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

We report measurements of the London penetration depth, $\lambda $, for different magnetic field and crystal orientations in fully deuterated $\kappa $ (ET)$_{2}$Cu[N(CN)$_{2}$]Br\textbf{, }an organic superconductor with T$_{C}$ = 11.9 K. $\lambda $ increases dramatically with deuteration and develops a strong magnetic field dependence. The superfluid density exhibits a power law temperature dependence indicative of a nodal order parameter. We discuss possible connections to nanoscale antiferromagnetic domains.