rotating field
The evolution of stripe patterns in type-I superconductors subject to a
rotating in-plane magnetic field is investigated magneto-optically. The
experimental results reveal a very rich and interesting behavior of the
patterns. For small rotation angles, a small parallel displacement of the main
part of the stripes and a co-rotation of their very ends is observed. For
larger angles, small sideward protrusions develop, which then generate a zigzag
instability, ultimately leading to a breaking of stripes into smaller segments.
The short segments then start to co-rotate with the applied field although they
lag behind by approximately . Very interestingly, if the rotation is
continued, also reconnection of segments into longer stripes takes place. These
observations demonstrate the importance of pinning in type-I superconductors.Comment: To appear in Phys. Rev.
A window into the neutron star: Modelling the cooling of accretion heated neutron star crusts
In accreting neutron star X-ray transients, the neutron star crust can be
substantially heated out of thermal equilibrium with the core during an
accretion outburst. The observed subsequent cooling in quiescence (when
accretion has halted) offers a unique opportunity to study the structure and
thermal properties of the crust. Initially crust cooling modelling studies
focussed on transient X-ray binaries with prolonged accretion outbursts (> 1
year) such that the crust would be significantly heated for the cooling to be
detectable. Here we present the results of applying a theoretical model to the
observed cooling curve after a short accretion outburst of only ~10 weeks. In
our study we use the 2010 outburst of the transiently accreting 11 Hz X-ray
pulsar in the globular cluster Terzan 5. Observationally it was found that the
crust in this source was still hot more than 4 years after the end of its short
accretion outburst. From our modelling we found that such a long-lived hot
crust implies some unusual crustal properties such as a very low thermal
conductivity (> 10 times lower than determined for the other crust cooling
sources). In addition, we present our preliminary results of the modelling of
the ongoing cooling of the neutron star in MXB 1659-298. This transient X-ray
source went back into quiescence in March 2017 after an accretion phase of ~1.8
years. We compare our predictions for the cooling curve after this outburst
with the cooling curve of the same source obtained after its previous outburst
which ended in 2001.Comment: 4 pages, 1 figure, to appear in the proceedings of "IAUS 337: Pulsar
Astrophysics - The Next 50 Years" eds: P. Weltevrede, B.B.P. Perera, L. Levin
Preston & S. Sanida
Numerical and asymptotic solutions of generalised Burgers’ equation
The generalised Burgers’ equation has been subject to a considerable amount of research on how the equation should behave according to asymptotic analysis, however there has been limited research verifying the asymptotic analysis. In order to verify the asymptotic analysis, this paper aims to run long time and detailed numerical simulations of Burgers’ equation by employing suitable rescalings of Burgers’ equation. It is hoped that this technique will make it possible to notice subtle changes in the shock structure which would otherwise be impossible to observe. The main aim of this paper is to validate the numerical methods used in order to allow further research into shock evolution where further relaxation effects will be included
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