Dimensional crossover of the vortex dynamics in κ-(ET)2X salts

We investigated the temperature and angular dependence of the irreversibility field, Birr, of the quasi-two-dimensional organic superconductors κ-(ET)2X with X = I3, Cu(NCS)2, and Cu[N(CN)2]Br by means of ac-susceptibility measurements. We find a crossover from an exponential temperature dependence at low temperatures and high fields, Birr∝ exp(−AT/Tc), to a power law close to Tc, Birr∝ (1−T/Tc)α, with α in the range of 1.3 to 2. Concomitantly, the angular dependence of Birr changes from two-dimensional (2D) to three-dimensional (3D) behavior, respectively. The exponential Birr(T) dependence is interpreted as the influence of the breakdown field of proximitycoupled superconducting ET layers on the pinning mechanism. The power law near Tc is explained by the usual 3D vortex pinning.