Fractal flux jumps in an organic superconducting crystal

Abstract

Angle dependant torque magnetization measurements have been carried out on the organic superconductor, κ-(ET) 2Cu(NCS) 2 at extremely low temperatures (25–300 mK). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps for magnetic field sweeps of 0–20 T. A fractal analysis of the flux jumps indicate that the instabilities do show a self similar structure with a fractal dimension of varying between 1.15 and 1.6. The fractal structure of the flux jumps in our sample shows a striking similarity to that of MgB 2 thin film samples, in which magneto-optical experiments have recently shown that the small flux jumps are due to the formation of dendritic flux structures. These smaller instabilities act to suppress the critical current density of the thin films. The similarity of the flux jump structure of our samples suggests that we may also observing the dendritic instability, but in a bulk sample rather than a thin film. If true, this is the first observation of the dendritic instability in a bulk superconducting sample, and is likely due to the layered nature of κ-(ET) 2Cu(NCS) 2, which results in a quasi-two dimensional flux structure over the majority it's mixed state phase diagram.