Topological Insulator State and Collapse of the Quantum Hall Effect in a Three-Dimensional Dirac Semimetal Heterojunction

Thin films promise new opportunities for the manipulation of surface states of topological semimetals with the potential to realize new states that cannot be obtained in bulk materials. Here, we report transport studies of gated Hall bar structures fabricated from approximately 50-nm-thick, (001)-oriented epitaxial films of cadmium arsenide, a prototype three-dimensional Dirac semimetal, in magnetic fields up to 45 T. The films exhibit a quantized Hall effect with pronounced odd-integer plateaus that is strikingly different from that of the more widely studied (112)-oriented films. We show that the unusual quantum Hall effect is a consequence of the inverted bulk band structure of cadmium arsenide that creates topological-insulator-like states at the bottom and top interfaces, each exhibiting a half-integer quantum Hall effect. A small potential offset between the two surfaces results in the crossing of the Landau levels and gives rise to the filling factor sequences observed in the experiments. Moreover, at large negative values of gate bias, the filling factor ν =1 is abruptly preempted by an insulating state that is accompanied by the collapse of the well-developed quantum Hall effect. We suggest that this new phase cannot be explained within a single-particle picture and discuss the role of Coulomb interactions between spatially separated surface states.