Optical conductivity of Ba0.6K0.4Fe2As2: The effect of in-plane and out-of-plane doping in the superconducting gapY. M. Dai, B. Xu, B. Shen, H. H. Wen, X. G. Qiu and R. P. S. M. LoboEPL, 104 4 (2013) 47006DOI: https://doi.org/10.1209/0295-5075/104/47006