Low-temperature penetration depth of kappa-(ET)(2)Cu[N(CN)(2)]Br and kappa-(ET)(2)Cu(NCS)(2)

We present high-precision measurements of the penetration depth of single c rystals of kappa-(ET)(2)Cu[N(CN)(2)]Br and kappa-(ET)(2)Cu(NCS)(2) at tempe ratures down to 0.4 K. We find that, at low temperatures, the in-plane pene tration depth (lambda\\) varies as a fractional power law, lambda\\ similar to T-3/2. While this may he taken as evidence for novel pair excitation pr ocesses, we show thar the data are also consistent with a quasilinear varia tion of the superfluid density, as is expected for a d-wave superconductor with impurities or a small residual gap. Our data for the interplane penetr ation depth show similar features and give a direct measurement of the abso lute value, lambda(perpendicular to)(0) = 100 +/- 20 mu m.