Anomalous suppression of the orthorhombic distortion in superconducting Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

The interplay between superconductivity, magnetism and structure has become a major theme of research in the iron arsenide families of superconductors. Here we report high-resolution x-ray diffraction measurements that reveal an unusually strong response of the lattice to superconductivity in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals. Below $T_{C}$ the orthorhombic distortion of the lattice is suppressed and, for Co-doping near x $\sim $ 0.063, the orthorhombic structure evolves smoothly to a reentrant 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. -- The work at the Ames Laboratory was supported by the US DOE, Office of Science, under contract No. DE-AC02-07CH11358.