21 research outputs found
Nucleation and propagation of thermomagnetic avalanches in thin-film superconductors
Stability of the vortex matter -- magnetic flux lines penetrating into the
material -- in type-II superconductor films is crucially important for their
application. If some vortices get detached from pinning centres, the energy
dissipated by their motion will facilitate further depinning, and may trigger
an electromagnetic breakdown. In this paper, we review recent theoretical and
experimental results on development of the above mentioned thermomagnetic
instability. Starting from linear stability analysis for the initial
critical-state flux distribution we then discuss a numerical procedure allowing
to analyze developed flux avalanches. As an example of this approach we
consider ultra-fast dendritic flux avalanches in thin superconducting disks. At
the initial stage the flux front corresponding to the dendrite's trunk moves
with velocity up to 100~km/s. At later stage the almost constant velocity leads
to a specific propagation regime similar to ray optics. We discuss this regime
observed in superconducting films coated by normal strips. Finally, we discuss
dramatic enhancement of the anisotropy of the flux patterns due to specific
dynamics. In this way we demonstrate that the combination of the linear
stability analysis with the numerical approach provides an efficient framework
for understanding the ultra-fast coupled non-local dynamics of electromagnetic
fields and dissipation in superconductor films.Comment: 16 pages, 20 figure
Nonlinearly driven Landau-Zener transition with telegraph noise
We study Landau-Zener like dynamics of a qubit influenced by transverse
random telegraph noise. The telegraph noise is characterized by its coupling
strength, and switching rate, . The qubit energy levels are driven
nonlinearly in time, \propto \sign(t)|t|^\nu, and we derive the transition
probability in the limit of sufficiently fast noise, for arbitrary exponent
. The longitudinal coherence after transition depends strongly on ,
and there exists a critical with qualitative difference between and . When the end state is always fully
incoherent with equal population of both quantum levels, even for arbitrarily
weak noise. For the system keeps some coherence depending on the
strength of the noise, and in the limit of weak noise no transition takes
place. For fast noise , while for slow noise and it
depends on . We also discuss transverse coherence, which is relevant
when the qubit has a nonzero minimum energy gap. The qualitative dependency on
is the same for transverse as for longitudinal coherence. The state after
transition does in general depend on . For fixed , increasing
decreases the final state coherence when and increase the
final state coherence when . Only the conventional linear driving is
independent of .Comment: 7 pages, 5 figure
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Flux Penetration in Superconducting Strip with Edge-Indentation
The flux penetration near a semicircular indentation at the edge of a thin
superconducting strip placed in a transverse magnetic field is investigated.
The flux front distortion due to the indentation is calculated numerically by
solving the Maxwell equations with a highly nonlinear law. We find that
the excess penetration, , can be significantly ( 50%) larger than
the indentation radius , in contrast to a bulk supercondutor in the
critical state where . It is also shown that the flux creep tends
to smoothen the flux front, i.e. reduce . The results are in very good
agreement with magneto-optical studies of flux penetration into an
YBaCuO film having an edge defect.Comment: 5 pages, 7 figure
Scaling Behavior of Quasi-One-Dimensional Vortex Avalanches in Superconducting Films
Scaling behaviour of dynamically driven vortex avalanches in superconducting
YBaCuO films deposited on tilted crystalline
substrates has been observed using quantitative magneto-optical imaging. Two
films with different tilt angles are characterized by the probability
distributions of avalanche size in terms of the number of moving vortices. It
is found in both samples that these distributions follow power-laws over up to
three decades, and have exponents ranging between 1.0 and 1.4. The
distributions also show clear finite-size scaling, when the system size is
defined by the depth of the flux penetration front -- a signature of
self-organized criticality. A scaling relation between the avalanche size
exponent and the fractal dimension, previously derived theoretically from
conservation of the number of magnetic vortices in the stationary state and
shown in numerical simulations, is here shown to be satisfied also
experimentally.Comment: 7 pages, 5 figure
Quasi-one-dimensional intermittent flux behavior in superconducting films
Intermittent filamentary dynamics of the vortex matter in superconductors is found in films of YBa2Cu3O7-δ deposited on tilted substrates. Deposition of this material on such substrates creates parallel channels of easy flux penetration when a magnetic field is applied perpendicular to the film. As the applied field is gradually increased, magneto-optical imaging reveals that flux penetrates via numerous quasi-one-dimensional jumps. The distribution of flux avalanche sizes follows a power law, and data collapse is obtained by finite-size scaling, with the depth of the flux front used as crossover length. The intermittent behavior shows no threshold value in the applied field, in contrast to conventional flux jumping. The results strongly suggest that the quasi-one-dimensional flux jumps are of a different nature than the thermomagnetic dendritic (branching) avalanches that are commonly found in superconducting films
Energy of dendritic avalanches in thin-film superconductors
A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment
Interaction between superconducting vortices and Bloch wall in ferrite garnet film
Interaction between a Bloch wall in a ferrite-garnet film and a vortex in a
superconductor is analyzed in the London approximation. Equilibrium
distribution of vortices formed around the Bloch wall is calculated. The
results agree quantitatively with magneto-optical experiment where an in-plane
magnetized ferrite-garnet film placed on top of NbSe2 superconductor allows
observation of individual vortices. In particular, our model can reproduce a
counter-intuitive attraction observed between vortices and a Bloch wall having
the opposite polarity. It is explained by magnetic charges appearing due to
discontinuity of the in-plane magnetization across the wall.Comment: 4 pages, 5 figure
Metal frame as local protection of superconducting films from thermomagnetic avalanches
Thermomagnetic avalanches in superconducting films propagating extremely fast while forming unpredictable patterns, represent a serious threat for the performance of devices based on such materials. It is shown here that a normal-metal frame surrounding a selected region inside the film area can provide efficient protection from the avalanches during their propagation stage. Protective behavior is confirmed by magneto-optical imaging experiments on NbN films equipped with Cu and Al frames, and also by performing numerical simulations. Experimentally, it is found that while conventional flux creep is not affected by the frames, the dendritic avalanches are partially or fully screened by them. The level of screening depends on the ratio of the sheet conductance of the metal and the superconductor in the resistive state, and for ratios much larger than unity the screening is very efficient