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Topological materials science

Kolloquium der Abteilung 2

Claudia Felser

1Max Planck Institute Chemical Physics of Solids, Dresden, Germany

(e-mail: felser@cpfs.mpg.de)

Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. Since there is a direct connection between real space: atoms, valence electrons, bonds and orbitals, and reciprocal space: bands, Fermi surfaces and Berry curvature, a simple classification of topological materials in a single particle picture should be possible. One important criterion for the identification of the topological material is, in the language of chemistry, the inert pair effect of the s-electrons in heavy elements, and the symmetry of the crystal structure [1]. Beyond Weyl and Dirac, new fermions can be identified in compounds that have linear and quadratic 3-, 6- and 8- band crossings that are stabilized by space group symmetries [2]. Binary phosphides are an ideal material class for a systematic study of Dirac, Weyl and new Fermion physics, since these compounds can be grown as high-quality single crystals. A new class of topological phases that have Weyl points was also predicted in the family that includes NbP, NbAs. TaP, MoP and WP2. [3-8]. In magnetic materials the Berry curvature and the classical anomalous Hall (AHE) and spin Hall effect (SHE) helps to identify potentially interesting candidates. As a consequence, the magnetic Heusler compounds have already been identified as Weyl semimetals: for example, Co2YZ [10-12], Mn3Sn [13-15] and Co3Sn2S2 [16]. The Anomalous Hall angle also helps to identify materials in which a QAHE should be possible in thin films. Even beyond this reciprocal Berry curvature, Heusler compounds with non-collinear magnetic structures also possess real-space topological states in the form of magnetic antiskyrmions, which have not yet been observed in other materials [17].

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[16]  Liu, et al. Nat. Phys. online (2018)

[17]   Nayak, et al., Nature 548, 561 (2017)