VORTEX DYNAMICS OF THE ORGANIC SUPERCONDUCTOR KAPPA-(ET)(2)CU[N(CN)(2)]BR

We report on ac-susceptibility and dc-magnetization measurements of th e quasi two-dimensional (2D) organic superconductor kappa-(ET)(2) Cu[N (CN)(2)]Br. We determined the temperature, frequency and angular depen dence of the irreversibility field B-irr for two single-crystalline sa mples with different superconducting transition temperatures (T-c = 11 .5 K and T-c = 11.0 K) and Meissner fractions (50% and 20%, respective ly). We find for both samples at T/T-c approximate to 0.75 a crossover from an exponential temperature dependence at low T, B-irr alpha exp( -AT/T-c), to a power law close to T-c, B-irr proportional to (1-T/T-c) (alpha) with 1.3 < alpha < 2. Concomitantly the angular dependence of B-irr changes from 2D to 3D behavior, respectively. The exponential B- irr(T) dependence is interpreted as evidence for the breakdown field o f the proximity-coupled superconducting ET layers. In the irreversible field-temperature range a logarithmic time dependence of the magnetiz ation is found, and analyzed with a thermally activated flux creep mod el in order to extract more reliable values for the lower critical fie ld B-cl. (C) 1997 Elsevier Science B.V.