Measurement of the c-axis optical reflectance of AFe$_{2}$As$_{2}$ (A=Ba, Sr) single crystals: Evidence of different mechanisms for the formation of two energy gaps Z. G. Chen, T. Dong, R. H. Ruan, B. F. Hu, B. Cheng, W. Z. Hu, P. Zheng, Z. Fang, X. Dai, and N. L. Wang We present the c-axis optical reflectance measurement on single crystals of BaFe$_{2}$As$_{2}$ and SrFe$_{2}$As$_{2}$, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the SDW state, only the smaller energy gap could be seen clearly for E ‖ c-axis. The very pronounced energy gap structure seen at a higher energy scale for E ‖ 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. PACS numbers: 74.25.Gz, 74.70.Xa, 75.30.Fv For quasi-two dimensional layered materials, strik- ing differences could exist in the in-plane and out-of- plane charge transport and dynamics. For example, in some high-T$_{c}$ cuprates, metallic in-plane charge trans- port coexists with nonmetallic conductivity along the c- axis[1, 2]. The contrasting behavior violates the con- ventional concept of band electron transport, and has been the subject of intensive study. Fe-pnictide super- conducting materials also crystalize in the layered struc- ture with Fe-As layers separated by alkaline metal ions or other insulator-like layers. Band structure calculations based on the local-density approximation (LDA) or gen- eralized gradient approximations (GGA) indicate domi- nantly two-dimensional (2D) cylinder-like Fermi surfaces (FSs) along the c-axis[3–5]. It is important to see whether or not the Fe-pnictides share similar anisotropic charge dynamical properties with cuprates. Optical spectroscopy is a powerful technique to investi- gate charge dynamics and band structure of a material as it probes both free carriers and interband excitations. In particular, it yields direct information about the energy gap formation in the broken symmetry state. Optical spectroscopy studies on the ab-plane properties of dif- ferent Fe-pnictides and chalcogenides systems have been reported by several groups.[6–18] For the parent com- pounds of Fe-pnictides, the measurements provide clear evidence for the formation of the partial energy gaps in the magnetic phase, supporting the itinerant picture that the energy gain for the antiferromagnetic ground state is achieved through the opening of a spin-density-wave (SDW) gap on the FSs.[7, 10, 14–16] For the supercon- ducting samples, the superconducting pairing gaps were also detected by the technique.[6, 17, 18] However, op- tical investigations have not been carefully done on the c-axis response of Fe-pnictide materials. There is only one work in the literature containing optical data along the c-axis.[10] Unfortunately, the data were limited to the high frequencies, above 700 cm − $^{1}$. Because of this limitation, neither the free-carrier response nor any fea- ture related to the SDW gap were observed. In fact, the Large-sized single crystals of BaFe 2 As 2 and SrFe 2 As 2 were grown from the FeAs flux in Al 2 O 3 crucibles sealed in quartz tubes. The growth procedure is similar to the description in our earlier work [19] except for one major difference: the crucible and quartz tube were placed in a direction of 45 degree relative to the vertical. After completing the growth procedure and breaking the cru- cible, we can easily find relatively thick crystals growing from the inner surface of the crucible along the direction at 45 degree relative to the crucible cylinder axis (i.e. crystals grow vertically). The dc resistivity measured by the four contact technique on cleaved ab-plane was found to be almost identical to the data presented in our early work, showing a sharp drops at 138 K and 200 K for BaFe 2 As 2 and SrFe 2 As 2 , respectively, being ascribed to the formation of SDW order.[7] We performed x-ray diffraction measurement to check the single crystalline nature of our samples and their orientation. The crystals were then cut in a direction perpendicular to the cleaved In this letter we present the c-axis ( E ‖ c ) optical re- flectance measurement over broad frequencies on thick BaFe 2 As 2 and SrFe 2 As 2 single crystal samples. We ob- served the SDW energy gap formation in the low-T mag- netic ordered state. However, different from the ab-plane response where two distinct energy gaps were identified for AFe 2 As 2 (A=Ba, Sr), only the gap corresponding to the smaller energy scale of E ‖ ab-plane could be clearly seen for the polarization parallel to the c-axis. The more pronounced gap structure at the higher energy scale for E ‖ ab-plane becomes almost invisible. The significant dif- ference between the two polarizations has important im- plication for the electronic structure of those compounds. A schematic picture for the band structure evolution across the structural/magnetic transition was proposed to understand the experimental findings. reported reflectance data appear to have extraordinarily low values. As information about the anisotropic charge dynamics is extremely important for understanding the materials, a detailed and careful determination of the c- axis optical response is highly necessary. arXiv:1001.1689v2 [cond-mat.supr-con] 29 Aug 2010 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China