Quirky response to magnetism presents quantum physics mystery
Magnetic topological insulators could be just right for making qubits,
but this one doesn't obey the rules

Date:
September 10, 2020
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists describe the quirky behavior of one such magnetic
topological insulator. The new article includes experimental
evidence that intrinsic magnetism in the bulk of manganese
bismuth telluride (MnBi2Te4) also extends to the electrons on
its electrically conductive surface. Such materials could be just
right for making qubits, but this one doesn't obey the rules.



FULL STORY ==========================================================================
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."
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.

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: "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 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.


========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. D. Nevola, H. X. Li, J.-Q. Yan, R. G. Moore,
H.-N. Lee, H.

Miao, P. D. Johnson. Coexistence of Surface Ferromagnetism
and a Gapless Topological State in MnBi2Te4. Physical Review
Letters, 2020; 125 (11) DOI: 10.1103/PhysRevLett.125.117205 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910171822.htm

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