New honeycomb-like material may help us develop quantum products

"Our follow-up effort in pursuing a better understanding of the phenomena led us to even more surprising discoveries."
Nergis Firtina
Graphic showing "loop currents," in light blue, flowing inside a honeycomb-like material, while electrons, in green, pass through.
Graphic showing "loop currents," in light blue, flowing inside a honeycomb-like material, while electrons, in green, pass through.

Oak Ridge National Laboratory 

A recently found, previously unseen phenomenon in a type of quantum material could be explained by a succession of buzzing, bee-like "loop-currents," thanks to University of Colorado Boulder scientists.

Published in Nature on October 12, the study could help engineers to develop new kinds of devices, such as quantum sensors or the quantum equivalent of computer memory storage devices.

The chemical formula for the quantum material is "Mn3Si2Te6." It's also known as "honeycomb" because its manganese and tellurium atoms form a network of interlocking octahedra that resembles beehive cells.

"It was both astonishing and puzzling," said Gang Cao, professor in the Department of Physics and corresponding author of the new study, which started in 2020.

"Our follow-up effort in pursuing a better understanding of the phenomena led us to even more surprising discoveries."

"Almost like ice melting into the water"

The substance acted largely like an insulator in most situations. In other words, it made it difficult for electrical currents to flow through it when the honeycomb was subjected to a magnetic.

Now, Gang Cao and his colleagues think they can explain that astonishing behavior.

New honeycomb-like material may help us develop quantum products
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The team reports that under some circumstances, the honeycomb is alive with tiny internal currents known as chiral orbital currents, or loop currents, based on tests in Cao's lab. Within each of the octahedra of this quantum substance, electrons zip around in loops.

Many known materials, including high-temperature superconductors, have been postulated to have such loop currents since the 1990s, but they have not yet been physically observed.

"We've discovered a new quantum state of matter," Cao said. "Its quantum transition is almost like ice melting into water."

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The honeycomb in question, however, is vastly different from those materials—the CMR occurs only when conditions avoid that same kind of magnetic polarization. The shift in electrical properties is also much more extreme than what you can see in any other known CMR material, Cao added.

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

Chaotic electrons

Cao and his team, including CU Boulder graduate students Yu Zhang, Yifei Ni, and Hengdi Zhao, wanted to find out the reason and launched forth.

They came up with the concept of loop currents along with co-author Itamar Kimchi of the Georgia Institute of Technology. The team's hypothesis states that numerous electrons constantly move about inside their honeycombs, tracing the boundaries of each octahedron.

These loop currents typically remain chaotic or flow in both clockwise and counterclockwise directions in the absence of a magnetic field. It resembles cars traveling simultaneously across two lanes of a roundabout.

That disorder can cause chaos for electrons traveling in the material, Cao said, increasing the resistance and making the honeycomb an insulator.

"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."

Cao also added that the work provides a new paradigm for quantum technologies.

Abstract:

Colossal magnetoresistance (CMR) is an extraordinary enhancement of electrical conductivity in the presence of a magnetic field. It is conventionally associated with a field-induced spin polarization that drastically reduces spin scattering and electric resistance. Ferrimagnetic Mn3Si2Te6 is an intriguing exception to this rule: it exhibits a seven-order-of-magnitude reduction in ab plane resistivity that occurs only when a magnetic polarization is avoided1,2. Here, we report an exotic quantum state that is driven by ab plane chiral orbital currents (COC) flowing along edges of MnTe6 octahedra. The c axis orbital moments of ab plane COC couple to the ferrimagnetic Mn spins to drastically increase the ab plane conductivity (CMR) when an external magnetic field is aligned along the magnetic hard c axis. Consequently, COC-driven CMR is highly susceptible to small direct currents exceeding a critical threshold, and can induce a time-dependent, bistable switching that mimics a first-order 'melting transition' that is a hallmark of the COC state. The demonstrated current-control of COC-enabled CMR offers a new paradigm for quantum technologies.

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