Temperature dependence of the Josephson plasma resonance between vortex phases in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Using a microwave cavity perturbation technique, we probe different regimes of vortex interactions within the mixed superconducting state of the title compound. A Josephson plasma resonance (JPR) is observed which is sensitive to correlations between pancake vortices in adjacent layers. Previous investigations have conclusively identified a transition between liquid-like and pinned solid or glassy vortex phases. Here, we study this behavior via field-cooled experiments, obtaining further evidence for the transition. These experiments also allow us to probe the JPR over a wider range in temperature and magnetic field within the liquid phase than was previously reported. Theoretical models predict a squared plasma frequency ( ω p 2 ) which follows a power law dependence on magnetic field (B) within both phases, i.e. ω p 2 ∝ B − μ . Our data agree well with this model, allowing a determination of the zero-field interlayer critical current density (J0) and the parameter μ. Indeed, this parameter (μ) is seen to follow a dependence on frequency which is indicative of a continuous transition between the two vortex phases.