Quirky Response to Magnetism Presents Quantum Physics Mystery – “There Must Be Some Exotic Physics Going on!”

Magnetic Topological Insulator Schematic

Schematic diagram showing both the magnetism and the conductive behavior on the surface of MnBi2Te4. The magnetism points uniformly upward, as shown by the red arrows, and the surface electrons, represented by the hourglass structures, are conductive because the top and bottom halves touch at the vertex with no ‘gap’ in the middle (see text). Both of these features are not expected to occur simultaneously, illustrating the need to further understand the material’s fundamental properties. Credit: Brookhaven National Laboratory

Magnetic topological insulators could be just right for making qubits, but this one doesn’t obey the rules.

The search is on to discover new states of matter, and possibly new ways of encoding, manipulating, and transporting information. One goal is to harness materials’ quantum properties for communications that go beyond what’s possible with conventional electronics. Topological insulators—materials that act mostly as insulators but carry electric current across their surface—provide some tantalizing possibilities.

“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.”

“There must be some exotic physics going on!” — Brookhaven Lab physicist Peter Johnson

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

“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,” said Dan Nevola, a postdoctoral fellow 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.”

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

Dan Nevola

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

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

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

Johnson leans toward the exotic physics explanation: “Dn 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 measurements

The scientists studied the material using various types of photoemission spectroscopy, where light from an ultraviolet laser pulse knocks electrons loose from the surface of the material and into a detector for measurement.

“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 around prior to doing the measurement,” Nevola explained. “It takes some of the electrons and kicks them [up in energy] to become conducting electrons. Then, in very, very short timescales—picoseconds—you do the measurement to look at how the electronic states have changed in response.” 

The map of the energy levels of the excited electrons shows two distinct surface bands that each display separate branches, electrons in each branch having opposite spin. 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, 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 anticipated part of the spectrum. 

“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 observation of a gap appearing in the regular surface state was definitive evidence of magnetic sensitivity—and evidence that the magnetism intrinsic in the bulk of this particular material extends to its surface electrons.

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

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

“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.

In spintronics, for example, the idea is to use different spin states to encode information in the way positive and negative electric charges are presently used in semiconductor devices to encode the “bits”—1s and 0s—of computer code. But spin-coded quantum bits, or qubits, have many more possible states—not just two. This will greatly expand on the potential to encode information in new and powerful 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.

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

Reference: “Coexistence of Surface Ferromagnetism and a Gapless Topological State in MnBi2Te4” by D. Nevola, H. X. Li, J.-Q. Yan, R. G. Moore, H.-N. Lee, H. Miao, and P. D. Johnson, 9 September 2020, Physical Review Letters.
DOI: 10.1103/PhysRevLett.125.117205

This work was funded by the DOE Office of Science.

7 Comments on "Quirky Response to Magnetism Presents Quantum Physics Mystery – “There Must Be Some Exotic Physics Going on!”"

  1. Magnetism is not a fundamental force but results from the electric force and the relativistic aspect of motion. If there is a some strange quantum effect going on with magnetism than there must be something strange going on with the electric force and/or relativity.
    Jacob Mettes Ph.D. ([email protected])

  2. Curious observer | September 13, 2020 at 11:57 am | Reply

    Not a trained quantum physicist by any means but… the act of measuring collapses the wave function. This occurs as expected on the surface state but not during measurements of the topological surface? Does this suggest no wave function collapse with attempt to measure topological surface because there is nothing there to measure or rather that what is expected to be there is not collapsing when measured?

    Thanks!

  3. I put an earth magnet on a very serious burn, and the pain stopped. The pain would return if I removed the magnet, so I left it on for an hour or so, afterwhich I had no blister and no pain. The next day it was just a pink mark on my skin, and it never even scabbed.. so weird. I use this for healing burns ever since. It’s like magic.

  4. Magnetic steering of water energises the electrons in water molecules. Must be a reason why natural flowing water is in constant gravitational motion.

  5. Edward Leedskalnin.

  6. Donald trumpia | November 7, 2020 at 1:02 am | Reply

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  7. Donald trumpia | November 7, 2020 at 1:53 am | Reply

    He was building the space force instead of saving the servant!Now the servant warned the earless champ emperor, heed the quest of the servant of the most high, he could neither hear not see! Now he wishes he could hear, even when given just a small chance, just a little vote to salvage his empire! But the servant says, game over! No more mercy, it is to late you have lost the chance!I the servant warned him,the pestilence, the lockdown will crumble the empire!! I shouted to the top of the mountain just to give him the pending danger. He was adamant I warned him the most high, is roused to anger by the servant’s denial of economic justice, he thought it was normal. The the emperor was rejected in the high places,to reigh, when the pestilence came to his courts and knocked him down. This was the point of no return, the rejection to reign: now many want the pestilence to go away!! Not until there is an abomination in the land! The pestilence the lockdown shall remain until there is justice for the servant. Until then the most high is dispilesed. The pestilence shall remain until the servant is happy. There is a convenant the servant made with the most high, yes the most just one you are sending me to the wilderness, will I survive that empty land? And the most high, responded, you are the apple of my eye! I will deliver you with my mighty hand! Now I can see the mighty hand and the deliverance at ✋. Please all you standing in the path of the servant, you are against the convenant.Do you want to flex with the mighty one? Unfortunately they do not know him. They will keep knocking against the wall like in darkness, with trouble and pestilence! They are blind, slow as 🐌 constantly being smitten yet not knowing what is striking them. There is an abomination of injustice which the Haven’s cannot take! With all the blindness, more will continue, may be drown like the pharaoh persuing the Israelites across the mighty raging river. The servant’s victory is certain!!!! Do not hold against the servant, or you will go down as a rejected people people,a rejected nation bound for trepidation. Yet there is a very clean path, to take away the pestilence, the scourge,the lockdowns, The path is paying the servant to the last penny! Then will the pestilence go away then the lockdown cease.otherwise the stalemate will continue.

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