Evidence for Josephson vortices in (BEDT-TTF)2Cu(NCS)2

We present evidence for and study the properties of coreless Josephson vortices (parallel to the superconducting layers) in the organic superconductor (BEDT-TTF${)}_2$Cu(NCS${)}_2$, using ac susceptibility measurements in the mixed state. We observe (1) extremely weak-pinning restoring forces for flux motion parallel to the layers, due to the absence of the vortex normal core; (2) single vortex (noncollective) pinning, due to weak interactions between Josephson vortices; (3) the ‘‘lock-in’’ of the vortices parallel to the layers when the dc field is applied at an arbitrary angle; and (4) highly nonlinear response to the ac field h in the lock-in state, above a threshold of h∼0.5 G. The behavior of tilted flux lines is dominated by the much stronger pinning and collective interactions of two-dimensional Abrikosov ‘‘pancake’’ vortices, and linear response is largely restored when the flux lines unlock from the layers. We measure the Josephson penetration depth to be ${\lambda }_{\mathrm{\perp }}$(T=5 K)∼200 μm, yielding a penetration depth anisotropy of γ=${\lambda }_{\mathrm{\perp }}$/${\lambda }_{\mathrm{\parallel }}$∼160-350.