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

Results of direct imaging of Abrikosov vortices in an organic superconductor by means of the decoration technique are presented. The vortices have been observed in the title compound at temperature of ≃0.5T_c at the magnetic fields 3.3–9.4 Oe directed perpendicular to the two-dimensional plane of the crystal. The decoration patterns reveal the sixfold coordination short-range order or slightly distorted triangular lattice of the 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 average size of the individual vortex images on the sample surface yields the upper limit estimation of the London penetration depth as 0.4 μm at the given temperature. Pronounced effects of pinning on the crystal defects and thermally activated mobility of the vortices have been observed.