Nontrivial Role of Interlayer Cation States in Iron-Based Superconductors

Abstract

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 -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 dp model Hamiltonians only.

Magnetic competition in iron-based germanide and silicide superconductors

“…Thus we see that the substitution of As with Ge does turn the striped antiferromagnetic CaFe 2 As 2 into the ferromagnetic CaFe 2 Ge 2 , as expected from our previous DOS analysis, and in line with the arguments outlined in Ref. [3] The further substitution of Ca with Y reduces again the DOS at the Fermi level, which consequently reduces the FM tendency and favors its competition with other magnetic metastable orders [7,8]. The end result is the non-magnetic YFe 2 Ge 2 , where superconductivity is possible again.…”

“…This allows, in particular, a qualitative discussion on the magnetic tendencies and on the competition between different possible instabilities (see e.g. [3,13]), as we do in this work. For a more quantitative discussion, however, an approach including the local many-body physics like dynamical mean-field theory should be used [14].…”

“…2. Compared to CaFe 2 As 2 and YFe 2 Ge 2 (see also [3,5,7,8]), the new compound CaFe 2 Ge 2 displays very similar features at the Fermi energy with all 5 bands of mainly Fe-3d character crossing the Fermi level. However, there is a shift upwards of these features that can be viewed as hole-doping in the iron plane.…”

“…In the case of YFe 2 Ge 2 , the system can be seen as an hole-doped version of CaFe 2 As 2 in its collapsed tetragonal phase [5] where the tendency towards ferromagnetism is due to a Stoner instability [3,7,8]. We exploit this connection and consider the intermediate compound CaFe 2 Ge 2 , reported for the first time in its pure parent phase in [9].…”

Emergent Magnetic Degeneracy in Iron Pnictides due to the Interplay between Spin-Orbit Coupling and Quantum Fluctuations

“…Usually, the existence of additional channels of magnetic fluctuations does not guarantee an enhancement of T c . On the contrary, in the case of ferromagnetic [57] or Néel fluctuations [58], they can cause pair breaking and promote competing superconducting states that suppress T c of the s þ− state. In our case, however, fluctuations associated with the C 2 and C 4 phases are peaked at the same wave vectors ðπ; 0Þ and ð0; πÞ, and thus support the same pairing state.…”