Loop Currents in Honeycomb

Molecular beehive: Physicists study ‘amazing’ morphing properties of honeycomb quantum material

By subjecting a honeycomb material to a specific magnetic field, yellow arrow, researchers can create order between the loop currents, light blue, within that material. Electrons, in green, can then pass through the material much more easily. Photo credit: Oak Ridge National Laboratory

A recently discovered, never-before-seen phenomenon in some kind of quantum material could be explained by a series of buzzing, bee-like ‘loop currents’. The discovery by University of Colorado Boulder (CU Boulder) physicists could one day help engineers create new types of devices, such as B. Quantum sensors or the quantum equivalent of computer storage devices.

The specific quantum material in question is known by the chemical formula Mn3If2You6. Or you could just call it a “honeycomb,” because its manganese and tellurium atoms form a network of interlocking octahedra that look like the cells in a beehive.

“We have discovered a new quantum state of matter. Its quantum transition is almost like melting ice into water.” — Gang Cao

When physicist Gang Cao and his colleagues at CU Boulder synthesized this molecular beehive in their lab in 2020, they received a shock: under most circumstances, the material behaved like an insulator. This means that electric currents could not easily pass through it. However, when they exposed the honeycomb to magnetic fields in a certain way, it suddenly became a million times less resistant to currents. It was almost as if the material had changed from rubber to metal.

“It was both amazing and enigmatic,” said Cao, corresponding author of the new study and professor at the Department of Physics. “Our subsequent efforts to better understand the phenomena led us to even more surprising discoveries.”

He and his colleagues now think they can explain this amazing behavior. The group, which included several CU Boulder graduate students, published their latest findings in the journal Nature on October 12th.

Drawing on experiments in Cao’s lab, the research group reports that under certain conditions, the honeycomb is traversed by 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. Since the 1990s, physicists have theorized that loop currents might exist in a handful of known materials, such as high-temperature superconductors, but have yet to observe them directly.

Cao said they may be able to drive surprising transformations in quantum materials like the ones he and his team stumbled upon.

“We have discovered a new quantum state of matter,” Cao said. “Its quantum transition is almost like melting ice into water.”

Colossal changes

The study focuses on a strange property in physics called colossal magnetoresistance (CMR).

In the 1950s, physicists realized that if they exposed certain types of materials to magnets that produce magnetic polarization, they could subject those materials to a shift — which would cause them to change from insulators to more wire-like conductors. Today, this technology shows up in computer drives and many other electronic devices, helping to control and transport electrical currents in various ways.

However, the honeycomb in question is very different from these materials – the CMR only occurs when conditions avoid the same type of magnetic polarization. Cao added that the shift in electrical properties is also much more extreme than any other known CMR material.

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

melting ice

He and his colleagues, including CU Bouldering grads Yu Zhang, Yifei Ni and Hengdi Zhao, wanted to find out why.

They came up with the idea of ​​loop currents along with co-author Itamar Kimchi from the Georgia Institute of Technology. According to the team’s theory, countless electrons are constantly circulating in their honeycombs, tracing the edges of each octahedron. In the absence of a magnetic field, these loop currents tend to remain disordered, or to flow both clockwise and counterclockwise. It’s a bit like cars going through a roundabout in both directions at once.

This disruption can cause “jam” for electrons moving in the material, Cao said, increasing resistance and making the honeycomb an insulator.

As Cao put it, “electrons like order.”

However, the physicist added that if you pass an electric current into the quantum material in the presence of a certain magnetic field, the loop currents start to circulate in only one direction. In other words, the traffic jams disappear. Once that happens, electrons can race through the quantum material, almost as if it were a metal wire.

“The internal loop currents that circulate along the edges of the octahedron are extraordinarily vulnerable to external currents,” Cao said. “When an external electrical current exceeds a critical threshold, it disrupts the loop currents and eventually ‘melts’, resulting in a different electronic state.”

He found that for most materials, the transition from one electronic state to another occurs almost instantaneously, or within trillionths of a second. But in his honeycomb, this transformation can take seconds or even longer.

Cao suspects that the entire structure of the honeycomb is beginning to change, with the bonds between atoms breaking and reforming in new patterns. This sort of rearrangement takes an unusually long time, he noted — a bit like what happens when ice melts into water.

Cao said the work offers a new paradigm for quantum technologies. For now, you probably won’t see this honeycomb in any new electronic device. Because the switching behavior only takes place at cold temperatures. However, he and his colleagues are looking for similar materials that will do the same thing under much more hospitable conditions.

“If we want to use this in future devices, we need materials that show the same behavior at room temperature,” Cao said.

Well, this kind of invention might be worth buzzing about.

Reference: “Control of chiral orbital currents in a colossal magnetoresistance material” by Yu Zhang, Yifei Ni, Hengdi Zhao, Sami Hakani, Feng Ye, Lance DeLong, Itamar Kimchi and Gang Cao, October 12, 2022, Nature.
DOI: 10.1038/s41586-022-05262-3

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