VORTEX PENETRATION DEPTH OF KAPPA-(ET)(2)CU[N(CN)(2)]BR

The magnetic field dependence of the in-plane penetration depth lambda (parallel to)(H) for single crystal kappa-(ET)(2)Cu[N(CN)(2)]Br has be en measured at 3, 9.6, and 36 MHz. Over a limited range, lambda(parall el to) scales with a characteristic field H (T) that coincides with a shoulder in the lambda(parallel to) vs. H curves. Above that field, l ambda(parallel to) increases sharply toward a second inflection point at H*(T), which is close to the irreversibility line measured by magn etization. For fields larger than H* the penetration depth diverges, suggesting that the vortex lattice has melted. The field dependence at one frequency agrees qualitatively with a model of pinned vortices at low fields giving way to flux flow at higher fields. However, the obs erved frequency dependence deviates significantly from the predictions of this model, suggesting that collective effects play a major role. Our technique also yields a new measurement for the interplane penetra tion depth lambda(perpendicular to)similar to-300 mu m, implying an an isotropy lambda(perpendicular to)/lambda(parallel to) > 200. (C) 1997 Elsevier Science B.V.