Importance of the 4p states of As and the interlayer cation states in LaFe2As2 and CaFe2As2

Abstract

The electronic structures of the UT and CT phases of both ${\mathrm{LaFe}}_{2}{\mathrm{As}}_{2}$ and the counterpart ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ are studied by density functional theory and tight-binding models. We find the 4p states of As contribute substantially to the relative instability between $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$ and $(0,0,\ensuremath{\pi})$ of Pauli susceptibility ${\ensuremath{\chi}}_{0}$, which may result in fluctuations mediating the pairing between electrons. This means that considering only the 3d states of Fe is not enough to capture the superconducting properties of iron-based superconductors. This finding is supported by the even higher relative instability at $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$ of interorbital Pauli susceptibility between the 3d orbital of Fe and 4p orbital of As compared with the total intraorbital and interorbital Pauli susceptibility from the 3d orbital of Fe, indicating that the 4p states of As affect ${\ensuremath{\chi}}_{0}$ considerably. Moreover, while the 3d states of Ca are found to contribute positively to the relative instability at $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$, the 5d states of La contribute negatively to the relative instability at $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$, illustrating the the 3d states of Ca and 5d states of La play contrary contributions to superconductivity. Our results provide qualitative criteria for the superconducting properties of ${\mathrm{LaFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$, and they reveal that both the 4p states of As and interlayer cation states should not be neglected in further studies on superconductivity of iron-based superconductors.