Granular-percolative superconductivity in organic materials: comparison with HTSC ceramics

We report investigations of the low temperature (2 K < T < 15 K) ac susceptibility and the magnetization of the fully deuterated organic superconductor κ‐(BEDT‐TTF)2Cu[N(CN)2]Br. Our measurements were performed after successive annealing at high temperature in the region 65 K < Ta < 110 K where the ethylene molecules undergo some kind of order disorder transition. The annealing time ta was varied from few seconds to about 2 × 106 s. The superconducting state can be described by a distribution of percolating clusters grown during annealing in the transition region. The spatial structure of such clusters and their relaxation times (in the transition region) obey essentially the same scaling laws and the same stretched exponential relaxation as in Ising spin glasses. The data are described by a granular model in which the grain radius r is equal to the average radius of the clusters. At the lowest fields and temperatures the clusters are coupled via Josephson junctions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)