Ultrasonic investigations on the iron-based superconductor Ba(Fe1-xCox)2As2

The simultaneous occurrence of a structural transition and long-range magnetic order in the proximity to the superconducting phase in iron-based superconductors raises the question whether the spin or the lattice degrees of freedom manifest the dominant low-energy excitations in the systems which are relevant for the superconductivity. Based on ultrasonic investigations both scenarios have been discussed in the literature. Ultrasonic experiments on Ba(Fe1-xCox)2As2 showed a huge softening of the transverse elastic constant c66 related to the structural tetragonal to orthorhombic/antiferromagnetic phase transition at TS/TN and a subsequent hardening upon entering the superconducting state at Tsc. We present measurements of the c33 and 1/2(c11+c12+2c66)-mode together with the ultrasonic attenuation on Ba(Fe1-xCox)2As2 single crystals with x=0 and x=0.06. In the undoped compound, two, slightly separated, step-like anomalies of nearly similar size, corresponding to the structural/magnetic transition at TS/TN are clearly visible in the c33-mode. According to our characterization measurements by resistivity and susceptibility, our crystal with x=0.06 shows neither a structural nor a magnetic transition but is superconducting at Tsc=24 K. Here the superconducting transition is visible in the c33 as well as in the 1/2(c11+c12+2c66)-mode.