Physicists probe the ‘amazing’ properties of metamorphosis of a honeycomb-like substance

Physicists probe properties

By exposing a honeycomb-like material with a specific type of magnetic field, the yellow arrow, researchers can create a system between toroidal currents, in bright blue, within that material. Electrons, in green, can pass through the material more easily. Credit: Oak Ridge National Laboratory

A series of noisy, bee-like “ring currents” could explain a recently discovered phenomenon that has never been seen before in a type of quantum material. The findings by researchers at the University of Colorado Boulder may one day help engineers develop new types of devices, such as quantum sensors or the quantum equivalent of computer memory storage devices.

The quantitative substance in question is known by the chemical formula Mn3bad2T6. But you can also call it “Beehive“Because the manganese and tellurium atoms form an interlocking octahedron-like network in a beehive.

Physicist Gang Kao and colleagues at CU Boulder made this molecular beehive in their lab in 2020, and they were in for a surprise: In most conditions, the material behaved much like an insulator. In other words, it did not allow electric currents to pass through it easily. However, when they exposed the honeycomb to magnetic fields in a certain way, it suddenly became millions of times less resistant to currents. It was as if the material had turned from rubber to metal.

“It was startling and baffling at the same time,” said Kao, a professor in the Department of Physics and corresponding author of the new study. “Our follow-up efforts in pursuit of a better understanding of phenomena have led to even more surprising discoveries.”

Now, he and his colleagues think they can explain this amazing behavior. The group, including several CU Boulder graduate students, published their latest findings on November 17 in the journal temper nature.

Drawing on experiments in Kao’s lab, the group reports that under certain conditions, the honeycomb is filled with tiny internal currents known as chiral orbital currents, or toroidal currents. Electrons move in rings within each of the octahedrons in this quantum material. Since the 1990s, physicists have assumed that toroidal currents can exist in a few known materials, such as high temperature superconductorsbut they haven’t directly monitored it yet.

They may be able to drive amazing transformations in quantum materials like the ones he and his team have found, Kao said.

“We have discovered a new quantum state of matter,” Kao said. “Its quantum transmission is almost like melting ice in water.”

Huge changes

The study focuses on a peculiar property of physics called colossal magnetoresistance (CMR).

In the 1950s, physicists realized that if they subjected certain types of materials to a magnet that generated magnetic polarization, they could cause those materials to undergo a transformation—causing them to shift from insulators to more wire-like conductors. Today, this technology appears in computer drives and many other electronic devices where it helps with control and navigation electric currents Along marked paths.

However, the honeycomb in question is very different from those of materials – CMR only occurs when conditions avoid the same type of magnetic polarization. transformation in electrical properties It’s also more extreme than you can see in any other known CMR material, Cao added.

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

melting ice

He and his colleagues, including CU Boulder graduate students Yu Zhang, Yifei Ni and Hengdi Zhao, wanted to know why.

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

can cause this disorder”traffic jam“For electrons that travel in the material, which increases the resistance and makes the honeycomb an insulator,” Kao said.

As Cao said: “Electrons are like a system.”

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

“Inner ring currents that run along the edges of the octahedron are extraordinarily vulnerable to external currents,” Kao said. “When an external electrician Stream Exceeding a critical threshold, it disables and eventually dissolves the loop currents, resulting in a different electronic state.”

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

Cao suspects that the entire structure of the honeycomb begins to shift, with bonds between atoms breaking and remodeling into new patterns. He noted that this kind of rearrangement takes an unusually long time — a bit like what happens when ice melts in water.

Kao said the work provides a new paradigm for quantum technologies. For now, you probably won’t see this honeycomb in any new electronic devices. That’s because the switching behavior only occurs at cold temperatures. However, he and his colleagues are looking for similar materials that would do the same under more hospitable conditions.

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

Now, this kind of invention could be worth the hype.

A method for controlling magnetic behavior in quantum matter

more information:
Gang Kao, Control of chiral orbital currents in a colossal magnetoresistance material, temper nature (2022). DOI: 10.1038 / s41586-022-05262-3.

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