Landau quantization and quasiparticle interference in the three-dimensional Dirac semimetal Cd₃ As₂

Condensed-matter systems provide a rich setting to realize Dirac ¹ and Majorana² fermionic excitations as well as the possibility to manipulate them for potential applications^3,4. It has recently been proposed that chiral, massless particles known as Weyl fermions can emerge in certain bulk materials^5,6 or in topological insulator multilayers⁷ and give rise to unusual transport properties, such as charge pumping driven by a chiral anomaly⁸. A pair of Weyl fermions protected by crystalline symmetry effectively forming a massless Dirac fermion has been predicted to appear as low-energy excitations in a number of materials termed three-dimensional Dirac semimetals^9-11. Here we report scanning tunnelling microscopy measurements at sub-kelvin temperatures and high magnetic fields on the II-V semiconductor Cd₃ As₂. We probe this system down to atomic length scales, and show that defects mostly influence the valence band, consistent with the observation of ultrahigh-mobility carriers in the conduction band. By combining Landau level spectroscopy and quasiparticle interference, we distinguish a large spin-splitting of the conduction band in a magnetic field and its extended Dirac-like dispersion above the expected regime. A model band structure consistent with our experimental findings suggests that for a magnetic field applied along the axis of the Dirac points, Weyl fermions are the low-energy excitations in Cd₃ As₂.