Superconductors — characterized by zero electrical resistance and the expulsion of magnetic fields — are known for their ability to conduct electricity without energy loss. They have applications in various fields, ranging from magnetic resonance imaging machines in the healthcare sector to high-tech quantum computers. Almost all the known superconductors are grown in laboratories by alloying or synthesis, which require resources and produce chemical waste. Therefore, finding superconductivity in a naturally occurring compound would be a key innovation.

Credit: adapted from H. Kim et al. Commun. Mater. 5, 17 (2024) under a Creative Commons licence CC BY 4.0

Now, Hyunsoo Kim and colleagues have demonstrated superconductivity below 5.4 K in a lab-grown sample of Rh17S15 (pictured), which is also found in nature as the mineral miassite (H. Kim et al. Commun. Mater. 5,17; 2024). Interestingly, the superconducting properties of this compound are quite different from those seen in most of the known superconductors.

In a conventional superconductor, an isotropic energy gap opens around the Fermi energy as soon as the system transitions to the superconducting state. However, Kim and colleagues found that in the case of miassite, the superconducting gap closes along certain symmetry directions referred to as nodes.

The team performed penetration depth measurements to determine the distance up to which a weak magnetic field can penetrate inside the superconductor from the surface. Although this length is temperature independent at low temperatures for conventional superconductors, Rh17S15 showed a linear temperature dependence — a sign of unconventional behaviour. Additionally, the superconducting properties strongly depended on the number of non-magnetic defects, unlike a conventional superconductor, where such defects hardly have any effects.

Thus, Rh17S15 turns out to be an unconventional superconductor, challenging the notion that unconventional superconductivity is solely a product of artificial synthesis.

Although Rh17S15 in its mineral form contains lots of impurities that hinder the manifestation of unconventional superconductivity, Kim and colleagues’ discovery is significant as it may change the focus of contemporary superconductivity research to consider sustainability more regularly. For example, the community could slowly switch from superconductors requiring the use of toxic elements such as arsenic, selenium and uranium towards naturally abundant minerals for understanding the mechanism of unconventional superconductors.