A cavity perturbation technique is used to study the microwave response of the organic superconductor -(BEDT-TTF) 2 Cu(NCS) 2 . Observation of a Josephson plasma resonance, below T c (ϳ10 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 GHz͒ indicate that the squared plasma frequency ( 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 ͑1995͔͒. Our data also suggests a transition in the vortex structure near the irreversibility line.
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