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The bizarre response to magnetism presents a mystery of quantum physics

The bizarre response to magnetism presents a mystery of quantum physics

Schematic diagram showing both magnetism and conductive behavior on the surface of MnBi2Te4. Magnetism uniformly points upward, as shown by the red arrows, and the surface electrons, represented by the hourglass structures, are conductive because the upper and lower halves touch each other at the vertex with no “space” in the center (see text). Both of these characteristics are not expected to occur simultaneously, demonstrating the need to further understand the fundamental properties of the material. Credit: Brookhaven National Laboratory

Research is underway to discover new states of matter and possibly new ways of encoding, manipulating and transporting information. One goal is to exploit the quantum properties of communications materials that go beyond what is possible with conventional electronics. Topological insulators – materials that primarily act as insulators but carry electrical current across their surface – provide some tantalizing possibilities.

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

For example, materials with this split insulator / conductor personality show a separation in the energy signatures of their surface electrons with opposite “spin”. This quantum property could be exploited in “spintronic” devices for encoding and transporting information. Going further, coupling these electrons with magnetism can lead to new and exciting phenomena.

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

In a new study just published and highlighted as Editor’s Tip in Physical Review Letters, Nevola, Johnson and their coauthors describe the bizarre behavior of one of these magnetic topological insulators. The paper includes experimental evidence that the intrinsic magnetism in most manganese bismuth telluride (MnBi2Te4) also extends to electrons on its electrically conductive surface. Previous studies had been inconclusive as to whether or not surface magnetism existed.

But when physicists measured the sensitivity of surface electrons to magnetism, only one of the two observed electronic states behaved as expected. Another superficial state, which should have had a broader response, acted as if the magnetism was not present.

“Is magnetism different on the surface? Or is there something exotic we just don’t understand?” Nevola said.

Johnson leans towards the explanation of exotic physics: “Dan did this very thorough experiment, which allowed him to observe the activity in the surface region and identify two different electronic states on that surface, one that could exist on any metal surface. and one that reflected the topological properties of the material, “he said. “The former was sensitive to magnetism, which proves that magnetism really exists on the surface. However, the other that we expected to be more sensitive had no sensitivity. So, there must be some exotic physics going on!”

The measurements

Scientists studied the material using various types of photoemission spectroscopy, in which light from an ultraviolet laser pulse pushes electrons off the surface of the material and sends them to a detector for measurement.

The bizarre response to magnetism presents a mystery of quantum physics

Dan Nevola, a postdoctoral fellow in the Brookhaven National Laboratory’s Division of Condensed Matter Physics and Materials Science, is the lead author of a new paper describing the bizarre quantum behavior of a magnetic topological insulator. Credit: Brookhaven National Laboratory

“For one of our experiments, we use an additional infrared laser pulse to give the sample a little kick to move some of the electrons before taking the measurement,” Nevola explained. “It takes some electrons and kicks them [up in energy] become conductive electrons. So in a very, very short time – picoseconds – take the measurement to see how the electronic states have changed in response. “

The excited electron energy level map shows two distinct surface bands showing each separate branches, the electrons in each branch having opposite spin. Both bands, each representing one of two electronic states, had to respond to the presence of magnetism.

To test whether these surface electrons were indeed sensitive to magnetism, the scientists cooled the sample to 25 Kelvin, allowing its intrinsic magnetism to emerge. However, only in the non-topological electronic state did they observe a “gap” opening up in the expected part of the spectrum.

“Within those gaps, electrons are forbidden to exist, and therefore their disappearance from that part of the spectrum is the signature of the gap,” Nevola said.

Observing a gap appearing in the regular surface state was the ultimate proof of magnetic sensitivity and proof that the intrinsic magnetism in most of this particular material extends to its surface electrons.

However, the “topological” electronic state studied by the scientists showed no such sensitivity to magnetism, no gaps.

“This raises a question mark,” Johnson said.

“These are properties that we would like to be able to understand and design, just as we design semiconductor properties for a variety of technologies,” continued Johnson.

In spintronics, for example, the idea is to use different spin states to encode information in the way that positive and negative electrical charges are currently used in semiconductor devices to encode the “bits” – 1 and 0 – of the code. Of computer. But spin-encoded quantum bits, or qubits, have many more possible states, not just two. This will greatly expand the potential for encoding information in new and powerful ways.

“Everything about magnetic topology isolators appear to be suitable for this type of technology application, but this particular material doesn’t completely obey the rules,” Johnson said.

So now, as the team continues their search for new states of matter and further insights into the quantum world, there is a new urgency to explain the bizarre quantum behavior of this particular material.

Take a look under the hood of the topological insulators

More information:
D. Nevola et al, Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4, Physical Review Letters (2020). DOI: 10.1103 / PhysRevLett.125.117205

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Brookhaven National Laboratory

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