Dendritic flux jumps in an organic superconducting crystal

TL;DR

This study reports the first observation of dendritic flux jumps in a bulk organic superconductor, revealing fractal and self-similar flux instabilities similar to those seen in thin films, linked to layered structure effects.

Contribution

It demonstrates the presence of dendritic flux instabilities in a bulk organic superconductor, a phenomenon previously observed only in thin films, highlighting the role of layered structure.

Findings

Observation of flux jumps with fractal dimension 1.15-1.6

Similarity to MgB2 thin film flux structures

First evidence of dendritic instability in bulk superconductor

Abstract

Angle dependent torque measurements have been carried out on the organic superconductor, kappa-(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 tesla. A fractal analysis of the flux jumps indicates that the instabilities do show a self similar structure with a fractal dimension varying between 1.15 - 1.6. The fractal structure of the flux jumps in our sample shows a stricking similarity to that of MgB2 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 these thin films. The similarity of the flux jump structure of our samples suggests that we are also observing the dendritic instability, but in a bulk sample rather than a thin film. This is the first observation of the dendritic instability in a bulk superconducing sample, and is likely due to the layered nature of kappa-(ET)2Cu(NCS)2, which results in a quasi-two dimensional flux structure over the majority of it's mixed state phase diagram.