Three-dimensionality and orbital characters of Fermi surface in Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_{2}$

We report a comprehensive study of the tridimensional electronic bands in the recently discovered Iron-selenide superconductor Tl$_{0.5}$Rb$_{0.3}$Fe$_{1.63}$Se$_2$ (T$_c$$\sim$32 K) with angle-resolved photoemission spectroscopy (ARPES). We determined the orbital characters and the $k_z$ dependence of the low-energy electronic structure by tuning the polarization and the photon energy of the incident photons. We observed a small 3D electron pocket near the Brillouin Zone (BZ) center and a 2D like electron pocket near the zone boundary. The photon energy dependence, the polarization analysis and the LDA calculations suggest a significant contribution from the Se 4$p_z$, Fe 3$d_{xy}$ and the Fe 3$d_{z^2}$ orbitals. Comparing with iron-pnictide superconductors, the emergence of Se 4$p_z$ states may be the cause of the different magnetic properties between iron-chalcogenides and iron-pnictides.