Graphic showing "loop currents," in light blue,听flowing inside a honeycomb-like material, while electrons, in green, pass through. (Credit: Oak Ridge National Laboratory)
A series of buzzing, bee-like 鈥渓oop-currents鈥� could explain a recently discovered, never-before-seen phenomenon in a type of quantum material. The findings from researchers at CU Boulder may one day help engineers develop new kinds of devices, such as quantum sensors or the quantum equivalent of computer memory storage devices.听
The quantum material in question is known by the chemical formula Mn3Si2Te6. But you could also call it 鈥渉oneycomb鈥� because its manganese and tellurium atoms form a network of interlocking octahedra that look like the cells in a beehive.听
Physicist Gang Cao and his colleagues at CU Boulder in their lab in 2020, and they were in for a surprise: Under most circumstances, the material behaved a lot like an insulator. In other words, it did听not allow electric currents to pass through it easily. When they exposed the honeycomb to magnetic fields in a certain way, however, it suddenly became millions of times less resistant to currents. It was almost as if the material had morphed from rubber into metal. 听
鈥淚t was both astonishing and puzzling,鈥� said Cao, professor in the Department of Physics and corresponding author of the new study. 鈥淥ur follow-up effort in pursuing a better understanding of the phenomena led us to even more surprising discoveries.鈥澨�
Now, he and his colleagues think they can explain that astonishing behavior. The group, including several graduate students at CU Boulder, online Oct. 12听in the journal Nature.听
Drawing on experiments in Cao鈥檚 lab, the group reports that, 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. Since the 1990s, physicists have theorized such loop currents could exist in a handful of known materials, such as high-temperature superconductors, but they have yet to directly observe them.听
Cao said they could be capable of driving startling transformations in quantum materials like听the one he and his team stumbled on.
鈥淲e鈥檝e discovered a new quantum state of matter,鈥� Cao said. 鈥淚ts quantum transition is almost like ice melting into water.鈥�
Colossal changes
The study homes in on a strange property in physics called colossal magnetoresistance (CMR).听
In the 1950s, physicists realized if they exposed certain types of materials to magnets that generate a magnetic polarization, they could make those materials undergo a shift鈥攃ausing them to switch from insulators to more wire-like conductors. Today, this technology shows up in computer disk drives and many other electronic devices where it helps to control and shuttle electric currents along distinct paths.听
The honeycomb in question, however, is vastly different from those materials鈥攖he CMR occurs only when conditions avoid that same kind of magnetic polalrization. The shift in electrical properties is also much more extreme than what you can see in any other known CMR material, Cao added.
鈥淵ou have to violate all the conventional conditions to achieve this change,鈥� Cao said.
Melting ice
He and his colleagues, including CU Boulder graduate students Yu Zhang, Yifei Ni and Hengdi Zhao, wanted to find out why.
They, along with co-author Itamar Kimchi of Georgia Institute of Technology, hit on the idea of loop currents. According to the team鈥檚 theory, 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. It鈥檚 a bit like cars driving through a roundabout in both directions at once.听
That disorder can cause 鈥渢raffic jams鈥� for electrons traveling in the material, Cao said, increasing the resistance and making the honeycomb an insulator. 听
As Cao put it: 鈥淓lectrons like order.鈥澨�
The physicist added, however, 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. Put differently, the traffic jams disappear. Once that happens, electrons can speed through the quantum material, almost as if it was a metal wire.
鈥淭he internal loop currents circulating along the edges of the octahedra are extraordinarily susceptible to external currents,鈥� Cao said. 鈥淲hen an external electric current exceeds a critical threshold, it disrupts and eventually 'melts'听the loop currents,听leading to a different electronic state."听
He noted, in most materials, the switch from one electronic state to another happens almost instantaneously, or in the span of trillionths of a second. But in his honeycomb, that transformation can take seconds or even longer to occur.
Cao suspects the entire structure of the honeycomb begins to morph, with the bonds between atoms breaking and reforming in new patterns. That kind of reordering takes an unusually long time, he noted鈥攁 bit like what happens when ice melts into water.
Cao said the work provides a new paradigm for quantum technologies. For now, you probably won鈥檛 see this honeycomb in any new electronic devices. That鈥檚 because the switching behavior only takes place at cold temperatures. He and his colleagues, however, are searching for similar materials that will do the same thing under much more hospitable conditions.
鈥淚f we want to use this in future devices, we need to have materials that show the same type of behavior at room temperature,鈥� Cao said.
Now, that sort of invention could be buzz-worthy.
