  High-resolution x-ray diffraction measurements reveal an unusually strong
response of the lattice to superconductivity in Ba[Fe(1-x)Cox]2As2. The
orthorhombic distortion of the lattice is suppressed and, for Co-doping near x
= 0.063, the orthorhombic structure evolves smoothly back to a tetragonal
structure. We propose that the coupling between orthorhombicity and
superconductivity is indirect and arises due to the magneto-elastic coupling,
in the form of emergent nematic order, and the strong competition between
magnetism and superconductivity.
