  The London penetration depth, $\lambda(T)$, has been measured in several
single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic,
electromagnetic, and structural characterization measurements confirm that
these crystals are of excellent quality. The observed low temperature variation
of $\lambda(T)$ follows a power-law, $\Delta \lambda (T) \sim T^n$ with $n=2.4
\pm 0.1$, indicating the existence of normal quasiparticles down to at least
$0.02T_c$. This is in contrast to recent penetration depth measurements on
single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which
indicate an anisotropic but nodeless gap. We propose that a more
three-dimensional character in the electronic structure of
Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap
with point nodes that would explain the observed $\lambda(T)$.

Superconductivity