‘Squishy’ compound reacts in remarkable ways to pressure

Remarkable things happen when a “squishy” compound of manganese and sulfide (MnS2) is compressed in a diamond anvil, say researchers from the University of Rochester and the University of Nevada, Las Vegas (UNLV). “We are showing remarkable physical transformations over a very, very short range of parameters, in this case pressure,” says Ashkan Salamat, associate professor of physics at UNLV. For example, as the pressure increases, MnS2, a soft insulator, transitions into a metallic state and then into an insulator again, the researchers describe in a paper flagged as an editor’s choice in Physical Review Letters.

“Metals usually remain metals; it is highly unlikely that they can then be changed back to an insulator,” says Ranga Dias, assistant professor of mechanical engineering and of 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.” Moreover, the transitions are accompanied by unprecedented decreases in resistance and volume across an extremely narrow range of pressure change—all occurring at about 80 degrees Fahrenheit. “The new phenomena we are reporting is a fundamental example of responses under high pressure—and will find a place in physics textbooks,” Salamat says.  Underlying the transitions described in this paper are the way the spin states (angular momentum) of individual electrons interact as pressure is applied. When MnS2 is in its normal insulator state, electrons are primarily in unpaired, “high spin” orbitals, causing atoms to actively bounce back and forth. This results in the material having higher resistance to an electrical 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 toward a metallic state—the electron orbitals “start to see each other, immediately come toward each other, and pairs of electrons start linking up as one,” Salamat says.

This opens up more space for individual electrons to move through the material—so much so that resistance drops dramatically by 8 orders of magnitude, as pressure is increased from 3 gigapascals (435,000 psi) to 10 gigapascals. This is a relative “nudge” compared to the 182 to 268 gigapascals required for superconducting materials. “Given the small range of pressure involved, a drop in resistance of this magnitude is really enormous,” Dias says. Low resistance is maintained even in the final phase—when the MnS2 reverts to an insulator—because the electrons remain in a “low spin” state.

Source: University of Rochester news release