Role of Hydrogen in the Electronic Properties of H-rich Pnictide Superconductors

The electronic and magnetic properties of the parent material CaFeAsH and its La/Co-doped compounds are investigated using first-principles calculations based on the generalized gradient approximation (GGA). We predict that the ground state of CaFeAsH is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. We find a sandglass-type hole pocket near the $\Gamma $point in CaFeAsH that is not present in CaFeAsF. In comparison with CaFeAsF, the sandglass-shaped pocket, mainly contributed from Fe d$_{\mathrm{xz}}+$d$_{\mathrm{yz}}$ orbitals, arises from the weak oxidization of CaH layers and the hybridization enhancement between FeAs layers. In contrast, the electronic properties of electron doped Ca$_{\mathrm{0.75}}$La$_{\mathrm{0.25}}$FeAsH and CaFe$_{\mathrm{0.75}}$Co$_{\mathrm{0.25}}$AsH indicate that La or Co doping almost does not affect the sandglass-type Fermi surface, while the suppression of Fermi surface nesting in Ca$_{\mathrm{0.75}}$La$_{\mathrm{0.25}}$FeAsH is weaker than that in CaFe$_{\mathrm{0.75}}$Co$_{\mathrm{0.25}}$AsH. This features may contribute to the higher T$_{\mathrm{c}}$ in La-substituted CaFeAsH.