Proximity-induced Superconductivity in Scalable Topological Insulator/Graphene/Gallium heterostructures*

Proximity-induced superconductivity in topological insulator (TI)/superconductor (SC) heterostructures is a potential platform to host Majorana zero modes. In this work, we synthesize and study high-quality, large area (Bi,Sb)₂Te₃/graphene(Gr)/2L-Ga heterostructures combining confinement heteroepitaxy (Briggs et al.  Nat. Mater. 19, 637–643 (2020)) and molecular beam epitaxy. This synthetic approach results in atomically sharp interfaces and the growth of the TI film preserves the superconductivity of our two-layer Ga film extremely well with a transition temperature of T_c ~ 4 K. We fabricated lithography-free van der Waals tunnel junctions using thin hexagonal Boron Nitride as tunnel barrier and performed transport tunneling spectroscopy on TI/Gr/Ga heterostructures. The tunneling spectra exhibit temperature and magnetic field dependences associated with two superconducting gaps. One gap agrees with the SC gap of the 2L-Ga. We attribute the second gap to proximity-induced superconductivity in the Dirac surface state of the TI film. In 5QL TI/Gr/Ga films, the induced gap is approximately 0.2 meV, which is about 40% of the SC gap in 2L-Ga. In addition, we observe discrete tunneling conductance jump corresponding to the addition of a single vortex when a magnetic field is applied. Our results open up new avenues for developing epitaxial two-dimensional systems for Majorana braiding and topological quantum computation.