A series of buzzing “loop-currents” could explain a recently discovered, never-before-seen phenomenon in a type of quantum material. The quantum material is known by the chemical formula Mn 3Si2Te6, but it’s safe to call it “honeycomb” because its manganese and tellurium atoms form a network of interlocking octahedra that resembles a beehive.
Professor in the Department of Physics Gang Cao and his colleagues at CU Boulder synthesized this molecular beehive in their lab in 2020, and they found that in most instances the material behaved a lot like an insulator; it did not allow electric currents to pass through it easily. When exposed to magnetic fields in a certain way, however, the honeycomb suddenly became millions of times less resistant to currents.
Now the team thinks it can further explain the behavior. Under certain conditions, the honeycomb is abuzz with tiny internal currents known as chiral orbital currents, or loop currents. Electrons zip around in loops within each of the octahedra in this quantum material. The work homes in on a strange property in physics called colossal magnetoresistance (CMR).
“We’ve discovered a new quantum state of matter,” Cao said. “Its quantum transition is almost like ice melting into water.”
The team’s theory states countless electrons circulate around inside their honeycombs at all times, tracing the edges of each octahedron. In the absence of a magnetic field, those loop currents tend to stay disorderly, or flow in both clockwise and counterclockwise patterns — akin to cars driving through a roundabout in both directions at once.
That disorder can cause “traffic jams” for electrons traveling in the material, increasing the resistance and making the honeycomb an insulator.
“Electrons like order,” Cao said.
The team said that if you pass an electric current into the quantum material in the presence of a specific kind of magnetic field, the loop currents will begin to circulate only in one direction. No more traffic jams.
“The internal loop currents circulating along the edges of the octahedra are extraordinarily susceptible to external currents,” Cao said. “When an external electric current exceeds a critical threshold, it disrupts and eventually ‘melts’ the loop currents, leading to a different electronic state.”
The team added that, in most materials, the switch from one electronic state to another happens almost instantaneously. But in his honeycomb, that transformation can take seconds or even longer to occur.
Cao said he suspects the entire structure of the honeycomb begins to morph, with the bonds between atoms breaking and reforming in new patterns.
The switching behavior only takes place at cold temperatures, but the team is searching for similar materials that will do the same thing under better conditions.
“If we want to use this in future devices, we need to have materials that show the same type of behavior at room temperature,” Cao said.
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