We numerically investigate the effect of an edge indentation on the threshold field of thermomagnetic instabilities in superconducting films subjected to a ramping magnetic field, applied perpendicular to the plane of the film. In particuar, we are able to address the question on whether edge indentations promote magnetic flux avalanches. For magnetic field-independent critical current densities model, the triggering of the first magnetic flux avalanche systematically occurs at the edge indentation. In contrast to that, for the more realistic field-dependent critical current density model the first flux avalanche can take place either at or away from the indentation. This selective triggering of magnetic flux avalanches is shown to result from the variation of the threshold magnetic field for the first flux avalanche triggered at the indentation and the reduction of the critical current density by large local magnetic fields at the tip of the indentation which translates in a lower power density dissipated near the tip. We demonstrate that this interplay can be tuned by varying the indentation size, ramp rate of applied fieldḢa, and working temperature T0. We build up a phase diagram in the µ0Ḣa − T0 plane with well-defined boundaries separating three distinct regimes of thermomagnetic instability.

No abstract

No abstract

Magnetic flux avalanches caused by thermomagnetic instabilities are a common phenomenon occuring in type II superconducting films. The unpredictability of these catastrophic events threaten the application of superconducting thin film equipment, such as high-temperature superconducting magnets. In the present work, through the fast Fourier transform method, we numerically investigate artificially triggered flux avalanches in superconducting films by a focalized laser, unveiling new features beyond naturally occurring avalanches. The numerical modeling is validated by reproducing previous experimental results. We investigate the effects of laser irradiation on the nucleation and evolution of flux avalanches for different cases, namely varying the laser irradiation position, laser power, laser-spot size, ramping rate of applied magnetic field and working temperature. We find that the laser irradiation can control and guide the position of flux avalanche at applied magnetic fields with small ramping rate, while the similar guidance effect cannot be observed at high ramping field. We demonstrate that such phenomenon can be tuned by environmental temperature, and the mechanism can be revealed by current crowding and local temperature around the laser spot. Furthermore, by considering a pair of laser spots, we observe two tunable scenarios by the laser power, (i) single flux avalanche triggered at one of laser spots and (ii) double flux avalanches triggered at both laser spots.

Topology is a crucial ingredient for understanding the physical properties of superconductors. Magnetic field crowds to adopt the form of a topologically-protected quantum flux lines which can lose this property when moving at high velocities. These extreme conditions can be realized when superconductors undergo a thermomagnetic instability for which the sample topology come also into play. In this work, utilizing the magneto-optical imaging technique, we experimentally study magnetic flux avalanches in superconducting films with multiply-connected geometries, including single and double rings. We observe a domino effect in which avalanches triggered at the outer ring, stimulate avalanches at the inner ring thus impairing the expected magnetic shielding resulting from the outer ring and gap. We implement numerical simulations in order to gain more insight into the underlying physical mechanism and demonstrate that such event is not caused by the heat conduction, but mainly attributed to the local current distribution variation near the preceding flux avalanche in the outer ring, which in turn has a ripple effect on the local magnetic field profile in the gap. Furthermore, we find that the domino effect of thermomagnetic instabilities can be switched on/off by the environmental temperature and the gap width between the concentric rings. These findings provide new insights on the thermomagnetic instability in superconducting devices with complex topological structures, such as the superconductor-insulator-superconductor (SIS) multilayer structures of superconducting radio-frequency (SRF) cavities.

Thickness uniformity is regarded as an important parameter in designing thin film devices. However, some applications based on films with nonuniform thickness have recently emerged, such as gas sensors and optimized materials based on the gradual change of film composition. This work deals with superconducting Pb thin films with a thickness gradient prepared with the aid of a diffuse stencil mask. Atomic force microscopy and energydispersive x-ray spectroscopy show variations in the range 90 nm-154 nm. Quantitative magneto-optical images reveal interesting features during both the abrupt and the smooth penetration regimes of magnetic flux, as well as the thickness-dependent critical current density (J c ). In addition, we observe a gradual superconducting transition as the upper critical field is progressively reached for certain thicknesses. Furthermore, the hysteresis observed for triggering flux avalanches when increasing and decreasing magnetic fields is also accounted for by the J c profile evolution along the thickness gradient. Numerical simulations based on the thermomagnetic model are in fair agreement with the experimental data. These findings demonstrate that wedge-shaped films are a viable approach to investigate, in a continuous fashion, thickness-dependent properties of superconducting materials.

No abstract

The time-varying magnetic field with electromagnetic perturbation is regarded as an important parameter to the thermomagnetic stability of superconducting film devices. In this work, using the thermomagnetic model, we investigate the sensitivity of thermomagnetic instability in the superconducting film exposed to a linear ramp magnetic field, superposed by additional AC magnetic perturbation with tunable amplitude and oscillation frequency. Surprisingly, we find that the thermomagnetic instability is a non-monotonic function with the increasing oscillation frequency of magnetic perturbation, depending on the working temperature and oscillation amplitude. The unexpected non-monotonic sensitivity of thermomagnetic instability is revealed by the characteristic oscillation of electric field which can not be aggravated by the AC magnetic perturbation with very high frequency. The findings of this paper demonstrate that the magnetic perturbation of very low or high frequency is not the main factor that triggers the thermomagnetic instability of superconducting films. Furthermore, using the magnetic moment measurement, we propose a possible electromagnetic interference detection by the superconducting film based on such non-monotonic sensitivity of the thermomagnetic instability, which can be used to detect the tunable target electromagnetic interference with characteristic frequency in a complex electromagnetic environment.