The quirky reaction to magnetism presents quantum physics secrets


Schematic diagram showing both magnetism and conductive behavior on the surface of MnBi2Te4. Magnetism is uniformly pointed upwards, as shown by the red arrows, and the surface electrons, represented by steeple structures, are conductive because the top and bottom parts have a ‘gap’ in the middle at the top. Touch with (see text). These two characteristics are not expected to occur simultaneously, reflecting the need to further understand the basic properties of the material. Sincerely: Brookhaven National Laboratory

The search continues to discover new cases, and possibly new methods of encoding, manipulation, and transport information. One goal is to exploit the quantum properties of materials for communication that can proceed with traditional electronics. Topological insulators – materials that act mostly as insulators, but carry electric current across their surface – offer some tantalizing possibilities.


“Energy of the Brookhaven National Laboratory, exploring topical materials as well as other intricate incidental phenomena such as magnetism and superconductivity for the focus of the US Department of Energy’s material science community” – is one of the most exciting and challenging fields. , A senior physicist in the condensed matter physics and materials science division in Brookhaven. “We are trying to understand these topical insulators because they have a lot of potential applications, especially in quantum informatics, an important new area for segmentation.”

For example, materials with this split insulator / conductor personality, with this spin on their surface indicate a dissociation in the energy signatures of the electrons. “This quantum property can potentially be used in” spintronic “devices for encoding and transport information. Going a step further, pairing these electrons with magnetism can lead to novel and exciting phenomena.

Dan Nevola, a postdoctoral fellow who worked with Johnson, said, “When you have magnetism near the surface, you can have these other foreign states of matter, which arise from the coupling of topical insulators with magnetism . ” “If we can find topical insulators with our own internal magnetism, then we need to be able to efficiently transport electrons of a particular spin in a particular direction.”

A new study was published and published just as an editor suggested Physical review letter, Nevola, Johnson, and their co-workers describe the bizarre behavior of one such magnetic static insulator. The paper contains experimental evidence that internal magnetism in the bulk of manganese bismuth telluride (MnBi2Te4) also propagates to electrons on an electrically conductive surface. Previous studies were inconclusive as to whether surface magnetism existed.

But when physicists measured the sensitivity of surface electrons to magnetism, only one of the two observed electronic states behaved predictably. Another surface condition, which expected a large response, such as magnetism, was not.

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

Johnson leans toward a foreign physics explanation: “Dan did this very careful experiment, which enabled him to observe activity in the surface area and identify two different electronic states on that surface, on any metal surface. May be present and one that reflects the topical properties of the material, ”he said. “The former was sensitive to magnetism, which proves that magnetism actually exists in the surface. However, the second one we expected to be more sensitive did not have any sensitivity at all. Therefore, there must be some foreign physics! ”

The measurement

Scientists studied the material using a variety of photosemission spectroscopy, where light from an ultraviolet laser pulse loosens electrons from the material’s surface and into a detector for measurement.

The quirky reaction to magnetism presents quantum physics secrets

Dan Nevola, a postdoctoral fellow at the Condensed Matter Physics and Materials Science Division at Brookhaven National Laboratory, is the leader on a new paper that describes the quirky quantum behavior of magnetic topological insulators. Sincerely: Brookhaven National Laboratory

“For one of our experiments, we use an additional infrared laser pulse to give the sample a slight kick to move some electrons around before performing the measurement,” explained Mongoose. “It takes some electrons and kicks them [up in energy] To conduct electrons. Then, in a very short time, picoseconds – you make measurements to see how electronic states have changed in response. ”

A map of the energy levels of excited electrons shows two distinct surface bands that display different branches in each branch, the ones with opposite spins of electrons. Both bands, each representing one of the two electronic states, were expected to respond to the presence of magnetism.

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

“Within such intervals, electrons are restricted from existing, and thus their disappearance from that part of the spectrum represents the signature of the gap,” Mongoose said.

Observations of the gap seen at regular surface conditions were definitive proof of magnetic susceptibility — and evidence that magnetism extends to its surface electrons in the bulk of this particular material.

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

“It throws in a bit of a question,” Johnson said.

“These are assets that we want to understand and engineer, just as we engineer the properties of semiconductors for a wide variety of technologies,” Johnson said.

For example, in spintronics, the idea is to use different spin states to encode the information of positive and negative electric charges in the current “semiconductor devices” bits “-1” and 0s – in computer code. To be encoded. But spin-coded quantum bits, or qubits, are many more possible states – not just two. This would greatly expand on the ability to encode information in new and powerful ways.

“Everything about magnetic static insulators seems like they are right for such a technical application, but this particular material does not follow the rules,” Johnson said.

Therefore, while the team will continue its exploration of new cases and move into the quantum world, there is a new urge to explain the bizarre quantum behavior of this particular material.


Take a look under the hood of topological insulators


more information:
D. Nevola et al., Coexistence of surface ferromagnetism and a gapless topological state in MnBi2Te4 Physical review letter (2020). DOI: 10.1103 / PhysRevLett.125.117205

Provided by Brookhaven National Laboratory

Quotes: Magnetic Response to Magnetism Retrieved 10 September 2020 from Quantum Physics Secrets (2020, 10 September) from https://phys.org/news/2020-09-quirky-response-magnetism-quantum-physics.html .

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