Two-dimensional vortex melting in BEDT organic superconductors and NMR relaxation induced by vortex structure defects

We present experimental studies and a theoretical model of the H-1 NMR relaxation rate (1/T-1) in the BEDT organic superconductor in magnetic fields B perpendicular to the conducting layers. The curves T-1(-1)(T,B = const) showa pronounced peak, whose position T-m(B) coincides with a phase transition line as it has been revealed by other methods. The peak sharpens remarkably with decreasing B when the family of curves T-1(-1)(T, B) converges to the field independent ''master curve'' below T-m. We show that the complex of experimental results cannot be interpreted as resulting from harmonic oscillations of a rigid vortex lattice, and suggest that the relaxation comes from the Brownian motion of highly diffusive point defects in decoupled 2D crystals of pancake vortices.