Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in copper-oxide compounds called cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Physicists at Ames National Laboratory have found compelling evidence of unconventional superconductivity in synthetic samples of Rh17S15, which is also found in nature as the mineral miassite.
Miassite is one of only four minerals found in nature that act as a superconductor when grown in the lab and the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form. Image credit: Paul Canfield.
Superconductivity is when a material can conduct electricity without energy loss.
Superconductors have applications including medical MRI machines, power cables, and quantum computers.
Conventional superconductors are well understood but have low critical temperatures.
The critical temperature is the highest temperature at which a material acts as a superconductor.
In the 1980s, scientists discovered unconventional superconductors, many of which have much higher critical temperatures.
“All these materials are grown in the lab,” said Ames National Laboratory researcher Ruslan Prozorov.
“This fact has led to the general belief that unconventional superconductivity is not a natural phenomenon.”
“It is difficult to find superconductors in nature because most superconducting elements and compounds are metals and tend to react with other elements, like oxygen.”
“Miassite is an interesting mineral for several reasons, one of which is its complex chemical formula.”
“Intuitively, you think that this is something which is produced deliberately during a focused search, and it cannot possibly exist in nature. But it turns out it does.”
Growing the miassite crystals was part of a larger effort to discover compounds that combine very high melting elements (like Rh) and volatile elements (like S).
“Contrary to the nature of the pure elements, we have been mastering the use of mixtures of these elements that allow for low temperature growth of crystals with minimal vapor pressure,” said Professor Paul Canfield, a physicist at Ames National Laboratory and Iowa State University.
“It’s like finding a hidden fishing hole that is full of big fat fish. In the Rh-S system we discovered three new superconductors.”
“And, through detailed measurements, we discovered that the miassite is an unconventional superconductor.”
The researchers used three different tests to determine the nature of miassite’s superconductivity.
The main test is called the London penetration depth. It determines how far a weak magnetic field can penetrate the superconductor bulk from the surface.
In a conventional superconductor, this length is basically constant at low temperature.
However, in unconventional superconductors, it varies linearly with the temperature.
This test showed that miassite behaves as an unconventional superconductor.
Another test the team performed was introducing defects into the material.
“This test is a signature technique his team has employed over the past decade. It involves bombarding the material with high-energy electrons,” Dr. Prozorov said.
“This process knocks-out ions from their positions, thus creating defects in the crystal structure.”
“This disorder can cause changes in the material’s critical temperature.”
Conventional superconductors are not sensitive to non-magnetic disorder, so this test would show no or very little change in the critical temperature.
Unconventional superconductors have a high sensitivity to disorder, and introducing defects changes or suppresses the critical temperature. It also affects the critical magnetic field of the material.
In miassite, the scientists found that both the critical temperature and the critical magnetic field behaved as predicted in unconventional superconductors.
Investigating unconventional superconductors improves scientists understanding of how they work.
“This is important because uncovering the mechanisms behind unconventional superconductivity is key to economically sound applications of superconductors,” Dr. Prozorov said.
The discovery is reported in a paper in the journal Communications Materials.
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H. Kim et al. 2024. Nodal superconductivity in miassite Rh17S15. Commun Mater 5, 17; doi: 10.1038/s43246-024-00456-w
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