Vortex penetration depth of κ-(ET) ₂Cu[N(CN)₂]Br

The magnetic field dependence of the in-plane penetration depth λ_∥(H) for single crystal κ-(ET)₂Cu[N(CN)₂]Br has been measured at 3, 9.6, and 36 MHz. Over a limited range, λ_∥ scales with a characteristic field H * (T) that coincides with a shoulder in the λ_∥ vs. H curves. Above that field, λ_∥ increases sharply toward a second inflection point at H * * (T), which is close to the irreversibility line measured by magnetization. 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 observed 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 penetration depth λ ⊥ ~ 300 μm, implying an anisotropy ⊥ ⊥/λ_∥ > 200.