Magnetic field is an important parameter to influence the electrons in metals. The continuous energy bands transform into discrete energy levels known as Landau levels in the presence of magnetic field, responsible for quantum oscillations in resistivity, magnetization etc. Usual metals require extremely large magnetic field (experimentally unachievable) to populate all the electron into the last Landau level (n = 0), a state called ultra-quantum limit. In contrast, semimetals owing to the small Fermi surfaces need relatively small magnetic fields to reach the ultra-quantum limit. How a system behaves in the highly degenerate ultra-quantum limit is still largely unexplored.
The recent discovery of topological Dirac and Weyl quasiparticles in semimetals has posed an exciting question, i.e., “What is the fate of such states in the ultra-quantum limit?” It is known for some time now that unlike the field-dependent parabolic dispersion in trivial metals, a Dirac spectrum supports a field-independent linear dispersion for the zeroth Landau level. Recently, Zhang et al. and Ramshaw et al. have provided evidence of the annihilation of Weyl nodes in TaP  and TaAs , respectively. What happens in the case of Dirac semimetals, for example Cd3AS2, where the ultra-quantum limit can be reached in pulsed magnetic fields is still unknown. Interestingly, by chemical substitution it is also possible to tune the quantum limit in these systems to much smaller magnetic field.
Our group has strong expertise in growing single crystals and understanding the transport properties of Dirac and Weyl semimetals [3,4]. In this proposal, we suggest to perform the crystal growth of several Dirac and Weyl materials, their characterization in the MPI-CPfS and extend their study in the ultra-quantum limit in collaborations with the HLD (High Magnetic Field Laboratory Dresden). It will be of strong fundamental interest, to observe novel electronic states emerging due to electron correlations in the ultra-quantum limit once the topological protection is lifted.
 C. Zhang et al. Magnetic-tunnelling-induced Weyl node annihilation in TaP,
Nat. Phys. AOP (2017).
 B. Ramshaw et al. arXiv:1704.06944 (2017).
 C. Shekhar et al. Nat. Phys. 11, 645-649 (2015).
 F. Arnold & C. Shekhar, Nat. Commun. 7, 11615 (2016).