Evidence for an electron nematic phase transition in underdoped iron pnictide superconductors

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.