1,629 research outputs found
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
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
Evolving solitons in bubbly flows
At the end of the sixties, it was shown that pressure waves in a bubbly liquid obey the KdV equation, the nonlinear term coming from convective acceleration and the dispersive term from volume oscillations of the bubbles.\ud
For a variableu, proportional to –p, wherep denotes pressure, the appropriate KdV equation can be casted in the formu t –6uu x +u xxx =0. The theory of this equation predicts that, under certain conditions, solitons evolve from an initial profileu(x,0). In particular, it can be shown that the numberN of those solitons can be found from solving the eigenvalue problem xx–u(x,0)=0, with(0)=1 and(0)=0.N is found from counting the zeros of the solution of this equation betweenx=0 andx=Q, say,Q being determined by the shape ofu(x,0). We took as an initial pressure profile a Shockwave, followed by an expansion wave. This can be realised in the laboratory and the problem, formulated above, can be solved exactly.\ud
In this contribution the solution is outlined and it is shown from the experimental results that from the said initial disturbance, indeed solitons evolve in the predicated quantity.\u
Avalanches and Self-Organized Criticality in Superconductors
We review the use of superconductors as a playground for the experimental
study of front roughening and avalanches. Using the magneto-optical technique,
the spatial distribution of the vortex density in the sample is monitored as a
function of time. The roughness and growth exponents corresponding to the
vortex landscape are determined and compared to the exponents that characterize
the avalanches in the framework of Self-Organized Criticality. For those
situations where a thermo-magnetic instability arises, an analytical non-linear
and non-local model is discussed, which is found to be consistent to great
detail with the experimental results. On anisotropic substrates, the anisotropy
regularizes the avalanches
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
Oscillations of a gas pocket on a liquid-covered solid surface
The dynamic response of a gas bubble entrapped in a cavity on the surface of
a submerged solid subject to an acoustic field is investigated in the linear
approximation. We derive semi-analytical expressions for the resonance
frequency, damping and interface shape of the bubble. For the liquid phase, we
consider two limit cases: potential flow and unsteady Stokes flow. The
oscillation frequency and interface shape are found to depend on two
dimensionless parameters: the ratio of the gas stiffness to the surface tension
stiffness, and the Ohnesorge number, representing the relative importance of
viscous forces. We perform a parametric study and show, among others, that an
increase in the gas pressure or a decrease in the surface tension leads to an
increase in the resonance frequency until an asymptotic value is reached
Bubble size prediction in co-flowing streams
In this paper, the size of bubbles formed through the breakup of a gaseous
jet in a co-axial microfluidic device is derived. The gaseous jet surrounded by
a co-flowing liquid stream breaks up into monodisperse microbubbles and the
size of the bubbles is determined by the radius of the inner gas jet and the
bubble formation frequency. We obtain the radius of the gas jet by solving the
Navier-Stokes equations for low Reynolds number flows and by minimization of
the dissipation energy. The prediction of the bubble size is based on the
system's control parameters only, i.e. the inner gas flow rate , the outer
liquid flow rate , and the tube radius . For a very low gas-to-liquid
flow rate ratio () the bubble radius scales as , independently of the inner to outer viscosity
ratio and of the type of the velocity profile in the gas, which
can be either flat or parabolic, depending on whether high-molecular-weight
surfactants cover the gas-liquid interface or not. However, in the case in
which the gas velocity profiles are parabolic and the viscosity ratio is
sufficiently low, i.e. , the bubble diameter scales as
, with smaller than 1/2
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.
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