The peculiar response to magnetism presents the mystery of quantum physics

The search for new states of matter and possibly enc encoding, manipulation and new ways to transfer information continues. One goal is to use the quantum properties of materials for communication that go beyond what is possible with traditional electronics. Topological Insulators – Materials that act mostly as insulators but carry an electric current on their surface provide some craving possibilities.

U.S. Exploring the intricacies of topological materials, as well as other interesting emerging phenomena such as magnetism and superconductivity, in exciting and challenging areas for the materials science community at the Department of Energy’s Brookwave National Laboratory, Peter Johnson said. , Senior physicist in the Department of Condensed Matter Physics and Materials Science at Brookwave. “We’re trying to understand these topological insulators because they have a lot of potential applications, especially in quantum information science, an important new field for that department.”

For example, this split insulator / conductor personality material versus “spin.” With the energy of their surface electrons showing a difference in the radiation signatures. This quantum property can potentially be used in “spintronic” devices for encoding and transporting information. Going one step further, combining these electrons with magnetism can lead to novelty and excitement.

“When you have a magnet close to the surface you may have these other foreign conditions that arise from the connection of the topological insulator with the magnetism,” said Dan Navola, a postdoctoral fellow working with Johnson. “If we can find a topological insulator with their own internal magnetism, we must be able to effectively transport electrons of a certain spin in a particular direction.”

Just published in a new study and as an editor’s suggestion Physical Review Letters, Navola, Johnson and their co-editors describe the peculiar behavior of such a magnetic topological insulator. The paper contains experimental evidence that the internal magnetism in the amount of manganese bismuth telluride (MnBi2Te4) also extends to the electrons on its electrically conductive surface. Previous studies have been inconclusive as to whether surface magnetism exists.

But when physicists measured the sensitivity of surface electrons to magnetism, only two of the observed electronic states behaved as expected. The second surface condition, which was expected to receive a large response, behaved as if there was no magnetism.

“Is magnetism different on the surface? Or is it something weird we just can’t understand?” Said Nevola.

Johnson leaned towards the explanation of foreign physics: “Dan performed this very careful experiment, which enabled him to observe the activity of the surface field and to identify the two different electronic states on that surface that exist on any metal surface and reflect one. “The topological properties of the material,” he said. “The former was sensitive to magnetism, proving that magnetism actually exists on the surface.” However, there was no sensitivity about the others whom we expected to be overly sensitive. So, there must be some foreign physics! “

Measurement

Scientists studied the material using a variety of photoelectric spectroscopy, where the light of an ultraviolet laser pulse pierces an electron from the surface of the material and brings it into a detector for measurement.

“For one of our experiments, we used an extra infrared laser pulse to give the sample a little kick to move some electrons before making measurements,” Navola explained. “He takes some electrons and kicks them [up in energy] To conduct electrons. Then, in a very short period of time – in picoseconds, you measure how the electronic states in the answer have changed. “

A map of the energy radiation levels of excited electrons shows two clear surface bands showing each of the specific branches, with each branch having an anti-electron spin. Both bands, each representing one of the two electronic states, were expected to respond to the presence of magnetism.

To test whether the electrons on this surface were really sensitive to magnetism, the scientists cooled a sample of 25 Kelvin to reveal its internal magnetism. However only in the non-topological electronic state did they observe the opening of a “gap” in the expected part of the spectrum.

“In such gaps, electrons are restricted to exist, and therefore their disappearance from that part of the spectrum represents the signature of the distance.” Said Nevola.

The observation of the gap appearing in the regular surface condition was definite evidence of magnetic sensitivity – and evidence that for the most part of this particular material the magnetism extends internally to the electrons of its surface.

However, the “topological” electronic state studied by scientists did not show such sensitivity to magnetism, no distance.

“It throws a little bit into the question mark,” Jones said.

“These are the properties we want to be able to understand and engineer, as we engineer the properties of semiconductors for a variety of technologies,” Johnson continues.

In spintronics, for example, the idea of ​​using different spin states to encode information is that semiconductor devices currently use positive and negative electric charges to encode “bits” —1 and 0s of computer code. But spin-coded quantum bits, or quibs, have many more modes – not just two. This will greatly expand on the possibility of encoding information in a new and powerful way.

“Everything about magnetic topological insulators seems to be suitable for this type of technical application, but these specific materials do not fully comply with the rules.”

So now, as the team continues to explore new states of objects and more insights into the quantum world, there is a new urgency to explain the peculiar quantum behavior of this particular material.