A team led by the Paul Scherrer Institute in Switzerland have discovered a new quasiparticles while examining a crystalline material – comprising aluminum and platinum atoms arranged in a specific pattern – that also offers electronic properties never previously witnessed. The discovery was made when they realized the compound contained exotic quasiparticles: Rarita–Schwinger fermions. Measurements also showed exotic electronic states on the surface of the material, four Fermi arcs, which are also significantly longer than any observed before.

As reported in Nature Physics [Schröter et al. Nat. Phys. (2019) DOI: 10.1038/s41567-019-0511-y], in the symmetrically repeating unit cells of the crystal, single atoms were offset from each other such that they took a spiral shape, bringing unique electronic behavioural properties for the crystal as a whole, with the Rarita–Schwinger fermions in its interior and very long and quadruple topological Fermi arcs on its surface.

The crystal, a small blackish/silverish cube of about half a centimeter in size, was produced to achieve a precise arrangement of the atoms in the crystal lattice. In crystals every atom has a specific position, and cube-shaped groups of adjacent atoms can form a unit cell that repeats itself in all directions, forming the crystal with its typical symmetries. In the aluminium–platinum crystal, single atoms in adjacent elementary cells followed a helical line, which meant the team were successful in their aim of producing a chiral crystal.

In chiral materials, the right-hand side is a mirror image of the left, and in chiral crystals this can mean some atoms runs clockwise and others counter-clockwise. Using X-radiation and photoelectron spectroscopy, the electronic properties inside the crystal were visible, with measurements allowing them to see the electronic structures on its surface. In this way, the crystal was shown to be not only a chiral material, but also a topological one.

The combination of chirality and topology brings unusual electronic properties that vary between surface and interior. As first author Niels Schröter said, “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, thus at the crystal surface, the number of holes is not well defined.”

A topological phase transition occurs at the crystal surface, resulting in the topological Fermi arcs, and it was clear these and the Rarita–Schwinger fermions were connected, as both result from it being a chiral topological material. The team are looking into similar materials that could show the unique properties in more detail, allowing them to investigate some fundamental issues regarding the nature of electrons at the surface of materials.

"Our result shows topological semimetals can exist in chiral crystals"Niels Schröter
Niels Schröter (left) and Vladimir Strocov at their experimental station in the Swiss Light Source SLS at PSI (credit: Paul Scherrer Institute/Mahir Dzambegovic)
Niels Schröter (left) and Vladimir Strocov at their experimental station in the Swiss Light Source SLS at PSI (credit: Paul Scherrer Institute/Mahir Dzambegovic)