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 experimentally 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.