Electronic states and material dependences of Fe-based superconductors

In this study, we will discuss the electronic states of Fe- based superconductors and its material dependences, in particular, by taking account of the bond angle dependences. First, we calculate the crystal field splitting ($\Delta$) of Fe 3d orbital coordinated by four As's. Next, the hopping integrals (t) are estimated by using the Slater and Koster fs method. Note that these parameters, t and $\Delta$ change with $\alpha$. Finally, we can obtain the dispersion relation as a function of $\alpha$. It is found that the spectral weights near the Fermi energy are dominated by yz, zx and x2-y2 orbitals. The yz and zx orbitals are higher in energy around the regular tetragonal geometry, in which $\alpha$ is almost 109$^{\circ}$. On the other hand, those two orbitals become lower in energy for the larger value of $\alpha$. Such an orbital crossing is crucial for the electronic states. The ground state phase diagram is obtained by the Hartree-Fock calculation of multi-band Hubbard model.