Novel Phenomenon in Quantum Physics Emerges

The search for new states of matter is currently ongoing, and possibly new methods of encoding, manipulating, and transporting information are also expected to find soon. One goal is to harness materials’ quantum properties for communications that do much more than what regular electronics can.

Topological insulators, which are materials that act mainly as insulators but transport electric current on their surface, offer some alluring possibilities.

Understanding Topological Insulators​

“Exploring the complexity of topological materials—along with other intriguing emergent phenomena such as magnetism and superconductivity—is one of the most exciting and challenging areas of focus for the materials science community at the U.S. Department of Energy’s Brookhaven National Laboratory,” said Peter Johnson, a senior physicist in the Condensed Matter Physics & Materials Science Division at Brookhaven. “We’re trying to understand these topological insulators because they have lots of potential applications, particularly in quantum information science, an important new area for the division.”

For instance, materials with this divided insulator/conductor property show a separation in the energy signatures of their surface electrons with opposite rotation. This quantum feature could eventually be harnessed in ‘spintronic’ devices for encoding and carrying information.

In addition, connecting these electrons with magnetism can trigger a novel and very exciting phenomenon.

“When you have magnetism near the surface, you can have these other exotic states of matter that arise from the coupling of the topological insulator with the magnetism,” explained Dan Nevola, a postdoctoral scientist working with Johnson. “If we can find topological insulators with their own intrinsic magnetism, we should be able to efficiently transport electrons of a particular spin in a particular direction.”

Sensitivity to Magnetism​

In new research published in Physical Review Letters, Nevola, Johnson, and their colleagues detail the strange behavior of one such magnetic topological insulator.

The paper also includes experimental evidence that innate magnetism in the bulk of manganese bismuth telluride (MnBi2Te4) spreads to the electrons on its electrically conductive surface. Earlier research had not been conclusive as to whether or not the surface magnetism existed.

However, when the physicists measured the surface electrons’ awareness of magnetism, only one of the two observed electronic states acted as expected. Another surface state, which was anticipated to have a larger response, behaved as if the magnetism wasn’t there.

Schematic diagram showing both the magnetism and the conductive behavior on the surface of MnBi2Te4. [Image Credit: Brookhaven National Laboratory]
“Is the magnetism different at the surface? Or is there something exotic that we just don’t understand?” Nevola asked.

Johnson shares the explanation: “Dan did this very careful experiment, which enabled him to look at the activity in the surface region and identify two different electronic states on that surface, one that might exist on any metallic surface and one that reflected the topological properties of the material,” he said. “The former was sensitive to the magnetism, which proves that the magnetism does indeed exist in the surface. However, the other one that we expected to be more sensitive had no sensitivity at all. So, there must be some exotic physics going on!”

The ​Evidence

​The researchers analyzed the material using different kinds of photoemission spectroscopy, where light from an ultraviolet laser strike knocks electrons loose from the surface of the material and into a detector for measurement.

​To test if these surface electrons were actually sensitive to magnetism, the team cooled the sample to 25 Kelvin, enabling its innate magnetism to appear. However, they observed a ‘gap’ emerging in the expected part of the spectrum only in the non-topological electronic state. 

“Within such gaps, electrons are prohibited from existing, and thus their disappearance from that part of the spectrum represents the signature of the gap,” Nevola said.

The finding of a gap showing up in the regular surface state was enough evidence of magnetic awareness. Also, it was evidence that the magnetism innate in the bulk of this specific material spreads to its surface electrons. Still, the ‘topological’ electronic state the researchers observed showed no such sensitivity to magnetism​, and no gap emerged.

“That throws in a bit of a question mark. These are properties we’d like to be able to understand and engineer, much like we engineer the properties of semiconductors for a variety of technologies,” Johnson continued.

Searching for New States of Matter

For instance, in spintronics, the concept is to use various spin states to encode information in the way positive and negative electric charges are currently used in semiconductor devices in order to encode the ‘bits’ – 1s and 0s – of computer code.

However, spin-coded quantum bits, or qubits, have multiple possible states, not just two. This will hugely expand on the potential to encode information in new and stable ways.

“Everything about magnetic topological insulators looks like they’re right for this kind of technological application, but this particular material doesn’t quite obey the rules,” Johnson said.

Therefore, now, as the team keeps searching for new states of matter and more insights into the quantum world, there’s a new necessity to explain this specific material’s eccentric quantum behavior.

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