New crystalline material boasts electronic properties never before seen

The study of a unique crystalline material, composed of aluminum and platinum atoms, has revealed a pair of electronic properties that have never been seen before.

The atoms in the new materials are crystallized in a special pattern, with each row offset from the other. The pattern creates a spiral staircase of aluminum and platinum atoms.

According to the new study, published this week in the journal Nature Physics, the material's unique crystalline structure produces Rarita-Schwinger fermions in its interior and extremely long quadruple topological Fermi arcs on its surface.

Rarita-Schwinger fermions are a type of quasiparticle first described by physicists in 1941. The phenomena describes material states that behave like actual particles.

Researchers at the Paul Scherrer Institute used the Swiss Synchrotron Light Source SLS to directly observe the quasiparticle for the first time.

"As far as we know, we are -- simultaneously with three other research groups -- among the first to see Rarita-Schwinger fermions," PSI researcher Niels SchrÃ¶ter said in a news release.

Inside the aluminum-platinum crystal, a repeating pattern of atoms -- typically cube-shaped -- forms a "unit cell" of one of the two elements. In most crystals, the pattern repeats itself in all directions, forming traditional symmetries, but inside the aluminum-platinum crystal, individual atoms inside adjacent cell units are slightly offset. The arrangement forms a spiral staircase, or a helical line.

The researchers had discovered a chiral crystal. Chirality describes a pair of mirror images, like a left hand and right hand.

"We researchers find chiral materials very exciting, because mathematical models make many predictions that exotic physical phenomena can be found in them," said Vladimir Strocov, a PSI researcher.

Using the SLS X-ray and photoelectron spectroscopy at PSI, scientists were able to observe unique electronic properties. In addition to being chiral, researchers determined that the crystal is also topological.

Topological materials display exotic, defect-resistant electrical properties. Researchers realized the aluminum-platinum crystal boasts unique topological properties on the outside and the inside.

"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," SchrÃ¶ter said. "Therefore, at the transition between crystal and air, thus at the crystal surface, the number of holes is not well defined."

"What is clear, however, is that this is where it changes," said SchrÃ¶ter. "We say that a topological phase transition takes place at the crystal surface. As a result, new electronic states emerge there: topological Fermi arcs."

Fermi arcs are long flows of massless quasiparticles. Researchers suggest the novel material and its Fermi arcs, with differing exotic properties inside and out, could be used to build high-tech electronics, like quantum computers.

"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," said SchrÃ¶ter. "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."

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