Nontrivial Role of Interlayer Cation States in Iron-Based Superconductors

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond angles or bond distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on $d$ or $d\text{−}p$ model Hamiltonians only.

Figure 1

Crystal structures of ${\mathrm{RbFe}}_2{\mathrm{As}}_2$, ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$, NaFeAs, and MgFeGe. The unit cells and interatomic distances are true to scale. The numbers next to the unit cells indicate the nominal valence of atoms at the same vertical positions.

Figure 2

Calculated Heisenberg exchange parameters for (a) the VCA interpolation between ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ and ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ [via ${\mathrm{SrFe}}_2({\mathrm{As}}_{0.5}{\mathrm{Ge}}_{0.5}{)}_2$] and (b) the VCA interpolation between NaFeAs and MgFeGe. Lines are guides to the eye. The error bars represent the statistical errors of the fit. The inset of (b) shows the structure of the two-dimensional Heisenberg model we use to fit the DFT energies. $J_1$ is the nearest-neighbor coupling in the square lattice of Fe atoms, while $J_2$ is the next-nearest neighbor coupling. $J_3$ and $J_4$ are longer-range exchange couplings. Positive values of $J$ correspond to antiferromagnetic exchange. Note that all calculations were performed in the crystal structures of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and MgFeGe, respectively.

Figure 3

Effective density of states in the extended Stoner model as a function of magnetic moment for (a) ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ and ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and (b) NaFeAs and MgFeGe. The colored bars on the right $y$ axis indicate the calculated inverse Stoner parameters $1/I$ for the respective case. All calculations were performed in the crystal structures of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and MgFeGe, respectively.

Figure 4

Total density of states calculated from DFT for (a) ${\mathrm{RbFe}}_2{\mathrm{As}}_2$, (b) ${\mathrm{YFe}}_2{\mathrm{Fe}}_2$, (c) NaFeAs, and (d) MgFeGe. The shaded areas below the curves correspond to the energy range needed to realize a moment of $1.0{\mu }_B$ or $2.4{\mu }_B$ per iron, respectively, within the extended Stoner model. All calculations were performed in the crystal structures of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ and MgFeGe, respectively.