Superconductivity and Proximity Effect in Topological Insulator/Graphene/Gallium Heterostructures

Proximity induced superconductivity in topological insulator (TI)/superconductor (SC) heterostructures is a potential platform to host Majorana zero modes. Through confinement heteroepitaxy (Briggset al. Nat. Mater. 19, 637–643 (2020)), we have achieved the intercalation of atomically thin Ga films at the interface of epitaxial graphene grown on SiC. In this work, we report on the superconducting properties of two-layer Ga using four-terminal transport and tunneling spectroscopy. We measure the temperature and magnetic field dependence of the superconducting Ga film (T_c~ 4 K) and evaluate its upper critical field as a function of temperature. The linear relationship between H_c2 and T suggests the 2D superconductivity of Ga. Transport tunneling spectroscopy on graphene/Ga films allows us to determine the superconducting gap of Ga. In finely controlled magnetic field sweeps, we observe evidence of single vortex motion and vortex trapping in the tunneling conductance. Subsequent molecular beam epitaxy growth of thin TI film (Bi_0.2Sb_0.8)₂Te₃ on graphene/Ga allows us to examine the superconducting proximity effect of the Dirac surface states. We present four-terminal transport and tunneling spectroscopy measurements on TI/graphene/Ga heterostructures.