Cuprates, Pnictides and Sulfosalts: Lessons in Functional Materials

TL;DR

This paper compares the electronic properties of cuprates, pnictides, and sulfosalts, highlighting how their orbital interactions influence high-temperature superconductivity and proposing a unified scenario for their superconducting mechanisms.

Contribution

It introduces a comparative analysis of sulfosalts, cuprates, and pnictides, revealing new insights into their orbital interactions and superconductivity mechanisms.

Findings

Sulfosalt murunskite isostructural to pnictides but similar to cuprates in electronic properties.

Localized holes on Cu in cuprates are crucial for high-temperature superconductivity.

A similar superconductivity scenario may exist in pnictides involving different Fermi liquid interactions.

Abstract

Murunskite K$_2$Cu$_3$FeS$_4$ is 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 $3d$ 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 $3d$ $e_{g}$ orbitals hybridized with the ligands set the lattice spacing. The $3d$ $t_{2g}$ orbitals overlap directly between the Fe atoms, resulting in several electronic bands appearing at the Fermi level. The ensuing Fermi liquid 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.