Interlayer electrodynamics of the vortex state in κ-(BEDT-TTF)2Cu(NCS)2

A microwave cavity perturbation technique is used to probe the interlayer electrodynamics within the vortex state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2. A Josephson plasma mode is observed which is extremely sensitive to correlations in the locations of vortices in adjacent layers and may, therefore, be used to gauge collective effects between vortices and crystal pinning sites in the title compound. Our previous investigations [Phys. Rev. B 62 (2000) 5965] revealed a transformation from a weakly pinned decoupled two-dimensional vortex solid phase, to either a depinned or liquid state. In this study, we carry out a detailed analysis of the magnetic field dependence of the Josephson plasma frequency within the two phases. Our findings agree favourably with recent theoretical models: within the liquid state, the squared plasma frequency (ωp2) decays with the inverse of the magnetic field strength, B; whereas, in the solid phase, a much slower decay is observed (ωp2∝B−0.35), which is indicative of extremely weak pinning.