  Electrical resistivity measurements of detwinned single crystals of the
representative iron arsenide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ reveal a dramatic
in-plane anisotropy associated with the tetragonal-to-orthorhombic structural
transition that precedes the onset of long-range antiferromagnetic order. These
results indicate that the structural transition in this family of compounds is
fundamentally electronic in origin.
