Quantum limit properties of Weyl semimetals*

Electrons confined to their 0^th Landau level by extreme magnetic fields—a regime known as the quantum limit—experience strong electron-electron interactions, making them unstable to the formation of new states of matter. The discovery of monopnictide Weyl semimetals has renewed interest in the high-field properties of 3D electrons, with the added twist of linear electronic dispersions. We use magnetic fields up to 95 Tesla to take the Weyl semimetals NbP and TaAs into their quantum limit. In electrical transport and torque magnetometry, we identify signatures of the 0^th Landau levels that are unique to Weyl fermions. In NbP, we show that Weyl fermions can be accessed in high fields, even when the zero-field chemical potential lies far from the nodes. In TaAs, we find that the left and right Weyl nodes are mixed by magnetic field, which opens a gap and suppresses the anomalous "ABJ"-induced conductivity. At the very highest fields in TaAs, we observe a thermodynamic phase transition to an as-yet unidentified state, indicating that Weyl semimetals are unstable to the formation of new states of matter.