Under extreme magnetic fields electrons in a metal are confined to a single highly-degenerate Landau level - a regime known as the quantum limit. Electrons under such conditions are unstable to the formation of new states of matter, such as the fractional quantum Hall states in two dimensions. The fate of 3D metals in the quantum limit, on the other hand, has been relatively unexplored. The discovery of monopnictide Weyl semimetals brings a new ingredient to the table - chiral "Weyl" fermions. These quasiparticles are similar to Dirac quasiparticles in graphene, but their "spin up" and "spin down" states are separated due to broken inversion symmetry and spin-orbit coupling. We use magnetic fields up to 95 Tesla to take the Weyl semimetal TaAs into its ultra-quantum limit, isolating its 0th Landau level from the rest of the electronic spectrum, and observe two transitions as a function of field. The first is accompanied by a two-order-of-magnitude increase in the resistivity, indicating a gapped state. The second transition is accompanied by a large increase in ultrasonic attenuation, suggesting the onset of a mesoscopically disordered state, perhaps reminiscent of the "bubble and stripe" phases seen in two dimensional electron gasses. At present we are combing microstructured transport, infrared spectrometry, and ultrasound to identify this potentially new state of matter that arises from Weyl fermions.
Brad Ramshaw
