Chirality is key: The discovery of chiral topological materials opens the door to exotic phenomena

Helical arrangement of the atoms in aluminium-platinum, indicating the chirality (also known as handedness) of the crystal structure

Researchers at the Max Planck Institute for Chemical Physics of Solids in Dresden used a special process to produce a novel material, a new aluminium-platinum crystal. Although the sample did not look particularly unusual to the naked eye - it was merely a silvery black cube half a centimetre wide - their aim was a tailor-made arrangement of the atoms in the crystal lattice.

Inside a typical crystal, groups of atoms form a basic structural element, called the unit cell. The unit cell is repeated in all directions, creating a crystal with its inherent symmetries. In this aluminium-platinum crystal, however, the atoms were arranged to follow a helical line, the same pattern as a spiral staircase. The result is a chiral crystal. 

Chirality is a concept first proposed by Lord Kelvin in the 19th century. It refers to objects that do not coincide with their mirror image. Chirality is also referred to as 'handedness', because the most obvious example is a pair of human hands. Chiral materials are of interest to researchers because theoretical models suggest that they will give rise to exotic physical phenomena. 

And this proved to be the case with the novel aluminium-platinum crystal. When an international team of researchers, led by the Paul Scherrer Institute in Switzerland, studied it, they found a new quasiparticle.

Niels Schröter, lead author of the study. (Photo: Paul Scherrer Institute/Mahir Dzambegovic)

Topology is a branch of mathematics used to describe properties that can only change in discrete steps, not a continuous flow. In 2016, the Nobel Prize in Physics was awarded to theoretical physicists for their work on topology, and its use in understanding exotic forms of matter. Topology can help us to understand materials with unusual electronic properties. Topological insulators, for example, conduct electricity across their surface but are insulators underneath that layer.

Topological materials could have useful applications in the future, including improved catalysts, new data storage media or more efficient electronic components.

When the research team examined the aluminium-platinum crystal using X-radiation and photoelectron spectroscopy, they were able to see the electronic properties inside the crystal. ARPES measurements on Diamond's I05 beamline showed the electronic structures on the crystal surface. Together, these results demonstrated that this novel crystal is not only a chiral material, but a topological one as well. 

Lead author Dr Niels Schröter of the Paul Scherrer Institute in Switzerland explained:

(A) Long fermi-arcs (indicated by the blue arrows) discovered at the Diamond Light Source on the crystal surface. (B) Exotic new fermion (indicated by the blue arrow) discovered in the bulk.

Complementary bulk sensitive measurements at the Paul Scherrer Institute revealed that aluminium-platinum not only hosts long Fermi-arc surface states, but also contains Rarita–Schwinger fermions. These exotic quasiparticles (states inside the crystal that behave like elementary particles) were predicted by William Rarita and Julian Schwinger in 1941, but have never been observed in experiments before. 

Dr Schröter said:

The team are continuing their exciting research, looking at similar materials that could show these exotic properties in more detail.

To find out more about the I05 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Cephise Cacho: cephise.cacho@diamond.ac.uk.

1. Hennemann L, New material also reveals new quasiparticles. Paul Scherrer Institute, 7 May 2019.