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| Hauptseite > Publikationsdatenbank > Spin- and orbital-dependent band structure of unconventional topological semimetals |
| Hauptseite > Publikationsdatenbank > Spin- and orbital-dependent band structure of unconventional topological semimetals |
| Book/Dissertation / PhD Thesis | FZJ-2023-03380 |
2023
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-701-1
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Please use a persistent id in citations: doi:10.34734/FZJ-2023-03380
Abstract: Topological semimetals host fermion quasiparticles with band crossing points in their bulk electronic structures. In Weyl semimetals, these crossing points are protected by symmetry and topology, forming a Fermi arc at the surface, which connects pairs of these points with opposite chiral charges. Recently, unconventional topological semimetals have emerged with strongly tilted Dirac cones, termed type-II Dirac/Weyl semimetals. Additionally, higher topological charges can be formed in structurally chiral crystals, referred to as chiral topological semimetals. In spite of the emergence of such new materials, the underlying spin texture and its link to topological properties even in conventional topological semimetals have still remained elusive. In this thesis, we studied the type-II Dirac semimetal NiTe2, the type-II Weyl semimetal MoTe2, and the chiral topological semimetal CoSi. Here, when the symmetries of the respective crystal structures are lower, a higher topological charge can be formed. Inversion-symmetric NiTe2 leads to a degenerate topological charge C = 0, while broken inversion symmetry in MoTe2 causes the splitting of topological charges with C = ±1. Chiral structured CoSi is characterized by C = ±2. By means of momentum microscopy together with an imaging spin filter, we revealed spin- and orbitaldependent electronic structures in connection with symmetry and topology. For inversion-symmetric materials like NiTe2, a spin polarization of bulk states is not allowed. An observed “hidden” spin polarization of the bulk Dirac cone, however, originates from the top Te atom of a Te-Ni-Te trilayer. This can be understood in a concept where the degenerate Dirac cone in NiTe2 is formed by a superpositionof two Dirac cones with opposite spin polarizations localized at the top and bottom Te atoms of the trilayer. In particular, we found the same scenario for NiTe2 and MoTe2: a pair of Weyl cones with opposite chirality exhibits a reversed spin polarization. Depending on the symmetry of the crystal structure, however, the cones are degenerate in k space for inversion-symmetric NiTe2 and separated for MoTe2 due to broken inversion symmetry. A strong circular dichroism with reversed sign gives a fingerprint of opposite chiral charges of the Weyl points in MoTe2. The sensitivity of the circular dichroism to the chirality of the system can be directly confirmed in the case of CoSi, where the dichroism reverses its sign between chiral crystals of the opposite structural handedness. The circular dichroism further revealed a different orbital texture of bands forming a higher-charge fermion in CoSi, which is attributed to their topology. In this thesis, we established a relationship between the spin and orbital texture, topology, and symmetry. Beyond the three studied materials, the results presented in this thesis significantly contribute to the understanding of unconventional topological semimetals, in general.
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