Revealing hidden Dirac cones in the 3D topological insulators

The first methods by which the topological surface states (TSS) of 3D topological insulators (TIs) can be measured is scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES). They cannot be used anymore when the TSS in a hybrid nanostructure is buried under another material. The technique by which the interface states can still be accessed is the ferromagnetic resonance. The dissipation of angular momentum upon electron tunneling from a ferromagnet into the adjacent 3D TI can be understood in terms of spin pumping across the interface. In Dirac systems the magnetic damping is proportional to conductance and the evolution of the magnetic damping in a hybrid ferromagnet/3D TI structure shows clear signatures of a Dirac point [1]. Measurements with finer energy resolution reveal the existence of two Dirac points at different energies; and only in chiral systems conductance through a series of barriers shows a clear peak between two dips, due to the characteristic chiral tunneling. These electrostatic barriers are created by substitutions and vacancies in the 3D TI which cause local variations of the Dirac point. When the 3D TI is severely mistreated, so that the surface becomes strongly corrugated, one would expect at most the destruction of the Dirac cone generated by the TSS. What happens instead is the appearance of an additional cone, extending over much larger momentum and energy ranges. Its origin is still unknown, because each hypothetical scenario for its generation fails to account for at least one of the experimental features [2].

[1] Pietanesi et al., “Tracing Dirac points of topological surface states by ferromagnetic resonance”, Phys. Rev. B109, 064424 (2024)

[2] Kronseder et al., "Generation of an anomalous linearly dispersing spin-polarized band in Bi-based topological insulators", arXiv:2605.22370v1 (2026)