High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1-xCox)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 magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

Single crystals of BaFe 2 As 2 and Ba 0.55 K 0.45 Fe 2 As 2 have been grown out of excess Sn with 1% or less incorporation of solvent. The crystals are exceptionally micaceous, are easily exfoliated, and can have dimensions as large as 3 ϫ 3 ϫ 0.2 mm 3 . The BaFe 2 As 2 single crystals manifest a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase near 85 K and do not show any sign of superconductivity down to 1.8 K. This transition can be detected in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In the Ba 0.55 K 0.45 Fe 2 As 2 single crystals this transition is suppressed and instead superconductivity occurs with a transition temperature near 30 K. Whereas the superconducting transition is easily detected in resistivity and magnetization measurements, the change in specific heat near T c is small, but resolvable, giving ⌬C p / ␥T c Ϸ 1. The application of a 140 kOe magnetic field suppresses T c by only ϳ4 K when applied along the c axis and by ϳ2 K when applied perpendicular to the c axis. The ratio of the anisotropic upper critical fields, ␥ = H c2Ќc / H c2 ʈc , varies between 2.5 and 3.5 for temperatures down to ϳ2 K below T c .

Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe2As2 splits into two transitions with Co doping. For Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS=60 K, followed by a second-order transition to AFM order at TN=47 K. Superconductivity occurs in the orthorhombic state below TC=17 K and coexists with AFM. Below TC, the static Fe moment is reduced along with a redistribution of low energy magnetic excitations indicating competition between coexisting superconductivity and AFM order.