Gap opening and quantum Hall effect in thin films of the three-dimensional Dirac semimetal Cd₃As₂

Three-dimensional Dirac semimetals, which are zero band gap materials with a linear dispersion relation at the Fermi energy, have attracted tremendous interest. While most studies are being performed on bulk crystals, fewer reports exist for epitaxial thin films. Moreover, predicted transitions into other topological phases induced, e.g., by confinement or symmetry breaking, have not yet been reported.
We report on the magnetotransport properties of epitaxial films of Cd₃As₂, grown by molecular beam epitaxy on III-V substrates. X-ray diffraction and transmission electron microscopy confirm the synthesis of epitaxial films in the low-temperature Dirac semimetal phase, which possesses the tetragonal I4acd structure. We show the emergence of the quantum Hall effect below a critical film thickness, which is a hallmark of a high-mobility, 2D electronic system. We show that an energy gap opens in the bulk states of sufficiently confined films and all carriers reside in 2D states. Sharp quantization of Hall plateaus and a vanishing longitudinal resistance demonstrates transport that is virtually free of parasitic bulk conduction. We discuss the nature of these surface states. The results demonstrate that heterostructure approaches can engineer quantum states in topological semimetals.