Avalanche-driven fractal flux distributions in NbN superconducting films

Abstract

Flux distributions in thin superconducting NbN films placed in a perpendicular magnetic field have been studied using magneto-optical imaging. Below 5.5 K the flux penetrates in the form of abrupt avalanches resulting in dendritic structures. Magnetization curves in this regime exhibit extremely noisy behavior. Stability is restored both above a threshold temperature T* and applied field H*, where H* is smaller for increasing field than during descent. The dendrite size and morphology are strongly T dependent, and fractal analysis of the first dendrites entering into a virgin film shows that dendrites formed at higher T have larger fractal dimension.Comment: 3 pages, 5 figure

Nanosecond voltage pulses from dendritic flux avalanches in superconducting NbN films

“…2,3 It can take place in bulk samples, wires, and tapes, but in thin films subjected to a perpendicular magnetic field, the instability takes a most spectacular form of dendritic avalanches propagating at ultra-fast speeds. The phenomenon has been reported to occur in a large number of materials important for practical applications, such as Nb, 4 NbN, 5 Nb 3 Sn, 6 and MgB 2 . 7 The avalanches are expected to generate strong electromagnetic noise and even destroy superconducting equipment.…”

“…7 The avalanches are expected to generate strong electromagnetic noise and even destroy superconducting equipment. 8 Whereas flux structures frozen in superconducting films after such events are routinely observed by magneto-optical imaging (MOI), [4][5][6][7] experiments providing insight to the time-resolved behavior of these ultrafast avalanches are extremely few. Previous work on films of YBa 2 Cu 3 O x has combined MOI and a femtosecond pulsed laser technique, where an avalanche was triggered by a laser-generated hot spot, and a few nanoseconds later laser light was used to record a snapshot of the evolving flux distribution.…”

Avalanches injecting flux into the central hole of a superconductingMgB2ring

“…Such avalanches were first observed in YBa 2 Cu 3 O x , where they must be triggered by a sudden point-like heatpulse 1,2 . Since then spontaneous dendritic avalanches have been observed 3,4,5,6,7 in Nb, Nb 3 Sn, YNi 2 B 2 C and NbN, as well as in patterned Pb thin films 8 . The recent interest in the phenomenon was largely triggered by the discovery 9 that in MgB 2 films the avalanches are ubiquitous below a threshold temperature T th ∼ 10 K. The dendrites disrupt electrical current flow and limit the overall current capacity of superconductors, 10 and are thus harmful for prospective applications.…”

Design of magneto-optical system for imaging of magnetic flux created during a conductor–superconductor phase transition

“…At temperatures significantly lower than T c , the magnetic field enters the superconducting film in dendrite-like fractal shape due to a thermo-magnetic instability (Rudnev et al 2005). A low-magnification measurement of the magnetic flux inside the sample is shown in figure 4.…”