Experimental evidence of Weyl and Dirac relativistic spectrum at topological Volkov-Pankratov heterojunctions

Weyl and Dirac relativistic fermions have attracted tremendous interest in condensed matter as they mimic relativistic high-energy physics. This resemblance enables phenomena like the chiral anomaly to occur in solid state, which therefore becomes a privileged template to experimentally probe and explore fundamental relativistic theories.

A striking example is the discovery of topological insulators, which are insulating in the bulk but exhibit a massless metallic chiral state (CS) at their boundaries. This CS obeys the Weyl equation and displays a relativistic energy-momentum relation. Using magneto-optical spectroscopy, we show in this work that massless Weyl and massive Dirac fermions intrinsically coexist on sufficiently smooth interfaces between a trivial and a topological material, an heterostructure called topological Volkov-Pankratov heterojunction. The emergence of the Dirac states, known as Volkov-Pankratov states, is directly observed, evidencing that their energy levels are perfectly controlled by the smoothness of the interface. Simultaneously, we reveal the optical absorption of the zero-energy chiral Weyl state, whose wavefunction is drastically transformed when going from smooth to the abrupt interface limit. Our work thus introduces a textbook system to explore in detail the rich relativistic energy spectra in condensed matter.