New chiral crystalline material exhibits new quasiparticles

Laura Hennemann for the Paul Scherrer Institute and Siv Schwink for Illinois Physics

University of Illinois Physics Professor Barry Bradlyn poses in his office at the Institute for Condensed Matter Theory on the Urbana campus. Photo by Siv Schwink for Illinois Physics
University of Illinois Physics Professor Barry Bradlyn poses in his office at the Institute for Condensed Matter Theory on the Urbana campus. Photo by Siv Schwink for Illinois Physics
Researchers at the Paul Scherrer Institute in Switzerland working with scientists at institutions in Germany, Great Britain, Spain, and the US, have investigated a novel crystalline material, a chiral semimetal, exhibiting never-before-seen electronic properties. These include so-called chiral Rarita-Schwinger fermions in the interior and very long, quadruple topological Fermi arcs on the surface. The crystal, synthesized at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, comprises aluminum and platinum atoms arranged in a helical pattern, like a spiral staircase. It’s the crystal’s chiral symmetry that hosts exotic emergent electronic properties.

These research findings, published online in the journal Nature Physics on May 6, 2019, validate a 2016 theoretical prediction by University of Illinois Physics Professor Barry Bradlyn (then a postdoc at the Princeton Center for Theoretical Science), et al., in the journal Science (vol. 353, no. 6299, aaf5037). That theoretical work was subsequently rounded out by a team of physicists at Princeton University, in research published in 2017 and 2018.

What are quasiparticles?

Niels Schröter (left) and Vladimir Strocov at their experimental station in the Swiss Light Source SLS at PSI. Photo credit: Paul Scherrer Institute/Mahir Dzambegovic
Niels Schröter (left) and Vladimir Strocov at their experimental station in the Swiss Light Source SLS at PSI. Photo credit: Paul Scherrer Institute/Mahir Dzambegovic
What the researchers at the Paul Scherrer Institute (PSI) have found is a new kind of quasiparticle—the so-called Rarita-Schwinger fermion—first predicted in 1941 by physicists William Rarita and Julian Schwinger. Quasiparticles are states in a material that behave as though they are actual elementary particles. The PSI researchers detected these quasiparticles for the first time in photoemission experiments at the Swiss Synchrotron Light Source (SLS) at PSI.

"As far as we know, we are—simultaneously with three other research groups—among the first to see Rarita-Schwinger fermions," says Niels Schröter, a researcher at PSI and first author of the new study.

The search for exotic electron states

The researchers discovered the quasiparticles while investigating a novel material: an aluminum-platinum crystal with a special type of symmetry. This new material was first predicted in the 2016 paper by Bradlyn et al.

“A few years ago, by our theoretical calculations,” Bradlyn notes, “we found a whole class of these materials that we predicted would host quasiparticle fermions and Fermi arcs. The thing that initially inspired this line of thought was just to see what was possible. We knew that you could have Weyl fermions with unit charge, and we knew that Dirac fermions existed, and what else could we get? Could something like a Weyl fermion exist with a charge higher than 1? The fermions we predicted have a charge of 4, and that is the highest that you can get.

“It turned out that to stabilize these emergent properties with such big charges, you need a lot of extra symmetry, so you need very special types of crystals. There aren’t very many materials that like to crystalize in this way, and those that do are hard to grow. The proposed material was computationally modeled by my colleague Maia Vergniory and her team in Spain. And then it took some time and a tremendous effort before the crystal could be materialized in Germany.”

“When viewed with the naked eye, our crystal was simply a small cube: about half a centimeter in size and blackish-silverish,” adds Schröter. “Our colleagues at the Max Planck Institute produced it using a special process.”

The aim of the Dresden researchers was to achieve a tailor-made arrangement of the atoms in the crystal lattice.

In a crystal, each atom has an exactly assigned place. An often cube-shaped group of adjacent atoms forms a basic element, called the unit cell. This unit-cell structure is repeated in all directions and thus forms the crystal with its typical symmetries, which are also visible from the outside. However, in the aluminum-platinum crystal of this investigation, individual atoms in adjacent elementary cells were slightly offset from each other so that they followed a helical line.

“It thus worked exactly as planned: We had a chiral crystal,” explains Schröter.

Chiral crystals have ‘handedness’

Chiral materials can be compared to our hands: The right hand is a mirror image of the left. In chiral crystals this means that in some specimens the imaginary spiral staircase of the atoms runs clockwise, in others it runs counter-clockwise. No matter how you turn the one crystal, it will always differ from the other-handed crystal.

“We researchers find chiral materials very exciting,” explains Vladimir Strocov, a PSI researcher and co-author of the current study, “because mathematical models make many predictions that exotic physical phenomena can be found in them."

And this was indeed the case with the aluminum-platinum crystal which the researchers investigated. Using the X-radiation of the SLS and photoelectron spectroscopy, they made the electronic properties inside the crystal visible. In addition, complementary measurements of the same crystal at the Diamond Light Source in Oxfordshire, England, allowed them to see the electronic structures on its surface.

These investigations showed that the special crystal was not only a chiral material, but also a topological one.

“We call this type of material a chiral topological semimetal,” Strocov comments. “Thanks to the outstanding spectroscopic abilities of the ADRESS beamline here at SLS, we are now among the first to have experimentally proven the existence of such a material.”

The world of donuts

Topological materials came into the public eye with the awarding of the 2016 Nobel Prize in Physics, when three researchers were honored for their investigations into topological phases and phase transitions.

Topology is a field of mathematics that deals with structures and forms that are similar to each other. For example, a ball of modelling clay can be formed into a die, a plate, or a bowl by merely pressing and pulling—these shapes are thus topologically identical. However, to obtain a donut or an 8-shaped form, you have to make holes in the clay: one for the donut, two holes for the 8.

This classification according to the number of holes and further topological properties have already been applied to other physical properties of materials by the 2016 Nobel laureates. Thus, for example, the theory of so-called topological quantum fluids was developed.

“The fact that our crystal is a topological material means that in a figurative sense the number of holes inside the crystal is different from the number of holes outside it. Therefore at the transition between crystal and air, at the crystal surface, the number of holes is not well defined. What is clear, however, is that this is where it changes,” explains Schröter. “We say that a topological phase transition takes place at the crystal surface. As a result, new electronic states emerge there: topological Fermi arcs.”

Quasiparticles inside, Fermi arcs on the surface

It is precisely the combination of these two phenomena, the chirality and the topology of the crystal, that leads to the unusual electronic properties, which differ inside the material and on its surface.

Side-by-side comparison of theory and experiment for fermi arc dynamics in a chiral semimetal. Left shows the first theoretical model, and right shows the new experimental validation in a material which realizes a doubled version of the model.
Side-by-side comparison of theory and experiment for fermi arc dynamics in a chiral semimetal. Left shows the first theoretical model, and right shows the new experimental validation in a material which realizes a doubled version of the model.
While the researchers were able to detect the chiral Rarita-Schwinger fermions inside the material, complementary measurements at the English synchrotron radiation source Diamond Light Source revealed other exotic electronic states on the surface of the material: four so-called Fermi arcs, which are also significantly longer than any previously observed Fermi arcs.

“It is quite clear that the Rarita-Schwinger fermions in the interior and these special Fermi arcs on the surface are connected. Both result from the fact that it is a chiral topological material,” says Schröter. “We are very pleased that we were among the first to find such a material. It's not just about these two electronic properties: The discovery of topological chiral materials will open up a whole playground of new exotic phenomena.”

New materials exhibiting exotic behavior of electrons have implications for potential applications in future electronics.

“These new chiral crystalline materials may be useful for catalysis,” notes Bradlyn. “Additionally, there may be spintronic or electronic applications.”

This news story is based on a press release from the Paul Scherrer Institute.

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