Researchers at PSI have investigated a novel crystalline material that exhibits electronic properties that have never been seen before. It is a crystal of aluminum and platinum atoms arranged in a special way. In the symmetrically repeating unit cells of this crystal, individual atoms were offset from each other in such a way that they - as connected in the mind's eye - followed the shape of a spiral staircase. This resulted in novel properties of electronic behaviour for the crystal as a whole, including so-called Rarita-Schwinger fermions in its interior and very long and quadruple topological Fermi arcs on its surface. The researchers have now published their results in the journal Nature Physics. Researchers at the Paul Scherrer Institute PSI have found a new kind of quasiparticle. Quasiparticles are states in material that behave in a certain way like actual elementary particles. The two physicists William Rarita and Julian Schwinger had predicted this type of quasiparticles in 1941, which came to be known as Rarita-Schwinger fermions. Exactly these have now been detected experimentally for the first time - thanks in part to measurements 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 Schroter, 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: a special aluminum-platinum crystal. "When viewed with the naked eye, our crystal was simply a small cube: about half a centimeter in size and blackish-silverish", says Schroter. "Our colleagues at the Max Planck Institute for Chemical Physics of Solids in Dresden produced it using a special process. In addition to the researchers in Dresden, scientists in Great Britain, Spain, and the US were also involved in the current study. 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, the so-called unit cell. This repeats itself in all directions and thus forms the crystal with its typical symmetries, which are also visible from the outside. However, in the aluminium-platinum crystal that was now investigated, individual atoms in adjacent elementary cells were slightly offset from each other so that they - as connected in the mind's eye - followed the shape of a spiral staircase, or in other words: a helical line. "It thus worked exactly as planned: We had a chiral crystal", explains Schroter.