A fresh twist in chiral topology

June 18, 2020

Electrons in “chiral crystals”, solid-state materials with definite “handedness”, can behave in unexpected ways. An interdisciplinary team from research institutions in Germany and China has realized now a theoretically predicted peculiar electronic state in a chiral compound, PtGa, from the class of topological materials. The study which was published in the journal Nature Communications allows a fundamental understanding of the electronic properties of this novel semimetal.

The concept of chirality is well-established in science: when an object cannot be superimposed on its mirror image, both the object and its mirror image are called chiral. In drug industry, for instance, more than 50% of the pharmaceutically active molecules used nowadays are chiral molecules. While one of the “enantiomers” is life-saving, its counterpart with opposite handedness may be poisonous. Another concept which has found widespread interest in contemporary materials science is topology as many so-called topological materials feature exotic properties. For example, topological materials can have protected edge states where electrons flow freely without resistance, as if a superconducting path of electrons were created at the edge of a material. Such unconventional properties are a manifestation of the quantum nature of matter. The topological materials can be classified by a special quantum number, called the topological charge or the Chern number.

Chiral topological materials have particularly unique properties which may be useful in future devices for quantum computers which could speed up computations considerably. An example for such a property is the long-sought large quantized photogalvanic current. Here a fixed dc current is generated in a chiral topological material once exposed to a circularly-polarized light, which is independent of the strength of incident radiation and its direction can be manipulated by the polarization of incident light. This phenomenon relies on the fact that the material possesses a high topological charge of 4, which is the maximum possible value in any material.

Solid-state chemists and physicists from the Max Planck Institute for Chemical Physics of Solids (MPI CPfS), the Leibniz Institute for Solid State and Materials Research (IFW), the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Helmholtz-Zentrum Berlin für Materialien und Energie  (HZB) and the University of Science and Technology of China, Hefei succeeded to realize this peculiar electronic state for the first time in the new chiral topological compound PtGa. Their results have been published in Nature Communications1.

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Counting of the topological charge, called the Chern number (C). Strong spin-orbit coupling is induced by heavier elements as indicated by the deeper background color of the pictured elements. Stronger spin-orbit coupling leads to more obvious split Fermi arcs. This allows the number of states crossing the closed loop to be counted, which determines the Chern number.

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