Ray optics in flux avalanche propagation in superconducting films

TL;DR

This paper demonstrates that dendritic flux avalanches in superconducting films exhibit wave-like behavior, including refraction consistent with Snell's law, suggesting they propagate as electromagnetic shock waves influenced by local dissipation.

Contribution

The study provides experimental evidence of wave properties in flux avalanches, showing their propagation can be described by optics principles and modeling as electromagnetic shock waves.

Findings

Dendritic avalanches refract at boundaries according to Snell's law.

Refractive index of about 1.4 was measured for the flux waves.

Voltage pulse measurements support the electromagnetic shock wave model.

Abstract

Experimental evidence of wave properties of dendritic flux avalanches in superconducting films is reported. Using magneto-optical imaging the propagation of dendrites across boundaries between a bare NbN film and areas coated by a Cu-layer was visualized, and it was found that the propagation is refracted in full quantitative agreement with Snell's law. For the studied film of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was close to n=1.4. The origin of the refraction is believed to be caused by the dendrites propagating as an electromagnetic shock wave, similar to damped modes considered previously for normal metals. The analogy is justified by the large dissipation during the avalanches raising the local temperature significantly. Additional time-resolved measurements of voltage pulses generated by segments of the dendrites traversing an electrode confirm the consistency of the adapted physical picture.