Topological insulator/superconductor heterostructures are a potential platform for topological quantum computing as they may support Majorana zero modes. In this talk, I will summarize our work in probing proximity-induced superconductivity in epitaxial (Bi,Sb)2Te3 (BST)/graphene (Gr)/2L-Ga thin films and graphene by building tunnel junctions and lateral Josephson junctions.

First, we developed a clean, lithography-free van der Waals tunnel junction on BST/Gr/2L-Ga thin films and performed transport tunneling spectroscopy measurements. We observed two superconducting gaps: One gap agrees with the SC gap of the 2L-Ga, and the second gap is attributed to the proximity-induced superconductivity in the Dirac surface state of the BST film. 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 jumps corresponding to the addition of a single vortex when a magnetic field is applied. This work establishes a large-area, potentially scalable topological insulator/superconductor platform for studies of topological superconductivity. Further studies combining tunneling spectroscopy measurements and Josephson junction experiments are needed to explore topological phase transitions and Majorana zero modes.

 Then I will discuss our work in developing superconducting via contacts. This technique allows us to make Josephson junctions using 3D superconductors but incorporates van der Waals transfer techniques. We first fabricated clean, lithography-free NbN/Pd-graphene Josephson junctions. We observed a gate-tunable supercurrent that reaches ~110 nA and the signature of a small proximity-induced gap in graphene. However, the IcRn product is much reduced compared to the gap of NbN/Pd. One potential reason is the low interface transparency, which needs to be further optimized. Our approach may be generalized to other van der Waals materials and opens up opportunities for studying novel superconducting heterostructures.