Interplane penetration depth in $\kappa $-(ET)$_{2}$Cu[N(CN)$_{2}$]Br

We report measurements of the interplane penetration depth $\lambda _{\bot }$(T)$_{ }$ in the organic superconductor $\kappa $- (ET)$_{2}$Cu[N(CN)$_{2}$]Br\textbf{ (}T$_{C}$ = 11.9 K). At low temperatures, the superfluid density $\rho _{\bot }$= [$\lambda _{\bot }$(0)/ $\lambda _{\bot }$(T)]$^{2} \quad \propto $ 1-AT$^{N}$ with N = 1.3 -- 1.5, close to the exponent measured for the in-plane superfluid density. This result adds support to a d-wave picture, but with transport between planes more coherent than is observed in similarly anisotropic copper oxide superconductors.