Measurement of the c-Axis Optical Reflectance of AFe2As2 (A = Ba, Sr) Single Crystals: Evidence of Different Mechanisms for the Formation of Two Energy Gaps

We present the c-axis optical reflectance measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the spin-density-wave (SDW) state, only the smaller energy gap could be seen clearly for E parallel to c axis. The very pronounced energy gap structure seen at a higher energy scale for E parallel to ab plane is almost invisible. We propose a novel picture for the band structure evolution across the SDW transition and suggest different driving mechanisms for the formation of the two energy gaps.