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The London penetration depth, $\lambda(T)$, has been measured in severalsingle crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic,electromagnetic, and structural characterization measurements confirm thatthese crystals are of excellent quality. The observed low temperature variationof $\lambda(T)$ 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 onsingle crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, whichindicate an anisotropic but nodeless gap. We propose that a morethree-dimensional character in the electronic structure ofBa(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gapwith point nodes that would explain the observed $\lambda(T)$.

Unconventional London penetration depth in Ba(Fe0.93Co0.07)2As2 single crystals