Materials

Weird compound jumps from conductor to insulator and back under pressure

Weird compound jumps from cond...
An illustration depicting how the new material works – normally, the manganese atomic ions (purple circles) and the disulfur molecular ions (figure-8s) are separated (left of frame). But under pressure, they move closer together (right of frame) changing the conductivity of the material
An illustration depicting how the new material works – normally, the manganese atomic ions (purple circles) and the disulfur molecular ions (figure-8s) are separated (left of frame). But under pressure, they move closer together (right of frame) changing the conductivity of the material
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An illustration depicting how the new material works – normally, the manganese atomic ions (purple circles) and the disulfur molecular ions (figure-8s) are separated (left of frame). But under pressure, they move closer together (right of frame) changing the conductivity of the material
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An illustration depicting how the new material works – normally, the manganese atomic ions (purple circles) and the disulfur molecular ions (figure-8s) are separated (left of frame). But under pressure, they move closer together (right of frame) changing the conductivity of the material

Normally metals and insulators sit at opposite ends of a spectrum of conductivity, but researchers have discovered a material that can switch between those states freely, even at room temperature. The material, a compound of manganese and sulfide (MnS2), starts off as an insulator but becomes conductive under pressure.

A material’s conductivity is a result of how easily electrons can move through it. Conductors, semiconductors and insulators are differentiated by their band gap, which essentially measures how much energy electrons need to move freely through the material. So in a conductor, that band gap is very low, in insulators it’s prohibitively high, and semiconductors fall in between.

For the new study, researchers at the University of Rochester and the University of Nevada, Las Vegas investigated compounds that act strangely. Specifically, it seems that pairing metals with sulfides brings out bizarre behavior under high pressure.

MnS2 is normally a soft insulator, but when the team compressed tiny amounts of it in a diamond cell anvil, it transitioned into a metallic, conductive state. That’s weird enough, but as the pressure increases further it switches back to an insulator at a certain point.

“Metals usually remain metals; it is highly unlikely that they can then be changed back to an insulator,” says Ranga Dias, an author of the study. “The fact that this material goes from an insulator to a metal and back to an insulator is very rare.”

On closer inspection, the team uncovered what’s happening inside the material during this process. In its usual insulator state, the electrons in the MnS2 are bouncing around somewhat randomly, leaving very little room for other electrons to move through the material and produce an electric current.

But as the pressure is applied and the material is physically compressed, the electrons are pushed closer together so that they start to link up in pairs. That means that individual electrons can now move through the material more freely – in fact, the resistance drops by eight orders of magnitude, the team says.

Finally, the material becomes an insulator again at even higher pressure because the electrons are kept in a low spin state.

Perhaps most importantly, the conditions required for the transitions are relatively simple, meaning it should be more practical to make use of the material. It can be done at room temperature of 27 °C (80 °F), and at pressures between 3 and 10 gigapascals (GPa). Normally, manipulating conductivity like this requires ultracold temperatures and high pressures of over 180 GPa, conditions that don’t translate well outside of the lab. That said, the team has also recently had success in making materials superconductive at room temperature.

As for what applications there might be for this material, the team has a few ideas.

“You could imagine having a logic switch or writing hard disk, where a very, very small permutation in strain or voltage could make something jump from one electronic state to another,” says Ashkan Salamat, an author of the study. “New versions of flash memory, or solid state memory, could permutate and take on a new approach using these types of materials.”

The research was published in the journal Physical Review Letters.

Source: University of Rochester

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