Josephson plasma resonance in κ-(BEDT-TTF)2Cu(NCS)2

A cavity perturbation technique is used to study the microwave response of the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). Observation of a Josephson plasma resonance, below T-c (similar to 10 K), enables investigation of the vortex structure within the mixed state of this highly anisotropic, type-II superconductor. Contrary to previous assumptions, frequency-dependent studies (28-153 GHz) indicate that the squared plasma frequency (omega(p)(2)) depends exponentially on the magnetic-field strength. Such behavior has been predicted for a weakly pinned quasi-two-dimensional vortex lattice [Bulaevskii et al., Phys. Rev. Lett. 74, 801 (1995)]. Our data also suggests a transition in the vortex structure near the irreversibility Line.