Under pressure, the “spongy” compound reacts remarkably

Remarkable things happen when a “spongy” compound of manganese and sulphide (MnS₂) is compressed into a diamond anvil, according to researchers at the University of Rochester and the University of Nevada, Las Vegas (UNLV).

“This is a new kind of charge transfer mechanism, and so from the point of view of the scientific community, it’s very, very exciting. We’re showing remarkable physical transformations over a very, very short range of parameters, in this case the pressure, ”says Ashkan Salamat, associate professor of physics at UNLV.

For example, when the pressure increases, MnS₂, a soft insulator, changes to a metallic state and then back to an insulator, the researchers describe in an article reported as an editor’s choice in Physical examination letters.

“Metals generally remain metals; it is highly unlikely that they could then be turned into an insulator,” says Ranga Dias, assistant professor of mechanical engineering and physics and astronomy at Rochester. “The fact that this material goes from an insulator to a metal and back to an insulator is very rare.”

Additionally, the transitions are accompanied by unprecedented decreases in resistance and volume over an extremely narrow range of pressure variation, all occurring at around 80 degrees Fahrenheit. The relatively low temperature increases the chances that the metallic transition process can eventually be harnessed for the technology, says Salamat.

In previous articles by Nature and Physical examination letters, the Dias and Salamat collaboration has established new benchmarks for achieving superconductivity at room temperature. A common denominator of their work is to explore the “remarkably bizarre” ways transition metals and other materials behave when paired with sulfides and then compressed in a diamond cell anvil.

“The new phenomena we are reporting are a fundamental example of high pressure responses and will find a place in physics textbooks,” said Salamat. “There is something very intriguing about the way sulfur behaves when attached to other elements. It has led to some remarkable breakthroughs.”

The breakthroughs made by the Dias and Salamat laboratories involved the compression of single picoliters of material, roughly the size of a single inkjet particle.

Rotation and pressure underpin the dramatic metal transition

The transitions described in this article underlie how the spin (angular momentum) states of individual electrons interact when pressure is applied, explain Dias and Salamat.

When MnS₂ is in its normal insulator state, electrons are mostly in unpaired “high spin” orbitals, causing atoms to actively bounce back and forth. As a result, the material has a higher resistance to an electric charge because there is less free space for individual electrons trying to pass through the material.

But as pressure is applied and the material is compressed to a metallic state, the electron orbitals “start to see each other, immediately come closer, and pairs of electrons begin to bond into one,” said Salamat.

This opens up more space for individual electrons to move through the material, so much so that the resistance drops dramatically by 8 orders of magnitude, when the pressure drops from 3 gigapascals (435,000 psi) to 10 gigapascals. This is a relative “boost” from the 182-268 gigapascals required for superconducting materials.

“Considering the small pressure range involved, a drop in resistance of this magnitude is really huge,” says Dias.

A low resistance is maintained even in the final stage, when the MnS₂ becomes an insulator again, because the electrons remain in a “low spin” state.

Fundamental science of materials, future technological advances

As often happens with new discoveries in basic science, the possible applications have yet to be explored.

However, says Salamat, a transition metal that, with a relatively small amount of stress, can go from one state to another – at room temperature, no less – is likely to be useful.

“You could imagine having a logical switch or a write hard drive, where a very, very small voltage or voltage swap could cause something to go from one electronic state to another. New versions of flash memory, or semiconductor memory, could swap and take on a new approach using these types of materials, ”says Salamat.

“You can perform maneuvers aggressive enough to drive these materials up to 300 Kelvin, which makes them potentially useful for technology.”

Lead author Dylan Durkee, a former undergraduate researcher at the Salamat Lab, now works as a graduate student with Dias. Other co-authors include Nathan Dasenbrock-Gammon and Elliot Snider in Rochester; Keith Lawler, Alexander Smith and Christian Childs at UNLV; Dean Smith at the Laboratoire National d’Argonne, and Simon AJ Kinder at the University of Burgundy.