Non-magnetic analogs to Fe-based superconductors
Iron-based pnictogen and chalcogenide superconductors often have “non-magnetic” analogs which are superconductors that crystallize in the same structure but consist of only non-magnetic elements. Often these analogs have a much lower superconducting transition temperature Tc, providing support for the argument that the magnetic element, viz. iron, is important for the enhancement of the Tc . For example, SnO crystallizes in the same crystal structure as FeSe, has a similar Fermi surface and exhibits the same pressure dependence of Tc, albeit with a much lower Tc . The analysis of SnO showed that the Fermi surface topology and the degree of nesting are important for the superconductivity, but that spin fluctuations are not essential for the appearance of superconductivity in the “11” structure type; rather, the spin fluctuations increase the coupling and thereby the Tc.
In the case of LiFeAs, the superconductor NaAlSi (Tc= 7 K)  can be viewed as its non-magnetic analog. The Fermi surfaces of the two compounds have been shown to have similarities . Our investigations of NaAlSi under high pressure  showed that Tc initially increases with p and is then suppressed rather quickly beyond p ≈ 5 GPa. This behavior cannot be attributed to a structural phase transition as proven by x-ray diffraction experiments under pressure. Although pressure has a strong effect on superconductivity, DFT calculations demonstrate that it does not significantly alter the electronic structure. Similarly, a comparison of NaAlSi with non-superconducting NaAlGe showed, that the electronic structure cannot explain the different behavior regarding superconductivity. The fact that the density of states does not change around the Fermi level enforces the idea of a non-BCS model for NaAlSi but does not prove it. Further experimental and theoretical work will be required to determine what the dominant factors are in determining Tc in these analogs of the “111” pnictide superconductors.
In a different project , we observed bulk superconductivity with Tc = 0.6 K in the intermetallic compound HfCuGe2 (Fig. 1) which is structurally related to the “1111” Fe-based superconductors but contains only non-magnetic elements. These findings indicates that superconductivity tends to run in certain structure types, and the very low Tc observed supports arguments that the presence of magnetic Fe is important for obtaining enhanced Tc in this family.