Cuprates, pnictides and sulfosalts: lessons in functional materials

Murunskite K₂Cu₃FeS₄is a representative sulfosalt, isostructural to the pnictides, but with electronic properties more similar to the insulating parent compounds of the cuprates. We use it as a bridge to compare the chemical and physical roles of metal and ligand orbitals in cuprates and pnictides. In cuprates, ionicity, covalency, and metallicity are tightly interwoven to give rise to high-temperature superconductivity (SC). Their most remarkable property is the interaction of an ionically localized hole on the copper (Cu) with a Fermi liquid (FL) on the oxygens (O), which is critically important for understanding all key properties of these materials. The localization is due to strong correlations on the Cu 3<i>d</i>orbital. We describe a scenario in which the localized hole gives rise both to SC by Cooper scattering of O holes, and to Fermi arcs, as observed in cuprate spectroscopy, the latter by a purely kinematic projection of the static local disorder, without invoking any residual interactions between the mobile O FL carriers. In the pnictides, the orbitals responsible for binding and metallic conduction appear to be separate. The Fe 3<i>d</i><i>e</i>_<i>g</i>orbitals hybridized with the ligands set the lattice spacing. The 3<i>d</i>t2gorbitals overlap directly between the Fe atoms, resulting in several electronic bands appearing at the Fermi level. The ensuing FL exhibits both charge and magnetic correlations. We argue that a similar SC scenario as in the cuprates is plausible in the pnictides, except that a light FL scatters on a slow nearly-antiferromagnetic (AF) one, rather than on localized holes as in the cuprates. In murunskite, the greater covalency of sulfur orbitals compensates for total disorder in the Fe atom positions, giving rise to an agglomeration of fractal magnetic clusters coexisting with perfect crystallographic order and exhibiting surprisingly robust quarter-zone AF. Because the open ligand orbitals play an important role in it, the essentially local quarter-zone AF of murunskite is more similar to the SC of the cuprates than to the half-zone AF of the pnictides, in which the ligands are passive.