Fermi surface and transport of Weyl semimetals
Weyl semimetals are materials where valence and conduction bands cross in single points, the Weyl nodes. When the Fermi energy is near these nodes, the electrons effectively behave as relativistic Weyl fermions with a linear energy dispersion and a well-distinguished chirality.
The existence of Weyl fermions was predicted in high energy physics in 1929 but never confirmed until 2015. In that year, researchers detected the surface states originating in the topological nature of Weyl nodes by angle resolved photoemission spectroscopy. A bulk signature of Weyl fermions is predicted to be a negative magnetoresistance induced by the so-called chiral anomaly, when electric current and magnetic field are parallel. One obvious condition to observe such a Weyl fermionic behaviour is that the Fermi energy in a material has to be close to the Weyl nodes. However, it is still not clear how close and what is the influence of the distance on the bulk transport properties. Measurements of the Fermi surface via quantum oscillations combined with band-structure calculations give us information on the distance of the Fermi energy to the Weyl nodes in a given material. Experimentally, we determine the oscillations of resistivity or magnetisation in magnetic field at low temperature and this will allow a determination of the Fermi surface topography of proposed Weyl semimetal candidates and give information about the effective mass and scattering. This way it can be seen in which materials the Fermi energy is close enough to the Weyl nodes so that the Fermi surface pockets contain separated Weyl nodes, which therefore have a well-defined chirality.
Tantalum arsenide is a Weyl semimetal where the electronic structure was studied profoundly and our results showed the presence of Fermi surface pockets in TaAs with separate Weyl nodes (see ref. Arnold et al., PRL117, 2016). In TaP, on the other hand, Fermi surfaces around Weyl-node pairs emerge that are not penetrated by an integer flux of Berry curvature (and are therefore topologically trivial). In NbAs however, the distance of the Weyl nodes to the Fermi energy seems to be the smallest in these compounds and it is therefore suited for further investigations of the chiral anomaly. Our latest research on TaAs investigates whether hydrostatic pressure can shift the Fermi level even closer to the Weyl nodes in this material and enables us to get experimental access to Weyl fermionic properties.
We are also interested in the behaviour of the physical properties of Weyl semimetals in a magnetic field so high that only the lowest Landau level is occupied. For this project we collaborate with the MagSup group at the Institute Néel in Grenoble, France.
Publications
Naumann, M.; Mokhtari, P.; Medvecka, Z.; Arnold, F.; Pillaca, M.; Flipo, S.; Sun, D.; Rosner, H.; Leithe-Jasper, A.; Gille, P. et al.
Fermi surface of the skutterudite CoSb3: Quantum oscillations and band-structure calculations
Physical Review B 103 (8), 085133 (2021)DOI
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Arnold, F.; Naumann, M.; Wu, S.-C.; Sun, Y.; Schmidt, M.; Borrmann, H.; Felser, C.; Yan, B.; Hassinger, E.
Chiral Weyl Pockets and Fermi Surface Topology of the Weyl Semimetal TaAs
Physical Review Letters 117, 146401 (2016)DOI
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Arnold, F.; Shekhar, C.; Wu, S.-C.; Sun, Y.; Donizeth dos Reis, R.; Kumar, N.; Naumann, M.; Ajeesh, M. O.; Schmidt, M.; Grushin, A. G. et al.
Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP
Nature Communications 7, 11615 (2016)DOI
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Naumann, M.; Arnold, F; Medvecka, Z; Wu, S.-C.; Süss, V.; Schmidt, M.; Yan, B.; Huber, N.; Worch, L.; Wilde, M. A.; Felser, C.; Sun, Y.; Hassinger, E.
Weyl nodes close to the Fermi energy in NbAs
Phys. Stat. Sol. B 259, 2100165 (2022) [1], DOI