Cherned up to the maximum

July 10, 2020

In topological materials, electrons can display behaviour that is fundamentally different from that in ‘conventional’ matter, and the magnitude of many such ‘exotic’ phenomena is directly proportional to an entity known as the Chern number. New experiments establish for the first time that the theoretically predicted maximum Chern number can be reached — and controlled — in a real material.

Controlling the Chern number by hand. Crystals of PdGa can be grown with two distinct structural chiralities (left and right column). The two enantiomers have mirrored crystal structures (second row), as seen in electron-reflection patterns (third row). Schröter et al. now demonstrate that the handedness are reflected as well in the structure of the Fermi surfaces (bottom row), which determine the electronic behaviour of the material. Both compounds display the maximal Chern number, but with opposite sign, +4 and -4, respectively. (Adapted from ref. 1.)

When the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2016 to David Thouless, Duncan Haldane and Michael Kosterlitz, they lauded the trio for having “opened the door on an unknown world where matter can assume strange states”. Far from being an oddity, the discoveries of topological phase transitions and topological phases of matter, to which the three theoreticians have contributed so crucially, has grown into one of the most active fields of research in condensed matter physics today. Topological materials hold the promise, for instance, to lead to novel types of electronic components and superconductors, and they harbour deep connections across areas of physics and mathematics. While new phenomena are discovered routinely, there are fundamental aspects yet to be settled. One of those is just how ‘strong’ topological phenomena can be in a real material. Addressing that question, an international team of researchers led by postdoctoral researcher Niels Schröter at the Paul Scherrer Institute, Switzerland, provide now an important benchmark. Writing in Science, they report experiments in which they observed that in the topological semimetal palladium gallium (PdGa) one of the most common classifiers of topological phenomena, the Chern number, can reach the maximum value that is allowed in any metallic crystal. That this is possible in a real material has never been shown before. Moreover, the team has established ways to control the sign of the Chern number, which might bring new opportunities for exploring, and exploiting, topological phenomena.

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