Observation of tunable Dirac interface states in topological crystalline insulator superlattice*

Topological crystalline insulators (TCIs) are unique topological systems. Their Dirac surface band structure - tightly linked to the crystal symmetries - can be tuned using a variety of external knobs. By stacking TCI Pb_1-xSn_xSe in a superlattice of alternating trivial and non-trivial topology, we dramatically enhance the optical response of its topological Dirac states and succeed in demonstrating the tuning of their energy gap with temperature. We use magnetooptical infrared Landau level spectroscopy to probe the band structure of those topological states in MBE grown high mobility TCI/trivial superlattices for magnetic fields up to 15T and temperatures between 4.2K and 200K. By simply varying the sample temperature, we show that one can tune the penetration depth of surface states into the TCI layer and the inter-surface hybridization across this layer. As a result, we observe a tuning of the Dirac gap over a range extending from 5meV to 60meV. We pave the way for further realization of tunable quantum phases using TCI superlattices.