Direct observation of vortices in the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2)

Results of direct imaging of Abrikosov vortices in an organic superconducto r by means of the decoration technique are presented. The vortices have bee n observed in the title compound at temperature of similar or equal to 0.5T (c) at the magnetic fields 3.3-9.4 Oe directed perpendicular to the two-dim ensional plane of the crystal. The decoration patterns reveal the sixfold c oordination short-range order or slightly distorted triangular lattice of t he vortices with the magnetic flux per vortex equal to the flux quantum, hc /2e. The slight deviation of the vortex lattice from the hexagonal symmetry is attributed to the in-plane anisotropy of the penetration depth. The ave rage size of the individual vortex images on the sample surface yields the upper limit estimation of the London penetration depth as 0.4 mu m at the g iven temperature. Pronounced effects of pinning on the crystal defects and thermally activated mobility of the vortices have been observed.