Positron probing of electron momentum re-distribution at the superconducting transition in Ba(Fe1−xCox)2As2 single crystals

Superconducting Ba(Fe0.943Co0.057)2As2 single crystals with Tc (R=0)=19.5 K, and non-superconducting BaFe2As2 single crystals have been studied by coincidence Doppler broadening of positron annihilation radiation line-shape (coincidence DBPARL) down to ~14 K. This appears to be the first reporting of positron probing of any Fe-based superconductor. The superconducting sample shows, on cooling below ~40 K and towards Tc, a sharp decrease of S, the line-shape parameter, which gives the fraction of suitably defined low momentum electrons as probed by the positrons. No such decrease of S for the non-superconducting sample indicates the effect to be induced by superconductivity. The ratio curve analysis of the coincidence DBPARL spectra suggests that in the superconducting state the positrons are annihilating less with the 3d electrons of Fe and Co and more with the 5s and 4d electrons of Ba. In addition, a novel double measurement of coincidence DBPARL for the a–b plane and planes normal to the a–b plane of the single crystalline sample shows a nearly isotropic distribution of the lower electron momentum in this Fe-based superconductor.