A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 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 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)], but has not so far been observed experimentally. Our data also suggests a transition in the vortex structure near the irreversibility line not previously reported for an organic superconductor using this technique.
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