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<img src='content_image/386843.jpg'>

important implication for the band structure evolution across the transition. We found that our data could be naturally explained by assuming that there are two types of FSs in the system: the 2D cylinder like FSs and a large-size 3D ellipsoid like FS, as schematically presented in Fig. 5. Presence of the large-size 3D ellip- soid FS with dominant Fe-3d 3 z $^{2$_{−}$}$r2 orbital is indicated by the recent ab-initio LDA+Gutzwiller calculations, where electron correlations are taken into account beyond LDA [23]. Recent transport[21], upper critical field[24], as well as ARPES experiments[25, 26] also suggest the presence of a 3D FS. 

If we neglect the small dispersion of those 2D FSs along the c-axis, the Fermi velocity along the c-axis be- comes zero and the electrons on the 2D FSs would only couple to the polarized light with E ‖ ab-plane. On the other hand, the electrons on the 3D FS can couple to the light polarized along any direction. We suggest that the 2D cylinder-like hole FSs around Γ and the 2D electron FSs around the corner of the Brillouin zone are strongly nested, which is the main driving force for the SDW in- stability of the system, while the 3D ellipsoid FS does not show any nesting with other FSs. Nevertheless, with the formation of the SDW order driven by the ( π, π ) nesting of 2D FSs, the new magnetic Brillouin zone boundary would cut the large-size 3D ellipsoid FS and result in an energy gap at the intersecting points as shown in Fig. 5 (b). This can be seen by optics in both E ‖ ab-plane and E ‖ c . Then we ascribe the larger gap seen most clearly for E ‖ ab-plane to the nesting-driven gap opened on 2D FSs, the smaller one to the gap formed on 3D FS which could be considered as the consequence of the SDW order. Be- cause the 2D FSs are more dramatically affected by the SDW instability than the 3D FS, we expect a reduction 

of the anisotropy in the magnetic ordered state. 

To summarize, we have successfully grown thick single c rystals of AFe 2 As 2 (A=Ba, Sr) and investigated their o ptical properties. Our study revealed a clear differ- e nce in optical conductivity for E ‖ ab-plane and E ‖ c-axis in the SDW state. The very pronounced energy gap s tructure seen at a higher energy scale for E ‖ ab-plane is almost invisible for E ‖ c-axis, whereas the smaller en- e rgy gap could be seen in both polarizations. We pro- pose a novel picture for the band structure evolution and s uggest different driving mechanisms for the two energy g aps. The strong ( π, π ) nesting between disconnected 2D c ylinder-like FSs is the main driving force for the SDW instability, leading to the opening of larger energy gap in the 2D FSs. The cutting of the magnetic Brillouin z one on the 3D FS leads to a smaller gap at the crossing region. 

This work is supported by the National Science Foun- dation of China, the Knowledge Innovation Project of the Chinese Academy of Sciences, and the 973 project of the Ministry of Science and Technology of China. 

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