Vortex dynamics of the organic superconductor κ-(ET) 2Cu[N(CN) 2]Br

Abstract

We report on ac-susceptibility and dc-magnetization measurements of the quasi two-dimensional (2D) organic superconductor κ-(ET) 2Cu[N(CN) 2]Br. We determined the temperature, frequency and angular dependence of the irreversibility field B irr for two single-crystalline samples with different superconducting transition temperatures ( T c=11.5 K and T c=11.0 K) and Meissner fractions (50% and 20%, respectively). We find for both samples at T/ T c≈0.75 a crossover from an exponential temperature dependence at low T, B irr ∝exp(− AT/ T c), to a power law close to T c, B irr∝(1− T/ T c) α with 1.3< α<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 of the proximity-coupled superconducting ET layers. In the irreversible field–temperature range a logarithmic time dependence of the magnetization is found, and analyzed with a thermally activated flux creep model in order to extract more reliable values for the lower critical field B c1.