Beside extensively studied topological insulators also some materials with zero energy gap -topological semimetals exhibit symmetry related properties leading to exotic behaviour of low energy excitations at certain points of the Brillouin zone where two electronic bands cross. Akin to topological insulators, crystal lattices of topological semimetals have to fulfil specific symmetry conditions: time reversal symmetry and/or inversion symmetry. Both symmetries must be accomplished in Dirac semimetals, whereas in Weyl semimetal one of them has to be broken. Topological semimetals exhibit many interesting features such as linear dispersion of charge carriers (Dirac and Weyl semimetals), inhibited back-scattering of bulk charge carriers (Weyl semimetals), chiral anomaly (Weyl semimetals), etc.

I will present experimental realizations of two topological semimetals grown as epitaxial layers by molecular beam epitaxy: NiTe₂ Dirac semimetal and TaAs Weyl semimetal. Until recently both materials have been obtained as bulk crystals only, we have been the first to report MBE-grown TaAs layers [1]; moreover we have observed new, hitherto unknown crystallographic phase of epitaxial TaAs, also leading to Weyl semimetal properties.

[1] J. Sadowski, J. Z. Domagała, W. Zajkowska, S. Kret, B. Seredyński, M. Gryglas- Borysiewicz, Z. Ogorzalek, R. Bożek, and W. Pacuski, Cryst. Growth Des. 22, 6039 (2022).

Link to lecture video