Observation of Weyl and Dirac fermions at smooth topological Volkov-Pankratov heterojunctions

Weyl and Dirac relativistic fermions are ubiquitous in topological matter. Their relativistic character enables high-energy physics phenomena like the chiral anomaly to occur in the solid state, which allows us to experimen-tally probe and explore fundamental relativistic theories. Here, we show that, on smooth interfaces between a trivial and a topological material, massless Weyl and massive Dirac fermions intrinsically coexist. The emergence of the latter, known as Volkov-Pankratov states, is directly revealed by magneto-optical spectroscopy, evidencing that their energy spectra are perfectly controlled by the smoothness of the topological interface. Simultaneously, we reveal the optical absorption of the zero-energy chiral Weyl state, whose wave function is drastically transformed when the topological interface is smooth. Artificial engineering of the topology profile thus provides a textbook system to explore the rich relativistic energy spectra in condensed matter heterostructures.