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Honeycomb Quantum Material's Transforming Properties Explored

A succession of buzzing, bee-like “loop-currents” could illustrate a newly discovered, unparalleled occurrence in a specific type of quantum material.

Honeycomb Quantum Material

Graphic showing "loop currents," in light blue, flowing inside a honeycomb-like material, while electrons, in green, pass through. Image Credit: Oak Ridge National Laboratory.

The conclusions from scientists at the University of Colorado Boulder may soon assist engineers in creating new types of devices, like quantum sensors or the quantum counterpart of computer memory storage platforms.

This specific quantum material is recognized by the chemical formula Mn3Si2Te6. However, one could also refer to it as a “honeycomb” because its tellurium and manganese atoms create a network of interlocking octahedra that resemble the cells in a beehive.

Physicist Gang Cao and his contemporaries at the University of Colorado Boulder manufactured this molecular beehive in their laboratory in 2020, and they were taken by surprise. Under a majority of circumstances, the material acted much like an insulator.

Simply put, it did not permit electric currents to travel easily through it. When the honeycomb was exposed to magnetic fields in a specific way; however, it unexpectedly became millions of times less resistant to currents. It was virtually as if the material had transformed from rubber into metal. 

It was both astonishing and puzzling. Our follow-up effort in pursuing a better understanding of the phenomena led us to even more surprising discoveries.

Professor Gang Cao, Corresponding Author and Physicist, Department of Physics, University of Colorado Boulder

Currently, he and his contemporaries believe they can clarify that surprising behavior. The team, including many graduate students at the University of Colorado Boulder, published its latest results in the October 12th, 2022, online edition of the journal Nature.

Based on experiments performed in Cao’s laboratory, the team reports that, under specific circumstances, the honeycomb is abuzz with minute internal currents called loop currents or chiral orbital currents. Electrons travel around in loops inside each of the octahedra in this quantum material.

Since the 1990s, physicists have hypothesized such loop currents could be present in a few identified materials, like high-temperature superconductors, but they still have to analyze them directly.

Cao stated they could be able to trigger astonishing transformations in quantum materials like the one he and his team uncovered.

We’ve discovered a new quantum state of matter. Its quantum transition is almost like ice melting into water.

Professor Gang Cao, Corresponding Author and Physicist, Department of Physics, University of Colorado Boulder

Colossal Changes

The research looks at a peculiar property in physics known as colossal magnetoresistance (CMR).

In the 1950s, physicists determined that if specific types of materials were exposed to magnets that produce magnetic polarization, they could cause those materials to go through a shift—causing them to transform from insulators to more wire-like conductors.

Currently, this technology is available in computer disk drives and several other electronic gadgets where it helps to regulate and transport electric currents along specific paths.

The honeycomb, however, is hugely diverse from those materials—the CMR happens when circumstances evade that same kind of magnetic polarization. Cao explained that the change in electrical properties is much more extreme than what one can observe in any other established CMR material.

“You have to violate all the conventional conditions to achieve this change,” Cao said.

Melting Ice

The team, including CU Boulder graduate students Yifei Ni, Yu Zhang, and Hengdi Zhao, were curious to learn why.

Together with Co-Author Itamar Kimchi of Georgia Institute of Technology, they chose to explore the concept of loop currents. According to the team’s theory, myriad electrons constantly travel around within their honeycombs, outlining the edges of each octahedron.

In the absence of a magnetic field, those loop currents are inclined to stay unsystematic or flow in clockwise and counterclockwise configurations. It is a little like cars driving around a roundabout in both directions simultaneously.

That chaos can result in “traffic jams” for electrons moving in the material, Cao said, boosting the resistance and turning the honeycomb into an insulator. 

As Cao illustrates it: “Electrons like order.”

The researcher added, however, that if an electric current is passed into the quantum material exposed to a specific kind of magnetic field, the loop currents will start to circulate in just one direction. Once that takes place, electrons can accelerate through the quantum material, virtually as if it was a metal wire.

The internal loop currents circulating along the edges of the octahedra are extraordinarily susceptible to external currents. When an external electric current exceeds a critical threshold, it disrupts and eventually 'melts' the loop currents, leading to a different electronic state.

Professor Gang Cao, Corresponding Author and Physicist, Department of Physics, University of Colorado Boulder

He observed that in most materials, the transformation from one electronic state to another occurs virtually immediately or in the span of trillionths of a second. However, in his honeycomb, that alteration could take seconds or longer.

Cao suspects the whole structure of the honeycomb begins to transform, with the bonds between atoms splitting and restructuring into new patterns. That sort of reordering takes an extraordinarily long time, somewhat like what takes place when the ice melts into water.

Cao said the study offers a new standard for quantum technologies. At present, one will not see this honeycomb in any new electronic gadgets. That is due to the switching behavior taking place only at cold temperatures. He and his contemporaries, however, are hunting for similar materials that will achieve the same thing under much more favorable circumstances.

If we want to use this in future devices, we need to have materials that show the same type of behavior at room temperature.

Professor Gang Cao, Corresponding Author and Physicist, Department of Physics, University of Colorado Boulder

That kind of invention could be exciting.

Journal Reference:

Zhang, Y., et al. (2022) Control of chiral orbital currents in a colossal magnetoresistance material. Nature.


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