Synthesis of Murunskite Single Crystals: A Bridge Between Cuprates and Pnictides

TL;DR

This study reports the synthesis of high-quality murunskite single crystals, revealing their potential as a new system bridging cuprates and pnictides, with properties conducive to future superconductivity research.

Contribution

It introduces murunskite as a novel interpolation compound between cuprates and pnictides, with successful synthesis and detailed characterization.

Findings

Murunskite shows semiconducting behavior and antiferromagnetic order at 100 K.

Spectroscopy and DFT calculations reveal partially open sulfur 3p orbitals.

Valence and conduction bands exhibit cuprate-like and pnictide-like characteristics, respectively.

Abstract

Numerous contemporary investigations in condensed matter physics are devoted to high temperature (high-$T_c$ ) cuprate superconductors. Despite its unique effulgence among research subjects, the enigma of the high-$T_c$ mechanism still persists. One way to advance its understanding is to discover and study new analogous systems. Here we begin a novel exploration of the natural mineral murunskite, K$_2$FeCu$_3$S$_4$, as an interpolation compound between cuprates and ferropnictides, the only known high-$T_c$ superconductors at ambient pressure. Because in-depth studies can be carried out only on single crystals, we have mastered the synthesis and growth of high quality specimens. Similar to the cuprate parent compounds, these show semiconducting behavior in resistivity and optical transmittance, and an antiferromagnetic ordering at 100 K. Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3$p$ orbitals are partially open, making them accessible for charge manipulation, which is a prerequisite for superconductivity in analogous layered structures. DFT indicates that the valence band is more cuprate-like, while the conduction band is more pnictide-like. With appropriate doping strategies, this parent compound promises exciting future developments.