S-N border instability, magnetic flux trapping and cumulative effect during pulsed S-N switching of high quality YBaCuO thin films

Abstract

Unexpected irreversible damage appeared in samples while investigating pulsed S-N switching in YBaCuO thin films. Examination of a scanning electron microscope image of a damaged film containing a defect revealed a step-like motion of the N-zone. Restoration of crack movement dynamics points to the formation of coherent jets. This would mean that cumulative convergence of unknown incompressible media took place. The known model of an edge barrier, when the S-N border, characterized by width λ eff , bounds a semi elliptical N-zone at the film edges, was applied for clarification of the cumulative convergence. In that model, counteracting forces cause a wave-like unstable form of the barrier S-N border, and further current growth increases the amplitude of wave-like distortions of the border. Then, current reconnection separates bulges from the N-zone and induces the appearance of the circular currents with a trapped magnetic flux. Heat release at the places with trapped flux cause an expansion in these locations and coherent jets originate from touch points due to cumulative convergence. We substantiate that specific properties of the S-N border at λ eff , such as high tensile strength, very high flexibility and also incompressibility, are responsible for the appearance of the cumulative effect. Thus, the N-zone propagation into S state begins by motion of the wave-like border. The non-damaging step-like N-zone propagation into the films without defects is clarified. It is concluded that both the critical current and the damage current are determined by the same barrier with the unstable S-N border, and, therefore, the irreversible damage of the films comes quickly after the current reaches the critical value.

Detailed look into the unstable S-N border during propagation of the N-zone into the thin YBaCuO film

“…But let us offer a phenomenological explanation. As in the paper [6], we prefer to look for comparison with the water jet case.…”

“…The critical current density j c and damage current density j d were determined assuming the uniform current distribution j c = I c /(w•d), where w is the film width and d is the film thickness. A detailed description of technological procedures and ns pulse measurement technique can be found in [6] and its citations.…”

“…As the j c value increases, the difference j d − j c gets smaller. As follows from [6], calculated j c values match whether the current distribution is homogeneous or nonhomogeneous. There is no such matching for j d .…”

“…It is shown in [6] that during the risetime of ns pulse narrow channels originate at opposite edges of a sample and join at its central part, forming a single narrow N-zone about 80 μm in width. After that, the YBaCuO thin film strip line samples of 2.6 mm width are switched when the formed integral channel spreads by increasing width.…”

“…As substantiated in [6], such an S-N border is not just a line separating the states but can be considered an object on its own, having unique properties. Partly these properties emerge due to the S state being a macroscopic quantum state.…”