Measurements of the interlayer penetration depth ${\lambda }_{\perp }$ have been performed on single crystals of the organic superconductors $\kappa \text{−}{\left(\text{ET}\right)}_2\text{Cu}\left[\text{N}{\left(\text{CN}\right)}_2\right]\text{Br}$ and $\kappa \text{−}{\left(\text{ET}\right)}_2\text{Cu}{\left(\text{NCS}\right)}_2$. We find that ${\lambda }_{\perp }\left(0\right)\approx 130\mu \text{m}$ for both materials. The normalized superfluid density ${\rho }_{\perp }={\left[{\lambda }_{\perp }\left(0\right)/{\lambda }_{\perp }\left(T\right)\right]}^2$ may be fit equally well to a power law $1-{\rho }_{\perp }\sim T^n$ with $n=1.3–1.5$ or to the form $1-{\rho }_{\perp }=\alpha \left(T^2/T_C\right)/\left(T+T^{\ast }\right)$, consistent with a $d$-wave pairing state with impurity scattering. The data imply coherent transport between conducting planes, in agreement with recent magnetoresistive measurements [J. Singleton, P. A. Goddard, A. Ardavan, N. Harrison, S. J. Blundell, J. A. Schlueter, and A. M. Kini, Phys. Rev. Lett. 88, 037001 (2002)] and in contrast to the copper oxides.

• Received 6 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.054519