Absence of Proximity-Induced Superconducting Gap on Bi₂Se₃/Graphene/Gallium Heterostructure

Superconducting gallium-intercalated epitaxial graphene on SiC (Gr/Ga/SiC), synthesized by confinement heteroepitaxy, offers a promising platform for exploring next-generation quantum devices. In this work, we employed molecular beam epitaxy to synthesize topological insulator Bi₂Se₃ thin films with varying thicknesses on Gr/Ga/SiC, forming Bi₂Se₃/Gr/Ga heterostructures. By performing in situ angle-resolved photoemission spectroscopy, we observed the formation of the Dirac surface states when the thickness of Bi₂Se₃ is greater than 5 quintuple layers (QLs), consistent with prior studies. Our in situ scanning tunneling microscopy and spectroscopy measurements down to 300 mK show that the proximity-induced superconducting gap is absent even on the surface of 1 QL Bi₂Se₃/Gr/Ga. The absence of the superconducting proximity effect might result from the incompatibility of the Rashba-type band splitting in Bi₂Se₃ and the Ising-type superconductivity in Gr/Ga/SiC.