When Dirac meets frustrated magnetism

Date:
July 31, 2020
Source:
Max-Planck-Institute for Microstructure Physics
Summary:
Scientists have discovered one of the largest anomalous Hall
effects (15,506 siemens per centimeter at 2 Kelvin) ever observed
in the new compound, KV3Sb5. This material has a never-before-seen
combination of properties: Dirac physics, frustrated magnetism,
2D exfoliatability, and chemical stability. Aside from future
fundamental research studying the interplay of these ingredients,
the unique combination has potential for next-generation computing
technologies like spintronics and quantum computing.



FULL STORY ==========================================================================
The fields of condensed matter physics and material science are intimately linked because new physics is often discovered in materials with special arrangements of atoms. Crystals, which have repeating units of atoms
in space, can have special patterns which result in exotic physical
properties.

Particularly exciting are materials which host multiple types of
exotic properties because they give scientists the opportunity to
study how those properties interact with and influence each other. The combinations can give rise to unexpected phenomena and fuel years of
basic and technological research.


==========================================================================
In a new study published in Science Advances this week, an international
team of scientists from the USA, Columbia, Czech Republic, England,
and led by Dr.

Mazhar N. Ali at the Max Planck Institute of Microstructure Physics in
Germany, has shown that a new material, KV3Sb5, has a never-seen-before combination of properties that results in one of the largest anomalous
Hall effects (AHEs) ever observed; 15,500 siemens per centimeter at
2 Kelvin.

Discovered in the lab of co-author Prof. Tyrel McQueen at Johns Hopkins University, KV3Sb5 combines four properties into one material: Dirac
physics, metallic frustrated magnetism, 2D exfoliability (like graphene),
and chemical stability.

Dirac physics, in this context, relates to the fact that the electrons
in KV3Sb5 aren't just your normal run-of-the-mill electrons; they are
moving extremely fast with very low effective mass. This means that they
are acting "light-like"; their velocities are becoming comparable to the
speed of light and they are behaving as though they have only a small
fraction of the mass which they should have. This results in the material
being highly metallic and was first shown in graphene about 15 years ago.

The "frustrated magnetism" arises when the magnetic moments in a material (imagine little bar magnets which try to turn each other and line up
North to South when you bring them together) are arranged in special geometries, like triangular nets. This scenario can make it hard for
the bar magnets to line up in way that they all cancel each other out
and are stable. Materials exhibiting this property are rare, especially metallic ones. Most frustrated magnet materials are electrical insulators, meaning that their electrons are immobile.

"Metallic frustrated magnets have been highly sought after for several
decades.

They have been predicted to house unconventional superconductivity,
Majorana fermions, be useful for quantum computing, and more," commented
Dr. Ali.

Structurally, KV3Sb5 has a 2D, layered structure where triangular vanadium
and antimony layers loosely stack on top of potassium layers. This allowed
the authors to simply use tape to peel off a few layers (a.k.a. flakes)
at a time.

"This was very important because it allowed us to use electron-beam
lithography (like photo-lithography which is used to make computer chips,
but using electrons rather than photons) to make tiny devices out of
the flakes and measure properties which people can't easily measure in
bulk." remarked lead author Shuo-Ying Yang, from the Max Planck Institute
of Microstructure Physics.

"We were excited to find that the flakes were quite stable to the
fabrication process, which makes it relatively easy to work with and
explore lots of properties." Armed with this combination of properties,
the team first chose to look for an anomalous Hall effect (AHE) in the material. This phenomenon is where electrons in a material with an applied electric field (but no magnetic field) can get deflected by 90 degrees by various mechanisms. "It had been theorized that metals with triangular
spin arrangements could host a significant extrinsic effect, so it was
a good place to start," noted Yang. Using angle resolved photoelectron spectroscopy, microdevice fabrication, and a low temperature electronic property measurement system, Shuo-Ying and co-lead author Yaojia Wang
(Max Planck Institute of Microstructure Physics) were able to observe
one of the largest AHE's ever seen.

The AHE can be broken into two general categories: intrinsic and
extrinsic.

"The intrinsic mechanism is like if a football player made a pass to their teammate by bending the ball, or electron, around some defenders (without
it colliding with them)," explained Ali. "Extrinsic is like the ball
bouncing off of a defender, or magnetic scattering center, and going to
the side after the collision. Many extrinsically dominated materials have
a random arrangement of defenders on the field, or magnetic scattering
centers randomly diluted throughout the crystal. KV3Sb5 is special in that
it has groups of 3 magnetic scattering centers arranged in a triangular
net. In this scenario, the ball scatters off of the cluster of defenders, rather than a single one, and is more likely to go to the side than if
just one was in the way." This is essentially the theorized spin-cluster
skew scattering AHE mechanism which was demonstrated by the authors in
this material. "However the condition with which the incoming ball hits
the cluster seems to matter; you or I kicking the ball isn't the same as
if, say, Christiano Ronaldo kicked the ball," added Ali. "When Ronaldo
kicks it, it is moving way faster and bounces off of the cluster with
way more velocity, moving to the side faster than if just any average
person had kicked it. This is, loosely speaking, the difference between
the Dirac quasiparticles (Ronaldo) in this material vs normal electrons (average person) and is related to why we see such a large AHE," Ali
laughingly explained.

These results may also help scientists identify other materials with this combination of ingredients. "Importantly, the same physics governing this
AHE could also drive a very large spin Hall effect (SHE) -- where instead
of generating an orthogonal charge current, an orthogonal spin current
is generated," remarked Wang. "This is important for next-generation
computing technologies based on an electron's spin rather than its
charge." "This is a new playground material for us: metallic Dirac
physics, frustrated magnetism, exfoliatable, and chemically stable all
in one. There is a lot of opportunity to explore fun, weird phenomena,
like unconventional superconductivity and more," said Ali, excitedly.


========================================================================== Story Source: Materials provided by Max-Planck-Institute_for_Microstructure_Physics. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. S.Y. Yang, Y. Wang et al. Giant, unconventional anomalous Hall
effect in
the metallic frustrated magnet candidate, KV3Sb5. Science Advances,
2020 DOI: 10.1126/sciadv.abb6003 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200731145141.htm

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